Sixth International Conference on Fault and Top Seals

Seal rock characterization became increasingly relevant in a geological storage context in recent years, as the suitability of proven hydrocarbon traps is not yet well understood for alternative pore fluids such as H2 or CO2. Structures which do not contain hydrocarbons as an indirect proof of their integrity may nevertheless be potential storage sites. Furthermore, the exploration of new hydrocarbon prospects still requires an understanding of all petroleum systems elements including the quality distribution of regional and local seal rocks such as mudstones. This work introduces a comprehensive seal rock investigation workflow including multiple petrophysical and image-based porosimetry techniques to capture and compare multiscale (nm to mm) porosity characteristics. Furthermore, geochemical evidence (based on Fluid Inclusion Chemostratigraphy and Rock-Eval pyrolysis) of hydrocarbon migration into and diffusion through a working Middle Miocene (Badenian) top seal in the Vienna Basin is presented. The main aims of this contribution are i) to highlight the capabilities of different state-of-the-art characterization techniques to visualize multi-scale changes in cap rock properties as a result of depositional variations, and ii) to present a new geochemical tool to prove migration and/or diffusion through low-permeable top seals.

A 3D subsurface structural model was built in a zone of the greater Bay du Nord Area, Flemish Pass Basin, offshore Newfoundland, to carry out a post-drilled, fault-seal analysis; aiming to test fault-seal predictions, calibrate computed static fault zone attributes and estimate hydrocarbon contact depths.

The hydrocarbon exploration campaigns in the greater Bay du Nord Area have targeted rotated fault blocks, where structural segmentation and compartmentalisation are common. Subsurface data analysis and special studies following the 2013–2016 exploration campaign were used to build a fault-seal model to support the 2020 exploration campaign.

The subsurface analysis following the 2020 exploration campaign provided open, base, and tight fault-seal cases for future prospect evaluation. Shale Gouge Ratios between 16% and 25% could be used as the transition between non-sealing and membrane sealing fault segments. It is recommended to revisit and update, if necessary, these cases when new sub-surface data and analysis are available.

The interplay of fault throws, and thickness and spatial distribution of reservoir units is the key control on the sealing behaviour of faults in the Greater Bay du Nord area. However, vertical leakage along faults and charge limitation could also regulate the hydrocarbon columns heights.

The role of hydrodynamics in capillary trapping has often been ignored in exploring for hydrocarbons in over-pressured basins. Conventional seal evaluation regards a top seal as static with seal capacity largely a function of the threshold capillary entry pressure, governed by the shale composition and its grain size distribution, and the relative fluid densities. Although possessing very low permeability, shale seals are not perfect seals and water can flow through them under an imposed hydraulic gradient. Likewise, water can also flow through hydrocarbon columns even though, because of the high hydrocarbon saturation, relative permeabilities may also be very low. The impact that this flow can have on the capillary seal capacity can, in theory, be quite profound and ideally should be considered in seal analysis workflows. This talk revisits the Manzocchi & Childs (2013) model for hydrodynamic seal as a means of evaluating the effect of excess pressure on seal capacity. How to implement the Manzocchi & Childs modelling approach in trap analysis is described and the technique then applied to several known over-pressured fields from the Norwegian continental shelf to assess its applicability. Results are shown to offer agreement between predicted and observed columns, potentially supporting the modelling approach.

In a purely top seal-controlled scenario the capillary entry or breakthrough pressure defines the maximum amount of buoyant fluid that can theoretically be accommodated. The seal capacity can be estimated based on normal compaction trends, therefore this work aimed at calibrating established normal compaction trends and resulting seal capacity models for Miocene (Pannonian, Sarmatian, and Badenian) seal rocks of the Vienna Basin.

41 core samples from the Vienna Basin covering a broad depth interval from 720 m to 3270 m were investigated with X-ray diffraction, mercury intrusion capillary porosimetry, He-pycnometry, and Rock-Eval pyrolysis. The results of this study show that established mudstone normal compaction trends are generally suitable to estimate seal capacity in the Miocene Vienna Basin fill. Theoretical column heights show a clear depth trend, and Badenian seals from burial depths > 2000 m show hydrocarbon column heights > 2000 m, implying that capillary seal failure is unlikely. Free hydrocarbon indicators (Rock-Eval S1 and PI) show clear signs of oil staining which correlates with the estimated seal capacity. That points to the validity of seal quality estimations and a potentially significant vertical mobility of fluids in the Vienna Basin which is not purely fault-controlled.

Developing an accurate understanding of fault growth can aid risk management of sites for CO2 storage purposes. Areas of fault interaction lead to differences in the stress field, resulting in an increased strain, which is often accommodated by a high intensity of deformation bands and/or fracturing alter the permeability of the faults. Hence, detecting areas of interaction of pre-existing structures may indicate locations of high risk in terms of the fault’s hydraulic properties. The Vette Fault Zone (VFZ), bounding the Alpha prospect within the potential CO2 Smeaheia storage site developed from a minimum of seven fault segments. Using data from the adjacent Tusse Fault Zone (TFZ), we can identify potential areas of high-risk, where fluids may have the ability to flow across or along the VFZ. The TFZ and VFZ have a high seal potential through gouge and smear mechanisms. Instead of any membrane breaching, areas of high risk have been identified at locations of relict fault-fault intersections where the initial displacement and area of overlap is high, and corresponds with the spill point of Troll East. Using the same assumptions for the VFZ, we can observe one potential high-risk zone, which occurs within the area of potential CO2 accumulation.

The pre-salt succession in the East Brazilian Margin consists of Ediacaran to Cambrian metamorphic basement overlain by Berriasian to Aptian mixed volcanics, clastics and carbonates, and topped by Aptian evaporites. The so called “Pre-Salt” reservoirs represent the upper part of this succession thus comprising the rift transition to post-rift tectonic stages and have been subject of several studies focused on sedimentology, petrography, geochemistry and diagenesis in the North of the Campos Basin. Despite the products of hydrothermal diagenesis being well known in the Pre-Salt reservoirs, their effects in the lower successions, the role of the faults as conduits for hydrothermal fluids, and the effects of lithology in the lateral extent of hydrothermal diagenesis have been only assumed though not fully understood. This works aims to provide evidence obtained through petrography, geochemistry, stable isotopes, fluid inclusion thermometry and U-Pb geochronology in carbonates for an extensive fault-related hydrothermal diagenesis throughout the basement and sedimentary section. The results are integrated through 3D seismic interpretation and inter-well correlations using lithogeochemical logs with recently published works that present, for the first time, comprehensive studies on the tectono-stratigraphic evolution and new age constraints for the deposition of the pre-salt succession.

The rejuvenation of hydrocarbon exploration activities in western Greece has raised an important question on the sealing efficiency of all Tertiary mudstone intervals and their behaviour upon strong tectonic deformation. The sealing efficiency of the mudstones was determined according to outcrop and core observations, combined with wireline log analysis of the brittleness of the mudstones. Mudstones in the overthrusted Oligocene Flysch Formation are assessed to be very brittle, as it is indicated by their brittleness index and the calcite-cemented fault planes on outcrops. Therefore, overthrusted Oligocene mudstones are considered to be prone to leakage, if strongly deformed. Their brittle behaviour is attributed to the strong uplift of the formation during the mid-Miocene thrusting event. On the other hand, wireline log analysis in subthrust locations show that if Oligocene mudstones are deeply buried, then they might regain ductile behaviour and act as efficient seals. Upper Miocene and Pliocene mudstones, although of uncertain extent, have sustained more limited uplift, resulting in the preservation of their ductility. Therefore, the sealing efficiency of such mudstones is less likely to become compromised during tectonic deformation.

Pressure management is a key issue for efficient utilization of CO2 storage within sedimentary basins of the world. We develop analytical approaches to pressure management for the case of multiple CO2 injection sites in a basin model with multiple fault compartments, using the case of the Smeaheia prospects offshore Norway. The analysis helps identify when and where key limiting factors are reached. Preliminary results show that the rock compressibility limits are only of concern in very small fault blocks, while pressure rise towards the geomechanical limits occurs across a range of fault block sizes. In very large fault blocks it is mainly the injectivity which controls final stored volumes. The study indicates that large scale CO2 storage in the Smeaheia prospects appears to be credible, with good potential for finding multiple injection sites with >50Mt storage per fault block. The methodology developed in this study should also be valuable for efficient early-stage screening of storage sites around the globe.

Structural cross sections along the southern Subandean Region of Bolivia shows two structural levels disconnected. A lower one dominated by fault-bend fold structures and an upper one dominated by detachment folding (e.g. Rojas Vera et al., 2019 and references therein). The question to address is; what has caused that rapid change in a short space and among sequences with similar sedimentation and lithologies. The answer should be within the Los Monos Formation, a 500m thick regional overpressured seal, so tight that is capable to maintain overpressure, hundreds of meters above the mean sea level.

This work will show the interaction among pore pressure, structural framework and stress evolution, considering the influence of an overpressured regional seal (Los Monos Fm) on the structural style of the Boyuy-San Alberto area in Bolivia.

To address the behavior and the influence of Los Monos Formation in the structural style of the southern subandean of Bolivia, a 2D geomechanical forward modelling along a section crossing the Suaruro and San Antonio Ranges, two of the Bolivian ranges holding giant gas fields, have been performed.

Discovering interpretational errors while undertaking fault seal analysis is a common experience for practitioners. Integrating quality control checks for identifying technical issues early on, help to understand and reduce uncertainty. From a commercial perspective, this can same time and help to keep projects on schedule. It is particularly important in areas where data quality or data coverage may be an issue. In this contribution, we present case studies that give insights into the potential for using fault topology for the evaluation of structural maps.