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Fifth International Conference on Fault and Top Seals
- Conference date: September 8-12, 2019
- Location: Palermo, Italy
- Published: 08 September 2019
76 results
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Numerical Simulation of Fault Compartmentalisation in CCS Reservoir
Authors L. Langhi, L. Ricard, J. Strand and D. DewhurstSummaryTo improve the South West Hub Carbon Capture and Storage Project (Western Australia) site characterisation and reduce uncertainties around CO2 capacity, injectivity and containment, conceptual fault hydrodynamic models are defined, and reservoir simulations are carried out to investigate the CO2 plume development and its interaction with faults. The conceptual fault hydrodynamic models are defined to incorporate host rock and fault properties accounting for fault zone lithology, cementation, and cataclastic processes. The primary focus was to study the impact of different faulting depth scenarios on reservoir compartmentalization and across-fault and up-fault migration of CO2. Selected flow simulations in the lower injection unit highlight a transition from partially sealing to totally sealing faults that occurs for the most probable faulting depth scenarios. This suggests some degree of compartmentalization in the injection unit resulting in potential restriction of reservoir volume affecting the plume distribution with a preferential CO2 migration aligned along the local N–S structural trend. The compartment bounding faults however, are expected to be composed of overlapping segments unlikely to represent extensively continuous barriers and lateral flow pathways are expected to intermittently exist between compartments reducing risk of pore pressure build up at depth.
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The CO2CRC Otway Controlled CO2 Release Experiment in a Fault: Geomechanical Characterisation Pre-Injection
Authors E. Tenthorey, E. Tenthorey, A. Feitz, A. Feitz, A. Credoz, M. Lavina, E. Coene, A. Idiart and S. JordanaSummaryThe CO2CRC Otway Research Facility is one of the world’s pre-eminent CO2 storage demonstration projects, focussed on carbon storage research and development. The latest injection test that is being planned is based around a small injection test into a shallow fault at depths less than 100 m. The main goal of the controlled CO2 release into the fault is to use a diverse array of geophysical and geochemical monitoring techniques to image the CO2 plume migrating into, around and ultimately up the fault toward the surface. In doing so, we expect to acquire significant learnings around fault permeability, the ease by which CO2 can migrate through minor faults and also gain some understanding about the portions of the fault that are most prone to allowing vertical gas migration. Preliminary geomechanical modelling presented here indicate that the Brumbys fault is a good candidate for a CO2 injection experiments, as it is expected to maintain its integrity and not introduce any geomechanical complications to the equation.
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Fault Seal Analysis Using Seismic Inversions: Pitfalls and Limitations
Authors V. Schuller and M. VögeleSummaryIf only few data available how can we assess the risk and uncertainty of a hydrocarbon accumulation which relies on the sealing capacity of a fault? We want to present a Fault Seal Analysis approach based on the strong integration of geophysical data. The purpose of this workflow is to deliver prediction on Fault Seal Capacity in areas and prospects where no well data is available or offset wells are too far away to be used for Fault Seal input (e.g. Vshale log). Our study suggests that 3D seismic inversion can be exploited to create a Vsh cube which can be used to “read-out” Vsh values along a planned exploration well path in an underexplored setting. The generated Vsh curve is used for a full 3D FSA which delivers distinct values on the fault sealing capacities and uncertainty ranges on expected hydrocarbon column heights. In order to verify the results of this approach, the inverted VSH data was subsequently compared to CPI data from a drilled exploration well in the AOI. Pitfalls and limitations, especially with regards to lithology and fluid effects, are highlighted.
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Fracture Reactivation in Shale: Laboratory Testing and Analytical Modeling
Authors B. Bohloli, J.C. Choi and M. SoldalSummaryThis paper presents a tool for first-order evaluation of fracture/fault reactivation due to fluid injection. It also provides frictional strength parameters of a shale from direct shear testing. The in-house tool, FracStress, was developed based on the analytical Mohr-Coulomb failure criteria and 3D transformation of in-situ stress conditions. The software accounts for the geometry of fracture/fault, 3D stress field, pore pressure and frictional properties. It calculates and visualizes the state of stress for any given fracture orientation. Friction and cohesion of a pre-fractured shale is determined through a direct shear test. The state of stress and frictional strength envelope of material are plotted to estimate the critical injection pressure for the material. The developed tool was tested using the stress condition and test results of Draupne shale in the Ling Depression, the North Sea. The analysis shows that for stress conditions present at the depth of 2582 m, Ling Depression, fractures dipping 60° are close to failure condition. However, fractures dipping 30° and 45° will sustain injection pressure of between 2 to 9 MPa depending on the state of horizontal stresses in the area and the friction coefficient of material.
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Sealing, Healing and Fluid Flow in Clay Rocks: Insights on Episodic Flow Events in Fault Zones
Authors V. Nenonen, P. Dick, J. Sammaljärvi, B. Johansson, M. Voutilainen and M. Siitari-KauppiSummaryThe porosity distribution and mineralogical changes in a clay-rich fault core from the Tournemire underground research laboratory are analyzed to determine the mechano-chemical processes in a small-scale vertical strike-slip fault. The results display significant spatial variations in porosity and mineralogy along different gouge zones due to a polyphased tectonic history combined with complex paleo-fluid migrations. Porosity values increase from the center of the gouges to their borders indicating diffusive sealing/healing effects and past hydrothermal activities. The healing and thus the strengthening of the fault is marked by an increase of calcium content, which is concurrent with lower porosities around the gouge zone. Chemical mapping in the gouges reveal clay alteration, iron zonality and the presence of zinc sulphide as well as barium sulphate inside the gouge, further suggesting past hydrothermal activity. Finally, even though the observed porosity variations only occur in subcentimeter-thick gouge bands, the higher porosity sections are pathways for fluid flow during fault activity.
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Effects of Cap Rock and Hydrodynamic on CBM Preservation A Case Study From Ordos Basin, China
More LessSummaryDifferent from other countries, more than 60% of total resources in China occur in medium- to high-rank coal basins and experienced complicated evolution in the long history of tectonic subsidence and uplifting. CBM preservation in these basins requires high sealing conditions. A series of investigations has documented factors influencing CBM enrichment and production. Among the factors, cap rock and hydrodynamic condition were identified as two primary control factors which has a significant influence on CBM preservation. The combination of low permeability mudstone with a high Young’s modulus in the roof and floor is very beneficial to the preservation of CBM. Type NaHCO3 and type NaCl formation water with the TDS (Total Dissolved Solids) value ranges from 1200mg/L to 4000mg/L is an indicator for a suitable preservation of CBM.
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Diagenetic Impact on Polyphase Fault Zones Drain Properties in Micro-Porous Carbonates (Urgonian – SE France)
Authors I. Aubert, P. Léonide, J. Lamarche and R. SalardonSummaryFault zones can act as drains or barriers depending of their architecture, mechanical and diagenetic evolution. Carbonates are very sensitive to diagenetic processes. When they are affected by faults, impact on reservoir properties is enhance. The study focalized on 2 polyphased faults affecting Lower Barremian microporous platform carbonates (Urgonian facies). We realized porosity measurements, observed 92 thin sections under cathodoluminescence, analyzed micrite micro-fabric with SEM and micro-sampled 189 O and C isotopic measurements to determine the diagenetic evolution of both host rock and fault zones. Our result reveal a strong decrease of porosity towards the fault planes. We identified seven cementation phases and 2 different micrit microfabrics from diagenetic analysis. The C and O measured isotopic values ranging from −7.2 to +1.42‰ for δ13C and from −10.40 to −3.65‰ for δ18O. The cross-cutting, overlap and superposition principles allowed to realize the fault zones and host rock paragenesis. Studied Urgonian carbonates have been affected by 3 diagenetic events divided in pre-faulting and fault-related stages impacting reservoir properties during the early stages of fault activity.
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Structural Diagenesis, Early Embrittlement and Fracture Setting in Shallow-Water Platform Carbonates (Monte Alpi Southern Apennines, Italy)
Authors V. La Bruna, J. Lamarche, F. Agosta, A. Rustichelli, A. Giuffrida, R. Salardon and L. MariéSummaryBed-perpendicular diffuse fractures are common features in carbonates that could be affected by early embrittlement process, which may enhance vertically persistent, opening-mode fractures during the first stages of diagenesis. Early-developed fractures could be independent of tectonics and form a background structural network at different scale. This study focuses on the structural, stratigraphic and petrographic characterization, from outcrop- to micro-scale of Lower Cretaceous, shallow-water, tight limestones pertaining to the Inner Apulian Platform paleogeographic/tectonic domain. These carbonates consist of a wide spectrum of inner platform-platform margin calcareous facies, namely mudstones, wackestones, packstones, grainstones, bindstones, floatstones and rudstones. The presented work is addressed on understanding the development of the early diagenetic features which occurred during the first stages of the sediment lithification processes and the description of the formation, geometry and distribution of the contemporaneous and subsequent structural elements. Field analyses in concert with laboratory analyses unrevealed the main structural-diagenetic events which affected the studied shallow-water carbonates. Petrographic analyses highlight early fractures are intrinsically related to the host rock pore type. Finally, evolution of the fracture network is summarized in a conceptual model by taking into account the several tectonic phases which affect the Monte Alpi sector
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Kinematic and Geological Significance of Contractional Deformation Bands in Oil-Bearing Sandstones: Example from Youshashan Anticline, China
More LessSummaryThe contractional deformation bands developed in oil-bearing sandstones of the fold related to the thrust fault in Qaidam basin, China. Deformation bands are organized as distributed and conjugated networks. The offsets of the deformation bands which are characterized by reverse-sense displacement is about 1mm-20mm. The dihedral angles of deformation bands vary in the range of 47–73°, and they bands are characterized by low S/C ratios in the range of 3.9–22.3. In comparison that of the host rock, the porosity of deformation bands decreased by approximately 11.0%–19.7%, the permeability decreases by 1–2 orders of magnitude. Deformation bands show grain “cataclasis” deformation, and can be categorized as contractional CSBs. The period of CSBs formation in the Youshashan anticline is consistent with or slightly later than the period of Youshashan anticline formation. The burial depth of the formation of deformation bands is approximately 2.5km–1.0km. The ability to store fluid of CSBs is worse than host rock, and oil-bearing content within CSBs is less than host rock. CSBs has a negative effect on hydrocarbon migration, resulting in higher oil saturation in the CSBs than the host rock.
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Stochastic Simulation of Sub-Seismic Faults Conditioned on Displacement Intensity Maps
Authors H. Goodwin, E. Aker and P. RøeSummaryWe present stochastic simulations of sub-seismic faults conditioned on displacement intensity and stress orientation maps generated from a geomechanical model. The simulations are performed in an iterative process where new faults are proposed and have a high probability of being accepted if the contribution of the new fault gives a better match with the input displacement intensity map. The algorithm is demonstrated on a synthetic test case and shows that displacement field and strike orientation of the simulated sub-seismic faults matches the pattern of the displacement intensity map and the orientation of the maximum horizontal stress from a geomechanical model.
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Seal Risking Reinvented Using 300 Year Old Math
Authors J.S. Davis, K.C. Hood, K.J. Steffen and J.M. MatthewsSummarySeal constitutes a primary geologic risk within petroleum exploration. Historically, translating seal analyses into risk commonly utilized risk matrices or tables, which combine measures of confidence and risk severity to reach a Chance of Success (COS). Unfortunately, pitfalls abound in the seal risking process, including failure to consider historical success rates, confusion between confidence and risk severity, poorly calibrated ‘gut-checks’, and DHI adjustments. These complications often resulted in inconsistent seal risking, an inability to distinguish more-risky from less-risky prospects, and poorly prioritized exploration portfolios. We present a new seal risking method – the Risking V – which helps to mitigate these obstacles. The method starts with calibrated base rates and explicit Lines of Evidence (LOEs) that address seal risk. Each LOE has clearly articulated confidence and risk criteria, defined by subject matter experts, which lead to Chance of Validity (COV) values. The risking minimum hurdle is a specified hydrocarbon column height. The user works through the LOEs, then applies sequential Bayesian updating to derive a robust COS specific to the characteristics of the prospect being evaluated. This new method results in a clearly documented and consistent seal COS and avoids many of the traditional risking pitfalls.
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Defining Fault Trap Leakage Pathways via Fault Element Seal Differentiation in Calibration Modelling of Complex Fields
Authors M. Smith and J. ParsonsSummaryDifferentiated seal calibration models of faulted multi-pay oil and gas fields, explaining reservoir fluid fill type in each faulted reservoir, provide a high-resolution quantitative calibration base to explain then predict the industry critical issue of the locations and column heights of fault trapped hydrocarbons. Field calibration models differentiate between working and failed fault seals (matrix and fracture properties) within a fields geologic and stress settings. The models allow testing of seal effectiveness in foot and hanging wall host rock, damage zone and fault core rocks. The calibration objective is to find a ‘best fit’ calibration model and sealing/leaking mechanism that best matches with the observed distribution of water sands, hydrocarbon sands and hydrocarbon column heights. Calibration modelling is conducted to be consistent with the characteristics of fault models as described by Caine et al., 1996 and others. In this method, no prior assumption is made as to the mechanisms of seal success or failure. Success and failure mechanisms vary with different settings and can only be inferred once a best fit model is defined. Calibrated field models are then used as the basis for forward modelling of, in field, exploration targets and for constraining fault zone fluid flow characteristics.
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Incorporating Uncertainty into Fault Seal Analysis
More LessSummaryAnalyzing the sealing capacity of fault rock distributions from clay content distributions modeled from Triangle Diagrams and from data exported from 3D models.
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Understanding the Dynamic Impact of Fault Models Using Fault Properties and Simple Streamline Simulations
More LessSummaryFault zones in porous sandstones can commonly be divided into two parts: a fault core and a damage zone. Both fault zone elements will influence sub-surface fluid flow and must be treated separately to create a geologically realistic model. The fault core can be implemented in the model as a transmissibility multiplier (TM) while the damage zone can be implemented by modifying the grid permeability in the cells adjacent to the model faults. Each of the input parameters used in calculating the TM and damage zone permeability modification is subject to geological uncertainty. We test the impact of varying input parameters through two methods: i) calculating flow indicator fault properties from the static model, and ii) employing a simplified flow-based connectivity calculation, returning dynamic measures of model connectivity. Our results indicate that fluid flow indicator fault properties are weakly correlated with measures of dynamic behaviour. In particular, models with low fault transmissibility show a much greater range of dynamic behaviour, and are less predictable, than models with high fault transmissibility. We ascribe this to the greater complexity of flow paths expected when a highly compartmentalized model contains faults that are likely to be baffles or barriers to cross-fault flow.
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Incorporating Ambiguity and Uncertainty into Subsurface Model Building
Authors J. Solum and K. OnyeagoroSummaryA geological model of a subsurface reservoir is unavoidably less complicated than the geological reality at depth. Two significant factors that contribute to this simplification are ambiguity of the seismic expression of faults and uncertainty caused by the presence of structural heterogeneities that are below the limit of seismic resolution. This is particularly important since fault systems are segmented, and fault architecture will vary with displacement. The fault ranking methodology described in this contribution is a useful way to help quantify interpretation uncertainty, and to identify which faults might need to be revised or might require mutiple realistic interpretations. The scaling relationships described in this work provide a means to estimate a realistic range of the dimensions of fault zone architectural elements, and therefore a way to calculate a realistic range of values for fault properties such as transmissibility.
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Fault Gouge Seal Models: Sensitivity Study of Petrophysical Inputs
Authors R. Locklair, M. Ascanio Regalado, A. Eakin and J. GageSummaryThe study presented here investigates uncertainty in the zonation and composition of host rock stratigraphy that are used in SGR models. For a given stratigraphic sequence, logging data from multiple wells were evaluated to characterize stratigraphic uncertainty related to lateral changes in zone thicknesses. Rock composition uncertainty was evaluated through construction of several derivative log curves in both the rock domain (Vshale) and the mineral domain (Vclay). SGR was calculated for a range of fault throws for all 40 combinations of well and log inputs. As expected, SGR outcomes varied dramatically between cases using Vclay and Vshale inputs, often by a factor of two. This difference could translate to nearly an order of magnitude difference in across-fault pressure differential using published SGR-pressure transforms. This study demonstrates that uncertainty regarding which well(s) and petrophysical input(s) to select for SGR analysis is large enough to influence interpretation of fault zone barriers, baffles, and leaks.
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A New Method of Accounting for Uncertainty and Temporal Variability in Studies of Fault Seal Capacity
Authors C. Reilly and M. ValcarcelSummaryA new workflow is presented here combining temporal analysis of fault seal capacity with a newly developed method of determining the uncertainties associated with fault seal studies. Recognised uncertainties in the geometry of geological models and in lithological data used for fault seal calculations are combined and possible deviations due to these uncertainties are run as a Monte Carlo simulation. Results enable quantification of all possible fault seal parameters. Temporal fault seal studies utilising structural restoration techniques to examine geological model configuration, and resultant fault seal throughout the history of hydrocarbon movement in the subsurface is also carried out. Incorporating our newly developed uncertainty modelling workflow with temporal fault seal modelling greatly reduces the risk inherent in resource definition which is dependent on faults for migration/seal. A number of global datasets are utilised to highlight the efficacy of this workflow combination, with this abstract focussing on the Cape Egmont Fault, Taranaki Basin, New Zealand.
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How to Consider Uncertainties in FSA: A-Posteriori Correction and Montecarlo Workflows for Hydrocarbon Column Height Evaluation
Authors M. Meda and L. ClemenziSummaryIn order to consider the main uncertainties related to the Fault Seal Analysis evaluation for the prospect volume prediction, two approaches have been proposed. The first one is based on a deterministic evaluation of the fault behavior, followed by a correction related to the quality and quantity of the available data. The second approach starts from a base case, modeled using Petrel software; according to defined ranges of values and alternatives for each main parameter, with a Monte Carlo approach, the base case is modified and a contact depth distribution is collected.
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Validation of Fault Seal Mechanisms in the Timor Sea: An Outcrop and Subsurface Perspective
Authors T. Murray, W. Power, S. Sosio de Rosar, Z. Shipton and R. LunnSummaryWhen risking faulted structures, across-fault juxtaposition and or membrane seal are key issues. Generally, this work is done on a “best-guess” model. The application of SGR methods in reservoir / seal systems that have moderate Vshale values artificially increases predicted column heights. In a risking processes these large columns are discounted through other geologic risk factors. Faults in Miri, Sarawak, have been systematically mapped in outcrop in great detail to measure the strike variability of fault rocks. This work greatly helps to understand the limitation of membrane seal algorithms. To illustrate the implications to subsurface risking, a validation will be presented in which observed hydrocarbon water contacts are compared with probabilistic models for both juxtaposition and SGR. A comprehensive review of a set of fields in The Timor Sea shows that probabilistic juxtaposition models more accurately predict hydrocarbon water contacts than calibrated SGR single “best” technical models.
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Sub-Seismic Deformation in Traps Adjacent to Salt Stocks/Walls: Observations from Green Canyon, Gulf of Mexico
Authors S. Wilkins, V. Mount, R. Davies, T. Butaud, B. Lindsey, C. Fenn, J. Syrek, H. Adiguna and I. MatthewsSummaryWe use core and image logs to characterize sub-seismic deformation at two adjacent sub-salt hydrocarbon fields, both of which are 3-way structural traps against salt. Microstructural analysis indicates that most of the deformation bands (DBs) are cataclasites, with a lesser amount of protocataclasites. Core calibration with high-resolution oil-based image logs allows interpretation of DBs that were previously unrecognized. Spatial distributions, orientations, and frequencies of DBs are established from structural interpretation of image logs acquired in wells at different structural positions along the salt-sediment interface of each field. The kinematics of the DBs are interpreted from orientation and displacements, and placed in a larger trap scale context to understand their origin and effect on subsurface fluid flow. Bulk effective permeabilities are estimated for each well by incorporating the host sand and DB permeabilities, as well as the reservoir thickness and volumetric fraction of DBs, and then compared to collected pressure and production data.
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Depth-Dependent Fault Sealing in the Greater Oseberg Area
Authors C. Hermanrud, G.M.G. Teige, T. Lien and M.O. OsnesSummaryWe have re-examined the hydrocarbon distribution in the Greater Oseberg Area to get a more complete understanding of migration and trapping in the area. This work has included observations of oil vs gas, fluid pressure and fault sealing in the area, as well as re-examination of previous modelling work. We suggest that diagenetic fault sealing trapped hydrocarbons in several structures in the Greater Oseberg Area. The faults were largely open when the reservoirs were at shallow burial depths. Therefore, oil could migrate to the Oseberg Alpha structure from the north, and to the Omega structures from the west. The faults started to seal off as the rocks entered the temperature zone that is favourable for quartz diagenesis. This resulted in fault sealing at some places, and open fluid communication along faults (possibly below the fluid contact positions) at other places. Further burial resulted in complete sealing of the fault planes and therefore in pressure build-up. The diagenetic reactions progressed despite the high fluid pressures and resulted in pressures that reached the fracturing limit at Huldra. These high pressures resulted in leaked and underfilled segments here.
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Fault Seal Prediction and Production Experience. Fault Behavior in Upper Jurassic Sands, Halten Terrace
Authors S. Ottesen, H.S. Solli Fossmark, R. Worthington, J. Van Hagen and N. FredmanSummaryThis study reports the findings of a regional investigation of fault seal prediction and production behaviour including several fields in the Halten Terrace area, on the Norwegian Continental Shelf. The Upper Jurassic quartz rich, shallow marine deposits of the Garn Fm are investigated. Faults where Garn Fm. is self-juxtaposed are considered and in short; the study compares production data (fault properties from dynamic flow history matching), 4D and initial pressure data) with fault permeability predictions from a combination of SGR calculations and petrophysical measurement of cored faults. One field also give a calibration point for smearing potential of the shale below the Garn Fm as the faults in this field only provide very limited communication across the shale even if Garn Fm is juxtaposed against deeper stratigraphy.
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Validation of the Membrane Seal Capacity Workflow Using a Case Study and MICP Database
Authors S. Martinez, T. Harrold, G. Saceda and K. MeyerSummaryIn this article a workflow to predict membrane seal capacity and maximum hydrocarbon column potential is applied in two exploratory wells in a new basin. The aim is to prove that the results from the workflow are in the range of the results obtained from laboratory MICP measurements. In addition to the workflow validation, the second objective is to present a regional database built over several years to help refine the hydrocarbon column prediction in new prospects and analog seals.
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Modelling how Faults Influence the Trapping of Oil & Gas
By N. GrantSummaryFaults are known to have a significant role in the migration and trapping of hydrocarbons, either offering conduits for, or barriers to, fluid flow. Faults may also influence fluid phase trapping and hence potentially phase fractionation in the subsurface. A Monte Carlo approach is used to model these effects for trap analysis. The aim its to show how varying both fault seal capacity, the fault orientation, the regional stress tensor, and the trap geometry can all affect how oil and gas are retained within a trap. The model reduces the problem to a 1D analysis with a structural description depth-referenced to the crest of the prospect. Both juxtaposition and membrane fault seal are included, together with hydrodynamic effects and fault reactivation risk. A trap scenario is modelled using input parameter distributions and governing equations. The potential of a prospect to trap hydrocarbons is then evaluated in a roll-up of results with the outputs including a predicted hydrocarbon column height distribution and column height control statistics. The technique also offers an insight into potential fluid phase partitioning that may occur dependant on the active leakage mechanism and spill control, enabling gas versus oil columns to be predicted in certain charge scenarios.
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Top Seal Membrane Capacity Prediction: A Practical Workflow for Exploration
Authors G. Saceda Corralo, T. Harold, S. Martinez and A. VayssaireSummaryAn empirical approach to estimate capillary pressure in conventional mud rock seals is proposed based on estimates of compaction state, porosity or effective stress of shales / mud rock sealing formations. This workflow covers how to infer or estimate shale porosity from different sources of data, the relationships to calculate the capillary entry pressure and maximum hydrocarbon columns. The equations and empirical relationships are based on published work and exploration industry software.
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Evolution of Clay-Gouge Volume and Sealing Capacity during Fault Growth Insight from Present and Paleo Oil-Column
More LessSummaryIt has been demonstrated that increased clay-gouge volume within a fault zone increases a fault’s ability to inhibit hydrocarbons migration across or along it. And Yielding et al. (1997) compared clay-gouge volume (such as shale gouge ratio) vs. the across-fault pressure difference in various fields around the world, calibrated of fault sealing capacity, the quantitative evaluation of fault sealing capacity was realized, and this method is currently widely used in petroleum exploration. However, the result of calibrating fault sealing capacity only reflect the sealing capacity of fault during the stable period, less attention is given to the errors of evaluating fault sealing capacity due to fault activation in field scale. In this paper, we calibrate of fault sealing capacity based on present oil-reservoir analysis from Qinan area, Bohai Bay Basin, China, and analyze the effect of fault reactivation on fault sealing capacity by means of identification of paleo-oil columns. Then we confirm the evolution of clay-gouge volume during fault growth and analyze the effect of fault reactivation on the nominal sealing capacity and the true sealing capacity. At last, we establish an evolution mode to illustrate the relationship between clay-gouge volume and sealing capacity during fault growth.
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Closing the SeAM - Linking Seal Analysis and Mechanics to Improve Fault Stability Prediction
Authors M. Kettermann, V. Schuller, A. Zamolyi, M. Persaud and B. GrasemannSummaryThe reactivation of critically stressed faults during production-induced pressure changes bears a big risk and limits the total recoverable volume as for safety reasons pressure changes must be kept lower than potentially possible. Reservoir pressure and regional stresses can be reasonably well measured and estimated before the start of production. However, to maximize recovery while minimizing the risk of fault reactivation, assessing fault strength as accurately as possible is crucial. The strength of a fault depends on the mechanical properties of the fault rock (friction angle, cohesion), which can vary strongly over short distances, depending on displacement, complexity of fault geometries and available host rock lithologies. Especially clay minerals entrained in the fault core can tremendously reduce the strength of faults. We develop a workflow that uses standard fault seal algorithms (e.g. Shale Gouge Ratio) to estimate shale content in the fault rock and subsequently provide corresponding frictional properties for each fault cell in a reservoir model. This will be a function of the statistical distribution of shale smears in a fault depending on mechanical properties at time of faulting, as well as present day effective stress.
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A Review of Fault Modelling Approaches
Authors A.E. Lothe, B.U. Emmel, A. Lavrov and P.R. CerasiSummaryDifferent approaches are used for large scale fault modelling for CO2 storage, pressure prediction on production scale or on geological time scale. In this contribution we give a short overview of several numerical methods targeting different aspects of fault properties on different scale. We show examples of fluid flow along and across faults, effect on pressure build up and stress changes due to production and injection in a neighbouring fault block, using four different simulation methods. These are a) 3D pressure basin modelling, b) reservoir modelling, c) modified discrete element modelling and d) finite element modelling. The different methods address different challenges related to fault behaviour for a) pressure build up over geological time scale, b) effect of pressure depletion in one part of a basin, and how it is controlled by fault transmissibilites, c) permeability and d) stress changes along a single major fault, due to pressure changes on production time scale.
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An Empirical Approach to Reduce Uncertainty when Predicting Hydrocarbon Column Heights during Prospect Evaluation
Authors I. Edmundson, R. Davies, L. Frette, E. Kavli, A. Rotevatn and A. DunbarSummaryPre-drill volume estimation in exploration involves a number of input parameters that carry a degree of uncertainty. The largest contributor to the pre-drill volume uncertainty is typically the hydrocarbon column height, which is controlled by both charge and seal behaviour. However, it is this parameter that E&P companies often find the hardest to predict, partly due to the lack of sufficient empirical data from existing fields and discoveries. A new empirical dataset has been compiled, which contains hydrocarbon column height, trap height, overburden thickness and trap fill percentage values for over 240 discoveries across the Norwegian Continental Shelf. The data has been aggregated into a probability tree to calculate the likelihood of a discovery containing different ranges of trap fill, depending on its burial depth and trap height. Results show that for a discovery with a given trap height, the probability of recording 100% fill increases when the overburden thickness increases. Equally, when the trap height increases for a given overburden thickness, the probability of discovery 100% trap fill decreases. These findings, amongst others strongly indicate the need to integrate trap height and overburden dimensions to assess suitable hydrocarbon column heights to use when estimating pre-drill volumes.
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Shale Smear Geometries and Implications for Fault Seal: An Outcrop Study from Mt Messenger, New Zealand
More LessSummaryFaults in reservoirs can act as both conduits and barriers to fluid flow. Fault seal arises due to a number of mechanisms including, juxtaposition of permeable and impermeable lithologies, fault cementation, and the production of low permeability fault rock by deformation of host beds. This presentation focuses on the production of low permeability fault rock by shale smear. The aim of this investigation was to gain a representative sample of the smearing behaviours of siltstone beds within the Mount Messenger Formation, a poorly lithified unit with a maximum burial depth of 1.5 km. Siltstone beds appear to be incorporated into fault zones by two end-member mechanisms; smearing and meso-scale synthetic faulting. Thin sections of these smears show that they are often deformed by brittle micro-faulting which is generally sub-parallel to the fault zone. Similarly, meso-scale faulting is most often characterized by multiple synthetic slip surfaces that displace the host beds across the fault zone. We suggest that shearing and incorporation of siltstone beds into fault zones is primarily a brittle process. This distinction is important for fault seal as it is more likely to produce variable silt source bed thicknesses within fault zones.
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The Influence of Fault Membrane Seal on Hydrocarbon Migration and Accumulation in the South Sesulu
Authors I. Lubis, H. Haryanto, L. Gultom, R. Sipayung and G. FauziSummarySouth Sesulu is tectonically complex hydrocarbon province in the southern part of Kutei Basin, Kalimantan. This study presents the role of fault seal on the interplay of migration and accumulation in the fault bounding trap in the northern part of the South Sesulu are. In particular, the study focuses in the deltaic reservoir, mapped as the “13.5 Ma” horizon that has 350 ft gross thickness. We focus the issues raised due to tectonic development prior to hydrocarbon generation and charge at about 11 Ma and subsequent structural reactivation. The structural reconstruction indicates that closure of the objective trap was generated within 13.5 to 9.8 Ma and structural modelling concludes that normal faulting continued to develop in the prospective trap. Displacement backstripping and fault seal analysis (FSA) shows that the bounding fault to the trap probably acted as a migration pathway when the 11.8 Ma horizon was deposited but later acted as seal after continued displacement. FSA has enabled us to predict the hydrocarbon water contact (HWC) in terms of a membrane seal of fault which is consistent with the hydrocarbon discovery in exploration wells. We have used the same analysis to predict as yet untested column height on other closures along the F8 bounding fault.
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Observations on Hanging-Wall Traps that Are Dependent on Fault-Rock Seal
Authors P. Bretan, G. Yielding and E. SverdrupSummaryHanging-wall traps are successful trapping styles with discoveries made in many sedimentary basins worldwide. Examples of hanging-wall traps are documented in the literature but there are very few case studies that describe the role played by fault-rock seal on trap integrity. This contribution focuses on hanging-wall traps that are dependent upon fault-rock seal. Detailed analysis of hanging-wall traps has revealed that the hydrocarbon column height trapped by fault-rock seal appears to be independent of burial depth and is typically less than 190m in height. A similar observation was observed in footwall traps that are dependent on fault-rock seal. Cross-plots of Shale Gouge Ratio (SGR) and buoyancy pressure from hanging-wall traps have a similar data distribution to published SGR – buoyancy pressure calibration plots. Gas-bearing hanging-wall traps are at or close to seal capacity when burial depths are less than 3500m. In contrast, gas-bearing footwall traps are at seal capacity at burial depths greater than 3500m. Published ‘global’ calibration plots of SGR vs buoyancy pressure can be used to evaluate the sealing or non-sealing risk of hanging-wall traps in the same manner as for footwall traps.
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An Integrated Calibration Method of Fault-Seal Capacity in Sand-Clay Sequences: Mechanisms, Principles, and Potential Pitfalls
More LessSummaryFault seal failure envelops calibrated by researchers can hardly be interpreted by the variation mechanisms of fault-rock permeability. We attempt to introduce an integrated calibration method of fault-seal capacity from rock deformation mechanisms and their controls on fault permeability and discuss the potential pitfalls in its application. Fisrtly, the sealing capacity of faults have a lower and upper bound which are controlled by cataclasis density and properties of low permeability host rocks. Secondly, periods of fault activation and main stage of oil accumulation should be clarified before calibration to identify whether hydrocarbon reservoirs are damaged by trap-bounding faults, in order to avoid overestimating the sealing capacity of reactivated trap-bounding faults. Then, AFPD-SGR data from faults reactivated after the main stage of oil accumulation can be never used in the calibration of SFE, which may cause the underestimation of sealing capacity of stable trap-bounding faults. Finally, the integrated calibration of SFE from previous compatible work could help not only to identify a potential reactivated fault on the geological time scale and also to understand the sealing capacity or permeability on the production time scale.
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Fault Zone Evolution and Architecture in Siliciclastic Turbidites and their Impact on Hydraulic Behaviour
Authors H. Riegel, T. Volatili, D. Jablonska, C. Di Celma, F. Agosta, L. Mattioni and E. TondiSummaryFaults in deep marine siliciclastic rocks are often characterized by great variability in fault zone architecture and relative permeability properties. Siliciclastic rocks found in turbidite successions are commonly represented by alternating layers of various thickness and grain size, which form successions with contrasting mechanical properties. For example, the alternation of sandstone and mudstone layers is responsible for the simultaneous occurrence of brittle (cataclasis) and ductile (clay smear) deformation. In this study, we will investigate the turbidite successions outcropping in Tuscany, Italy, as the study areas contains several faults with varying values of displacement that cut through heterolithic layers. Results of field and laboratory analysis are consistent with both bed thickness and grain size distribution of alternating beds dictating the fault zone architecture in terms of fault core composition and texture, damage zone thickness, fracture intensity and height. Furthermore, presence of thin siltstone/shale layers in alternating patterns with thicker sandstone bed intervals likely enhanced the development of clay smear structures in the investigated outcrops. The analysis of deformation mechanisms and resulting fault zone architecture carried out in the selected siliciclastic rocks allow to better understand the role of inherited depositional, diagenetic, and structural heterogeneities on fault zone permeability properties.
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Effect of Shale Volume on the Porosity of Clastic Reservoirs. Case-Study from Mkuki-1 Reservoir, Offshore Tanzania
Authors D.E. Mbaga and G. MwendenusuSummaryThe presence of clay minerals or shale in porous formations presents problems from the interpretation of wireline logs. This mainly affects the porosity and water saturation quantification of the reservoir which result into gross effect in the calculation of hydrocarbon in place (STOOIP). Most of the sand reservoirs show some degree of shaliness. It has therefore become vital to determine the effect of shale content in most of the clastic reservoirs. This project is dedicated in describing the relationship between shale content and the porosity of clastic reservoirs and derive any trend between the two parameters using wireline logging data from Mkuki-1 well, offshore Tanzania.
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Fault Growth and Linkage: Implications for Trap Integrity Associated with Fault Reactivation in the Qinan Slope, Huanghua Depression of Bohai Bay, China
More LessSummary1) Under the differential activities of faults, tilting is the formation mechanism of antithetic fault traps, and fault segment growth and linkage controls the formation and evolution of synthetic fault traps.
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Gas Physical Properties and their Implication on Gas Saturation and Leakage in Deep Reservoirs- A Case Study of Cambrian Gas Field, Sichuan Basin, China
More LessSummaryBased on series of experiments and numerical calculation, the gas physical properties (i.e. density, gas-water interfacial tension) were obtained and their implication on hydrcarbon saturation was studied. Furthermore, and evaluation model was established based on the identification of different types of residue water to obtain gas/water saturation in reservoirs. It is concluded that the lower interfacial tension between gas and water helps the gas charges into reservoirs and thus higher gas saturation was observed in the results of calculation. In addition, the more complicated hydrcarbon saturation variation during burial history was studied. It is found that the seal rock is thin thickness(∼50m) and gas in the reservoir tended to loss during the uplift in burial history.
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Sealing Properties of Tight Rocks – Experimental Challenges
By A. AmannSummaryThe sealing capacity of sedimentary rocks has been discussed recurrently over several decades and in different contexts. These include the stability and longevity of petroleum and natural gas reservoirs as well as the safe permanent storage of radioactive waste or carbon dioxide in the subsurface.
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New Measurements of the Petrophysical Properties of Top and Fault Seals
Authors Q. Fisher, C. Grattoni, S. Allshorn and P. GuiseSummaryThe paper presents new measurements on key petrophysical properties of fault rocks and shale top seals at subsurface conditions. In particular, results are presented from a new instrument that can make mercury injection capillary pressure measurements at reservoir conditions and accurately measure the threshold pressure. Absolute gas permeability from shale caprock samples measured using an extended pressure transient experiment are presented. New relative permeability measurements are also presented from fault rocks. The stressed MICP measurements suggest that the threshold pressures of fault rocks and top seals could be far higher than has previously been assumed. The transient gas permeability measurements indicate that fractures are often present within shales that may not be present in the subsurface indicating that traditional methods may result in an overestimation of shale permeability. The gas relative permeability behaviour of fault rocks appears to be partially related to their absolute permeability. The oil-water relative permeability behaviour of fault rocks seems highly variable and requires further investigation.
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Clay Smear: How Predictive Will we Ever Be? Insight From Sandbox Models and Seismic
Authors P.D. Richard and J. UraiSummaryClay smear is an important fault seal mechanism which controls fluid distribution in hydrocarbon fields. Although the idea is four decades old and extensive research and application in the subsurface has improved our understanding, predicting fault seal by clay smear remains a challenge. The complex architecture of fault zones and the difficulties of critically testing subsurface predictions calls for a challenge to the reliability of current methods. Empirical predictions rely on local calibration, but commonly actual findings are quite different from predictions. The objective of this paper is to share some pioneering sandbox experiments 25 done years ago to determine the structural processes and physical parameters that control clay smear formation. Two modes of clay smear formation have been modelled: injection and smear. The mode is determined by the contrast in strength between the clay and the surrounding rocks. If the strength contrast is low (relatively strong clay layer), no injection occurs, and the fault zone is a sheared equivalent of the faulted stratigraphy. If the strength contrast is high enough (soft clay), injection of clay into the fault zone is triggered. These experimental results led to the development of the squeezing block concept of clay injection.
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Influence of Host Rock Lithology Contrast on Permeability Reduction in Fault Zones (Vienna Basin, Austria)
Authors T. Schröckenfuchs, V. Schuller and A. ZamolyiSummaryIn order to calibrate fault seal capacities to a specific basin, faults were analysed using core material. This calibration was conducted on several Neogene hydrocarbon fields in the Vienna Basin, Austria. Laboratory results showed a significant permeability reduction in all fault rocks compared to the host rocks. The highest and lowest sealing capacities were observed in rocks classifying as cataclastic bands. Although these cataclastic bands showed a broad variation in sealing capacity, the reason for the strong variation could neither be related to the depth at time of faulting or secondary cementation nor the shale/phyllosilicate content or the maximum burial depth of the host rock. Therefore all samples underwent further microscopic investigation. The examined samples revealed that the sealing potential of the fault rocks is strongly linked to the detrital carbonate content in the host rock. Our study shows that addressing the lithology variations in the host rock should not be neglected for fault seal analysis.
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Laboratory Investigations of the Properties of Volcanic Ash Seals
Authors T. Vanorio, J. MacFarlane and A. ClarkSummaryAn essential part of understanding seals includes studying the physical and mechanical behaviors of seals upon chemical interaction with fluids in the subsurface and, specifically, how chemical cementation and fluids being potentially aggressive impact the mechanical behavior of the bonded fabric. In this paper we focus on the chemistry of the cementation process of volcanic ash, the resulting formation of fibrous minerals and their impact on strength, and the role of CO2 in undermining such a cementation process. Intriguingly, volcanic ash and many of the fluids present in the subsurface (i.e., lime, alkalis, and sulfur) have been used to produce ancient mortars and still used in modern cementitious binders. This intertwining of the cementation of ash-based mortars and ash beds in the subsurface is an opportunity for cross-fertilizing knowledge across the Geosciences and Engineering, which is crucial to understand how fluid chemistry controls or undermines the cementation and strength of seals in the subsurface.
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Visualising Hydraulic Fractures in Bedded and Fractured Shales: A Series of Analogue Experiments
Authors A. Wiseall, R. Cuss and E. HoughSummaryThe rapidly growing shale gas industry, especially in the United States, has led to a growing number of research questions associated with understanding the controls on hydraulic fracturing. Having a better understanding of these controls will lead to an improved industry for both regulators and industry. Classic fracture mechanics states that hydraulic fractures will propagate in the direction of the maximum stress, which at these depths is often vertical. However, micro-seismic data from the industry shows that hydraulic fractures often have a considerable lateral extent. The natural heterogeneity of shales and there often heavily fractured nature has been hypothesised to explain this. To test this hypothesis a series of analogous hydraulic fracture visualisation experiments have been carried out using high speed photography of up to 1000 frames per second. Both layered arrangements of Bowland Shale and kaolinite clay discs and fractured arrangements have been examined. Hydraulic fractures were shown to become lithologically bound dependent on material properties. Furthermore, complex interactions between hydraulic fractures and natural fractures was observed. Further work is required to understand the way material properties govern fracture propagation. Results from this can be combined with well characterised shale formations to produce more accurate hydraulic fracture propagation models.
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Textural Controls on the Permeability and Structure of Fault Zones in Shallow Burial Limestones, Malta
Authors A. Cooke, Q. Fisher, E. Michie and G. YieldingSummaryThe lack of a predictive tool for calculating fault permeability in carbonate reservoirs has led to an increasing amount of research towards the permeability structure of carbonate-hosted fault zones. However, a better understanding of fault rock distributions and their potential petrophysical properties is required to predict the impact of faults in carbonate reservoirs. This research combines structural, microstructural and petrophysical data from a series of carbonate-hosted fault zones in Malta, enabling an understanding of the fault zone permeability structures in various lithofacies, whilst highlighting the heterogeneity on all scales of carbonate-hosted fault zones. From the studied exposures, fault displacements of 30 m are required for a continuous fault core, but 100–200 m displacement is required for a continuous cataclasite veneer. Fault rocks have reduced macro-scale heterogeneity compared to host rocks, whilst the outcrop scale heterogeneity is increased. Fault rock permeability measurements show that permeability is reduced relative to the host rock for all faulted lithofacies. Cataclasite exhibits the lowest permeability (geometric average = 10−3 mD). These reductions are large enough to have a noticeable impact on fluid flow over production scales for the c.60% of high porosity fault rocks, however less than 10% of fault rocks derived from low porosity host rocks exhibit these permeability reductions. 30% of fault rocks derived from high porosity host rocks exhibit permeability reductions sufficient to behave as a significant barrier to fluid flow.
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Impact of Laboratory-Induced Deformation and Naturally-Occurring Faults on Fluid Flow in Carbonates
Authors I. Kaminskaite, Q.J. Fisher and E.H. MichieSummaryFaults were studied in a broad range of carbonates and cover all carbonate rock types from Dunham classification: from mudstones to crystalline, with porosities ranging from <1 to 52%. Protolith samples were used for the triaxial testing. Laboratory-deformed and naturally-faulted samples were compared with the host rocks in a petrophysical, macrostructural and microstructural sense. The results show that transition from dilatant to compactive cataclastic flow in carbonates occurs at the porosity of c.10%. Samples with porosity <10% typically dilate during failure and show an increase in permeability after the deformation, whereas samples with porosity >10% compact and show reduced permeability. Deformation bands formed in outcrops with carbonates with a porosity >50% may form significant barriers to fluid flow because cataclasis is often accompanied by recrystallization and cementation which could not be produced in the laboratory. Fault cores in low-porosity outcrops (<1%) consisting of cataclasites and cemented chaotic breccias may form seals to fluid flow because they reduce the permeability by up to a magnitude of 6 and are not cross-cut by fractures. Transitional shear planes or breccias formed both in the laboratory and in damage zones of naturally-formed fault zones show enhanced fracture connectivity, which creates conduits to flow.
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Integrated Assessment of InSAR and Passive Seismic Monitoring, Addressing Caprock Integrity within a SAGD Project
Authors C. Hindriks, S. Azri, K. Bisdom and R. RahmouneSummaryThis extended abstract describes a study explaining and demonstrating a method to identify spatial and temporal changes in fault stability from a combined assessment of InSAR and micro-seismic monitoring data. The method uses reservoir deformation inferred from InSAR data to stochastically model a micro-seismic catalogue which is compared to the measured catalogue. Discrepancies and variations in space and time between the modeled and measured catalogue are addressed by variations in stochastic parameters that are related to fault proximity, fault activation and reservoir strain. Knowledge on the transient behavior of these parameters aid in highlighting high risk zones and planning for future reservoir development options.
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Fault Architecture, an Integrated Study
More LessSummaryFault geometric attributes and fault rock properties are essential components of fault seal analysis and understanding the fluid flow within faulted reservoirs. Therefore, studying the fault geometry and properties are important for different applications such as petroleum exploration and production, CO2 storage, and geothermal energy management. We use an integrated approach to provide a more realistic geometry and architecture of faults. In this approach, fault imaging through seismic attributes is integrated with comparable outcrop studies. Using frequency decomposition and choosing higher frequency seismic data for the attribute analysis, we image faults beyond seismic resolution. Our data covers both siliciclastic and carbonate rocks. Utilizing the compiled fault geometric attributes data measured on outcrop and seismic and comparing them with the previously published data; we investigate the fault scaling relations. The scaling relations are to predict fault dimensions and to better understand the fault growth mechanism. This will reduce the uncertainty related to fault prediction in any kind of reservoir.
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Top Seal Understanding by Integrating 3D Resistivity with Seismic and Wells: Lessons from Gulf of Mexico
Authors V. Ricoy-Paramo and F. RothSummaryThis work illustrates three representative cases of the integration of 3D CSEM derived resistivity with seismic data within the top seal section with the aim of investigating the resistivity response of top seals that (i) have failed, (ii) work, and (iii) are leaky but can still contain hydrocarbons in the prospect. In the past 10 years, CSEM has been used in exploration programs of various offshore basins throughout the world where many wells have been drilled. Although CSEM data have too low resolution to determine the properties close or at the seal reservoir interface, we are getting information from the bulk rock properties of the top seal rock lithology. This has provided a significant number of case examples where the resistivity of the top seal section can be investigated and related to exploration outcome. This integration has shed light on typical patterns of expressions of seismic and resistivity for discoveries vs. failures. We present three such case examples and conclude that the integration of resistivity with seismic allows to enhance our understanding of top seal by providing information on fluid distribution and containment.
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Seismic Fault Damage Zone Characterisation for Reservoir Modelling Using Advanced Attribute Analysis
Authors C. Botter and A. ChampionSummaryFaults play a key role in reservoir connectivity by enhancing or restricting fluid flow. While seismic data is one the main ways of subsurface investigation, faults are still interpreted as surfaces as their internal structure is often at the limit of seismic resolution. In order to populate reservoir models of fault volumes using fault facies techniques, we introduce an innovative seismic attribute analysis to characterise fault core and damage zone from undamaged surrounding rocks in siliclastic settings. We use a total horizontal derivative attribute in addition to the standard seismic attributes applied to fault analyses, and use statistical methods to establish the outer limits of the damage zone. We apply our workflow in porous silisclastic normal faulting in the Thebe Gas Field, Exmouth Plateau of the Canarvon basin, offshore Northwest Australia. Based on this analysis, we are able to define seismic facies calibrated with well and analogue data that can be used for spatial conditioning of fault-facies based reservoir modelling. Our work provide a visual and quantitative tool to define the internal structure of the fault damage zone in order to model fluid flow more accurately in reservoir grids.
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Strain and Flow Pathways in a Shale Fault Zone: An In-Situ Test of Fault Seal Integrity
Authors P. Henry, Y. Guglielmi, C. Gout, R. Castilla, P. Dick, F. Donzé, A. Tsopela, D. Neyens, L. De Barros and J. DurandSummaryA series of small scale (decametric) injection tests were performed in a small fault at IRSN Tournemire underground laboratory in Toarcian shales. Pressure, induced strains and flow rate were monitored at the injection borehole. Monitoring systems comprising strain sensors and a resistivity streamer were installed in observation boreholes within the fluid invaded zone. A micro-seismic network was also deployed. This experiment shows complex interactions between flow and strain as both appear to be distributed between major discontinuities and the fracture network in the damage and core zones. Permeability variations can be approximated as exponential functions of fluid pressure, with different coefficients below and above a threshold, defining the Fracture Opening Pressure. Hydraulic opening is typically associated with dilatant shearing of fractures. However, the associated strain appears small and largely reversible. Rupture on the main fault plane is triggered after several hours of injection, resulting in a permanent change of flow pathways and flow rate. Numerical modeling based on these experimental results suggest fluid channeling along a fault zone could occur in the subcritical Coulomb regime without fault activation at the larger scale.
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Significance of Fault Seal in Assessing CO2 Storage Capacity and Leakage Risks - An Example from Offshore Norway
Authors L. Wu, R. Thorsen, P. Ringrose, S. Ottesen and K. HartvedtSummaryUnderstanding of fault seal is crucial for assessing the storage capacity and leakage risks of a CO2 storage site, as it can significantly impact the project on across-fault and along-fault migration/leakage risking, as well as reservoir pressure predictions. Here we present a case study from Smeaheia offshore Norway to illustrate the importance of fault seal assessment. Leakage risk assessment has been systematically conducted for Smeaheia using the Evidence Support Logic and the Bow-tie methodologies. The results show that the Alpha structure has low across-fault and along-fault leakage risks, thus has a potential value to be added as extra volume for the CO2 storage project. The Beta structure shows large fault-related leakage risks, since it is a 3-way closure juxtaposed to the faulted/fractured Precambrian basement, with large uncertainties of across-fault and along-fault permeabilities of the Øygarden Fault Zone. Fault seal analysis of the relay ramps along the Vette Fault Zone shows that the Smeaheia site can be affected by the pressure drawdown from Troll depletion. However, Smeaheia also has pressure recharging potential from sea bed through Quaternary sediments. Regional dynamic modelling with input from fault seal studies and new pressure measurements can help narrow the storage capacity uncertainty window.
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Deep Galerkin Model in Batchflow
Authors A. Koryagin and S. TsimferSummaryRecently, a lot of papers proposed to use neural networks to approximately solve partial differential equations (PDEs). Yet, there has been a lack of flexible framework for convenient experimentation. In an attempt to fill the gap, we introduce a DEEPGALERKIN-model from BATCHFLOW-framework, open-sourced on GITHUB. Coupled with capabilities of BATCHFLOW, DEEPGALERKIN-model allows to 1) solve partial differential equations from a large family, including heat equation and wave equation 2 ) easily search for the best neural-network architecture among the zoo, that includes RESNET and DENSENET 3) fully control the process of model-training by testing different point- sampling schemes. With that in mind, our main contribution goes as follows: implementation of a ready-to-use and open-source numerical solver of PDEs of a novel format, based on neural networks.
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3D Pore Pressure Analysis for Seal Risk Identification
Authors N. Ordonez Pérez, C. Cobos, F. Obregon and G. Zamora ValcarceSummarySeals represent barriers with different pressures at either sides of the sealing feature (no matter if it is a fault, a slide surface or a change in lithology or facies). Therefore recognizing changes in pressure becomes a strong tool to identify sealing zones and possible traps.
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Advanced Geomechanical Techniques for Natural Fracture Prediction
Authors J.C. Menescal, Y. Bezerra, J.A. Souza, A. Hussein, R. Plateaux, X. Garcia, T. Falcao, A. Rodriguez, L. Maertan and R. StohlerSummaryCarbonate rock reservoirs can be complex and difficult to model. Different processes can affect porosity and permeability during and after deposition. Post deposition process includes diagenesis ( Okubo et al. 2015 ) and deformation related to faults and bending ( Giuffrida et al. 2019 ). Most of the reservoirs are somehow deformed due to multiple processes that occurs in geological time. In carbonate rocks this deformation can act opening or closing fractures, generating conducts or baffles that controls fluids flow behaviour ( Pimenta et al. 2013 ). Because of this impact, modelling 3-D fracture networks has become a critical step for fluid flow simulation. In this paper, it is proposed the use of an advanced workflow which incorporates natural fracture prediction techniques blending with structural restoration and geomechanical forward modelling concepts to evaluate the fractures distribution and then use it to build a fracture model to a giant oil field in Campos Basin, Brazil. The studied reservoir consists mostly of oolitic grainstones from Quissamã Formation deposited on a carbonate platform similar to the rocks described by Okubo et al. (2015) . The reservoir structure is a raft generated by the carbonate platform break above the moving salt ( Vendeville and Jackson 1992 ).
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Top Seal Undrained Pore Pressure Response - Sensitivity to Skempton’s A
Authors M. Duda, R.M. Holt and J.F. StenebråtenSummaryChange of pore pressure in a reservoir modifies stress state in the reservoir itself and in its surrounding, which may further cause an undrained pore pressure response in reservoir’s overburden governed by Skempton’s parameters A and B. This sensitivity study aims to determine the importance of Skempton’s parameter A, often assumed to be 1/3 as predicted by isotropic linear elasticity, and explores the consequences of including experimental, angle-dependent values of this parameter in geomodelling. A top seal shale, assumed to be a representative example of impermeable overburden rocks, has been laboratory-tested in undrained conditions with different stress paths, i.e. ratios between the maximum and the minimum principal stresses. The results of the experimental data analysis have been used to model Skempton’s parameter A distribution and resultant undrained pore pressure response in an existing oil reservoir’s top seal. The modelling indicates that Skempton’s A values computed for a real reservoir geometry and stress state differ significantly from the typically assumed 1/3. The resultant differences in the undrained pore pressure response are significant and usually largest in the direct proximity of fault zones, suggesting that including anisotropic Skempton’s A in geomodelling may help in predicting failure risk in top seal rocks.
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Impact of Fault Permeability on Geomechanical Models of CO2 Injection in the Petrel Sub-basin, Northern Australia
Authors D. Dewhurst, Y. Zhang, P. Schaubs and L. StalkerSummaryThe Petrel sub-basin has been assessed as potentially suitable for the geological storage of carbon dioxide. One of the elements required in characterising an area for geological CO2 storage involves geomechanical modelling of the region. A preliminary set of simple geomechanical models were performed to evaluate the risk of fault reactivation and potential degree of uplift for CO2 injection in the Petrel Sub-basin. A number of injection scenarios were run, ranging from likely in situ injection rates (1–5 million tonnes of CO2 per year) to very large injection rates (∼80 million tonnes of CO2 per year). Overall model results suggest that injection at 1 to 5 million tonnes/year does not result in fault reactivation or host rock failure. Partial fault failure can occur at unrealistically high injection rates of 20 million tonnes/year or above. No fault reactivation occurs at any injection rates under strike slip stress conditions, fault reactivation only happens when the stress regime is on the normal fault-strike slip fault boundary. Varying fault permeability by up to two orders of magnitude changes modelled flow patterns, uplift and pore pressure distributions slightly but does not significantly affect fault reactivation potential.
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A Review of Deformation Bands in Carbonates
By A. CilonaSummaryDeformation bands are narrow tabular zones mm- to-cm thick, which can accommodate shear and/or volumetric strain by means of gran rearrangement, consequent pore collapse, and cataclasis. These structural features are very common in clastic sediments, but they can form also in granular carbonate media. Deformation bands can represent buffers for cross-flow and may cause compartmentalization or decrease in productivity of hydrocarbon reservoirs. This contribution presents an overview of the literature on carbonate-hosted deformation bands, compares them to the clastic hosted deformation bands and highlights the main differences between natural and laboratory deformation bands
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Pressure Equilibration Induced by Reactivation of Deep Carbonate Faults
Authors M. Lesueur, T. Poulet and M. VeveakisSummaryFluid production is known to induce stress changes in the reservoir that can be large enough to reactivate nearby dormant faults. Interestingly, following the reactivation of a fault, fluid pressure equilibration between the two sides of the fault can sometimes be observed. A sealing fault then becomes a flow channel, provoking leakage of the reservoir into the adjacent layer or fluid invasion leading to problematic early water breakthrough. In order to characterise this highly coupled, non-linear Thermo-Hydro-Mechano-Chemical (THMC) behaviour of the fault and quantify fluid invasion, we introduce a multi-scale numerical framework using the REDBACK finite element simulator. This approach links the reservoir (km) scale - implementing a poro-elastic model - to the fault-scale (m) - implementing a THMC reactivation model. The complex behaviour of the fault is upscaled into an interface law that links the stress state of the fault to its response in terms of slippage and permeability. Using this framework, we investigate the fault reactivation scenario in the case of a production from a well located next to a sealing fault in a high P,T carbonate reservoir.
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Pore Space Evolution within Carbonate Fault Rocks
Authors F. Ferraro and F. AgostaSummaryWe investigate five extensional carbonate fault zones responsible of destructive earthquakes in peninsular Italy to understand the control exerted by cataclasis and diagenesis on their petrophysical properties. The study fault rocks were exhumed from depths ≤1.5 km during Plio-Quaternary times. They are made up of calcitic and dolomitic survivor grains, carbonate matrix, and possible calcite cements. The grain-supported fault rocks localize within the outermost portions of the fault cores, whereas the matrix-supported in the innermost portions, near the main slip surfaces. By integrating the results of optical microscopy, digital image, ultrasonic, and petrophysical analyses carried out at both room and increasing confining pressures, we are able to assess the pore type, geometry, connectivity and overall network properties in the different fault rock textures. All grain-supported and dolomiterich, matrix supported fault rocks include soft, crack-like pores, which constitute well-connected connected networks. Differently, within the calcite-rich, matrix-supported fault rocks conspicuous precipitation of euhedral calcite that occurred under vadose conditions occluded the largest and well-connected pores, leaving only stiff, sub-spherical pores. The results of this work permit to establish, for the first time, the relative control exerted by deformation mechanisms and diagenetic processes on the pore characteristics in carbonate fault rocks.
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Carbonate Fault Seal: How Can we Improve its Predictability?
Authors E. Michie, Q. Fisher, B. Freeman, A. Cooke, I. Kaminskaite and G. YieldingSummaryA significant knowledge gap exists when analysing and predicting the behaviour of faults within carbonate reservoirs. To improve this, a large database of carbonate fault rock properties has been collected as part of a consortium led by the University of Leeds and Badleys. This Carbonate Fault Rock project has been successful in discovering key controls on fault rock development, including their petrophysical properties. Many tens of faults within carbonates have been analysed from a range of lithofacies, tectonic regimes, burial depths and displacements, and porosity and permeability measurements from over 300 samples have been made, with the goal to find trends controlling fault rock development. Factors have been examined to assess their control on fault rock permeabilities, such as tectonic regime, displacement, depth of burial, host lithofacies, lithofacies juxtaposition, host porosity and host permeability. The results show which factors have the more significant influence on fault rock properties, and are used to generate a predictive algorithm to compute fault permeability and transmissibility multipliers in carbonate reservoir models.
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Parameterising and Modelling Sub-Seismic Fault Structure for Assessing Compartmentalisation and Juxtaposition Risk
Authors T. Manzocchi, C. Childs, V. Roche, A. Heath, I. Telles, V. Papanikolaou, G. Camanni, M. Carneiro and E. DelogkosSummaryPartitioning of fault displacement onto multiple segments within a fault mapped as a single structure on seismic can result in flow paths between seemingly unconnected fault-bound compartments. The problem can be tackled using a quantitative fault zone description underlain by three parameters that have been characterised for a variety of natural fault systems. Methods for deriving these parameters for subsurface datasets are outlined, and a range of analytical and modelling approaches for assessing the likely importance of sub-seismic fault segmentation in an exploration and production context are discussed and illustrated.
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The Three-Dimensional Geometry of Relay Zones within Segmented Normal Faults
Authors V. Roche, G. Camanni, C. Childs, T. Manzocchi, J. Walsh, J. Conneally, M.M. Saqab and E. DelogkosSummaryNormal faults often comprise arrays of fault segments. The boundaries between adjacent fault segments, relay zones and relay ramps can be of crucial importance to fluid migration pathways across and along faults and for across-fault reservoir juxtaposition. However, despite the increased number of studies of faults in high quality 3D seismic data, the three dimensional geometric or kinematic properties of relay zones or the controls on their 3D geometry are still unclear. This contribution investigates the characteristics of three dimensional segmentation based on an extensive collection of normal faults and relay zones in different geological settings. Our analysis shows that relay zones most often develop by bifurcation from a single fault surface but can also arise from the formation of segments which are disconnected in 3D from their inception. Relay zones generally occur between fault segments that step in the dip or strike direction, and intermediate oblique relay zones are less frequent. When combined together, these different geometries can form a complex spectrum of fault segmentation, whose characteristics can be related to underlying geological controls, such as the mechanical heterogeneity of the faulted sequence and the influence of the basement structure.
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Amalgamation of Silt Smears on Normal Faults at Outcrop Scale
More LessSummaryPresentation examines the role of silt smear amalgamation in low permeability fault-rock generation for two different siliciclastic sequences in New Zealand. Data for outcrop-scale normal faults indicate that the geometries and occurrence of smears is highly variable. In sequences where silt smear is common, fault rock is primarily derived from fine-grained beds in the wall rock with frequent smear amalgamation. In these cases published algorithms may provide useful measures of fault-seal potential. In sequences where smear of silt beds is not ubiquitous and the thickness of smears is highly irregular, smears are only rarely amalgamated. In such cases the majority of fault rock appears to be generated by cataclastic processes and existing algorithms may not accurately describe the contribution of low-permeability wall rock to fault zones. The distinction of these two smear models is key for fault-seal analysis and is partly attributed to sequence architecture and silt-bed composition, where higher phyllosilicate content is associated with greater smearing.
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Scaling of Fault Damage Zones and Implications for Naturally Fractured Reservoirs
Authors S. Mayolle, R. Soliva, Y. Caniven, C. Wibberley, G. Ballas, S. Dominguez and G. MilésiSummaryIn this study, we focus on fault damage zones in carbonate rocks. We analyse fracture damage from small-scale outcrops to map-scale analyses, using scanlines adjacent to fault cores ( Figure 1a, b ) far from fault tip zones, and aerial images. This enabled the detailed characterisation of 12 faults, on which we define the Displacement-Thickness relationship. We analyse the resulting scaling laws and discuss their properties with respect to the map scale observations and previous models proposed in the literature. Finally, we propose new explanations based on fault segmentation and linkage and discuss their application to NFR.
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Fault Leakage Rates Estimated In-Situ: Implications for Fault Seal Evaluation
By C. WibberleySummaryThis presentation examines the main controls on Darcian leakage through fault seals and investigates the evaluation of hydrocarbon column heights in such cases. Firstly, case studies are used to illustrate how fault seal permeability may be estimated in-situ from aquifer compartmentalisation data, improving on existing algorithms currently based mainly on empiric relationships. Secondly the quantitative estimation of hydrocarbon column heights is investigated from the dynamic interplay between charge rates into the trap versus seal leakage rates. A particular consideration is placed on the geometric and petrophysical properties of the leak paths, such as across-fault flow through a fault zone barrier or up-fault conduit leakage. Finally, a consideration is made of the relative importance of aquifer hydrodynamics and dynamic hydrocarbon flow on controlling hydrocarbon contacts. It is concluded that both hydrodynamic flow in the aquifer, and dynamic charging and leaking of hydrocarbons in an active petroleum system, can control hydrocarbon column heights trapped against a seal. Whilst an understanding of the pressure distribution is critical to prospect evaluation in such settings, sensitivity to the geometric parameters of the trap and charge / leak pathways such as fault conduits remains a key uncertainty in the quantitative analysis.
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DEM Modeling of Pressurized Fluid Flow within a Fault Zone in Shale Based on In-Situ Tests
Authors F.V. Donzé, A. Tsopela, Y. Guglielmi, P. Henry and C. GoutSummaryFracture interaction mechanisms and reactivation of natural discontinuities under fluid pressurization conditions inside fault zone can represent critical issues in risk assessment of caprock integrity. A field injection test, carried out in a damage fault zone at the decameter scale i.e. mesoscale, has been studied using a Distinct Element Model. Considering the complex structural nature of a fault zone, the contribution of fracture sets on the bulk permeability has been investigated during a hydraulic injection. It has been shown that their orientation for a given in-situ stress field plays a major role. However, if homogeneous properties are assigned to the fracture planes in the model, the limited irreversible displacements cannot be reproduced. Despite these limited displacements (40 μm maximum), the transmissivity increased by a factor of 10–100. These results provide insights in fracture-controlled permeability of fault zones depending on the geometrical properties of the fractures and their resulting hydro-mechanical behavior for a given in–situ stress field.
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Validation of Fault Seal Prediction Tools, a Case Study in the Permian Rotliegend, Dutch North Sea
Authors K. Van Ojik, A. Silvius, Z. Shipton and Y. KremerSummaryTwo case studies of Permian Rotliegend fields from the Dutch offshore area have been carried out to investigate the validity of property transformation functions estimating threshold entry pressures of fault rock ( Bretan et al. 2003 , Sperrevik et al. 2002 ). These SGR (Shale Gouge Ratio) based functions have been calibrated to Jurassic, Brent type of rock and field data, with clay smearing as the dominant fault sealing mechanism. The theoretical foundation for these algorithms however does not appear to capture the (cataclastic) mechanisms likely to be at play in the fault rocks observed to be present in core at Permian Rotliegend level. One case study is presented here in which the estimated threshold entry pressures are compared against actual pressure data from two wells on either side of a partly-sealing fault, both on a geological and a production timescale. Conclusions from this case study are a.o. that the two functions tested appear to predict required threshold entry pressure levels accurately, within an uncertainty range, which was somehow unexpected in view of the above.
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In-Situ Stress and Fault Reactivation in Bozhong 34-2 Oilfield of Huanghekou Sag, Bohai Bay Basin
By Y. JinSummaryThis study have applied Fault Analysis Seal Technology to the Bozhong 34-2 oilfield, located the relative risk areas of faults, and try to guide water injection in the process of oilfield development. The “Huang model” is used to obtain the in-situ stress. The stress magnitudes indicate a normal faulting regime in the study area, and the direction of maximum horizontal principal stress is N70°E. The fault stability is controlled by fault orientation obviously, and the faults with strike near east-west, dip angle close to 60° have a relatively high risk. All faults are stable under the hydrostatic condition, but the processes of injection and production have a strong influence on the pressure. Therefore, for accurate determination of fault risk, it is necessary to simulate the current pore pressure distribution.
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Fracture Stratigraphy and DFN Modelling of Tight Carbonates, the Case Study of Monte Alpi (Southern Italy)
Authors A. Giuffrida, F. Agosta, P. Castelluccio, E. Panza, V. La Bruna, A. Rustichelli, E. Tondi, M. Eriksson, S. Torrieri and M. GiorgioniSummaryRecent geological and structural analyses of the Monte Alpi carbonate massif focused on the dimensional properties and growth mechanisms of large-scale, high-angle fault zones. In this work, we focus on the role played by primary heterogeneities on the vertical growth of high-angle structural elements such as joints, sheared joints, persistent fracture zones, and small faults. By combining traditional field structural analyses with 3D photogrammetric analysis, we are able to document the attitude and height distribution of individual structural elements. Data are statistically elaborated in order to assess their multi-scale dimensional properties, which are discussed in light of the abutting and crosscutting relationships with respect to the primary heterogeneities. Results of this work fill the gap that exists between the in situ and seismic resolution analyses, and provide new insights into the mechanical stratigraphy of tight, platform-related carbonates of the Inner Apulian Platform.
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Imbricated Structure and Hydraulic Path Induced by Strike Slip Reactivation of a Normal Fault in Carbonates
Authors I. Aubert, J. Lamarche, P. Richard and P. LeonideSummaryFault zones strongly impact basin reservoir properties as they can act as drains or barriers depending of their structural and diagenetic properties. In the case of reactivated faults, newly develop fracture and fault systems reworked and cross-cut preexisting structures generated by the first fault activity potentially result in new fluid pathways. The study focalized on the polyphase Castellas fault affecting Barremian calcarenites (Urgonian facies). We performed a 870m-long high resolution mapping along the Castellas fault zone with a data of 586 measurements, including 56 fault planes, 460 fractures and 70 bedding planes. We realized a geological map and a stratigraphic log and fifteen cross-sections perpendicular to the fault zone, to capture fault architecture variations. The Castellas fault structure, observed on map and cross-sections exhibit large lateral variations of fault core and fault geometry. It is composed of a complex anastomosed architecture made of 8 main fault planes striking from N047° to N078°, with secondary fault planes, horse tail structures and imbricated lenses. Fault zone hydraulic signature change from a porous fault rock with a tight damage zone during normal faulting to a fracture network and fault core enhanced permeability during strike-slip deformation.
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The Cause of Hydrocarbon through Salt in Kuqa Basin, Northwest China: Evidence from Mechanics Laboratory Results and Oil-Gas Exploration Verification
By Z. Qin-GongSummaryBoth anhydrite rocks and salt rocks have embrittlement at low temperatures and ductility at high temperatures. The brittle-ductile transition depth of evaporites in Kuqa Basin is 3000m or so. In the late tectonic active period of Kuqa Era, the strong tectonic compression from South Tianshan will generate fractures and faults in evaporites to let hydrocarbon immigrate upward if burial depth shallower than 3000m. However, after evaporites were continuously buried deeper than 3000m, previous faults would be welded to keep good seals. so early oil accumulations were destroyed and late high mature coal gas accumulations were preserved.
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Fault Seal Analysis (FSA): Benefits and Limitations in Recent Wintershall Drilling Projects
Authors J. Chevallier-Messbacher and P. BretanSummaryShould fault rock attribute analysis be wider implemented within Wintershall Dea and if so, under which data conditions should it be recommended to estimate fault seal potential using Shale Gouge Ratio (SGR) and when to compute hydrocarbon column heights? This question has been addressed through a comprehensive and detailed project, evaluating Pre-Drill and Post-Drill data analysis and results, run on existing Wintershall prospects. The pre-post drilling comparison of fault seal analysis results demonstrates that understanding the sedimentology is equally important as understanding the structure. It is important to be aware that uncertainties in the input will directly be reflected in the predictive capabilities of the fault seal analysis. For this reason, the integration of a fault seal analysis results in the prospect assessment needs to be decided in dependency of the quality and completeness of the input data. And as each variation of the prospect, will have an impact on the fault seal prediction, the fault seal analysis needs to be in very close cooperation with the prospect geologist to ensure consistency between the prospect model used for prospect evaluation and the fault seal analysis model.
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The Impact of Faults on Fluid Flow in Travertine
Authors D. Drayton and Q. FisherSummaryUnderstanding continental carbonate deposits is of increasing importance due to their great hydrocarbon reservoir potential, particularly with the development of continental carbonate plays within the ‘pre-salt’ of the South Atlantic ( Carminatti et al., 2008 ). However, these deposits possess significant heterogeneities related to complex depositional and diagenetic processes ( Delle Piane et al., 2017 ). The nature of faulting within continental-type carbonates and its impact on fluid flow is crucial when considering reservoir characterisation and quality. Unlike established concepts of fault and fluid flow models within clastic rocks ( Knipe 1992 ; Yeilding et al., 1997 ; Fisher & Knipe, 1998 ), the heterogeneous nature of carbonates means that faulting within these deposits is difficult to model and predict (Michie et al., 2017) . To this end, we have carried out an integrated petrophysical and geomechanical study of shallowly buried travertine samples from Italy. The study aims to characterise heterogeneities, examining their control on fault chracteristics and, ultimately, the impact on fluid flow.
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A Strategic Company Approach towards Understanding Sealing Mechanisms and their Impact on the E&P Value Chain
Authors J. Schmitt, F. Schaefer, S. Aartmrg, J. Chevallier and R. HeldSummaryWintershall Dea has kicked off a strategic initiative towards a better handling and fit-for-purpose application of the various seal related topics, including fault seal as well as top, base and intraformational seals. The project targets at establishing a holistic view to seal risk as basis for more informed discussion between the technical community and management. An improved understanding of fault sealing aspects will impact projects throughout the E&P lifecycle. Anchoring fault seal analysis with all its components and tasks such as de-risking exploration prospects, clarification of maximum sustainable hydrocarbon column heights or compartmentalization will create value at relatively low costs as it is involving merely standard data sets.
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Assessment of Fault Sealing in the Nanbaxian Area of Qaidam Basin, Western China
By Q. LiSummaryFault sealing is mainly restricted by many factors, such as lateral sealing and vertical sealing. Based on the structural characteristics of faults in the northern margin of Qaidam Basin, the evaluation methods of fault sealing are analyzed and summarized.
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Seal Integrity in the Uplifted Basins in the Greater Hoop Area on the Northern Barents Shelf
Authors R. Paulsen, S. Grundvåg, K. Senger and E. StuelandSummaryThe geological evolution of the northern Barents Shelf is complex, and the area has undergone several phases of tectonic activity leading to a mosaic of several deep and shallow basins, platforms, salt-cored domes and structural highs. The area is highly gas prone with some very few commercial oil discoveries, and only two fields currently in production. Deep burial and subsequent Late Cretaceous–Cenozoic exhumation and isostatic rebound following the last glaciation have strongly influenced the petroleum system This have in particular lowered the cap rock integrity over large parts of the northern shelf. Thus, we wish to evaluate the regional cap-rock properties in the area in order to lower future geological exploration risk. Additionally we aim to establish a robust structural framework and sequence-stratigraphic models, highlighting the tectonic and stratigraphic evolution of regional cap-rock units on the northern Barents Shelf.
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