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4th EAGE North African/Mediterranean Petroleum and Geosciences Conference and Exhibition Tunis 2009
- Conference date: 02 Mar 2009 - 04 Mar 2009
- Location: Tunis, Tunisia
- ISBN: 978-90-73781-99-3
- Published: 02 March 2009
1 - 50 of 71 results
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The Paleocene in North Africa – Sea-level Changes and Paleoproductivity in Tunisa, Libya and Egypt Using Microfossils
More LessThe Paleocene is a time of warm oceans and generally high sea levels. In North Africa, vast epicontinental seas extended far inside the African continent. In this paper we correlate sea level, primary paleoproductivity and facies distribution of the proximal, carbonate-dominated Paleocene deposits of the northern Sirte Basin with the distal, marl-dominated strata of Tunisia and Egypt. The Paleocene depositional record in North Africa is dominated by microfossil-rich hemipelagic marls that form important seals for petroleum accumulations in Tunisia, Libya and Egypt. In the investigated successions, sea level changes can be discerned through variations in sedimentological and faunal composition and, rarely, in lithological changes. By integrating analysis of quantitative distribution patterns of microfossils and sedimentological patterns, our study aims to reconstruct paleoenvironmental changes with emphasis on sea level and paleoproductivity fluctuations. Our data recognized a third-order cycle during the middle Paleocene, which correspond to a change in the paleobathymetric setting and to increased paleoproducitvity, with locally associated anoxia. This third-order cycle in the middle Paleocene, seems to be a suitable event not only for a regional correlation, but probably also to the global scheme.
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The Onshore Tunisia Numidian Flysch
Authors S. Riahi, U. Patel, M. Soussi, D.A.V. Stow, I. Croudace, C. Flides, K. Ben Isamil Lattrache and K. BoukhalfaThe study concerns the Oligo-Miocene aged Numidian Flysch which forms the highest structural unit in northern Tunisia and consists of an interbedded unit of highly quartzose sand and shale, which are of turbiditic affinities. An integrated petrographic and geochemical study of this formation was carried out to infer the sandstones provenance, and reservoir characteristics. On the basis of the framework composition (point counting) and whole-rock geochemistry (major elements), the sandstones are classified as quartzarenite, and sublitharenite types. The modal analyses imply a recycled orogen provenance. Moreover, petrographic point count data indicate quartz-rich sedimentary (recycled), middle to high-grade metamorphic and plutonic parent rocks for Numidian sandstones. According to the plate and tectonic reconstructions of the western Mediterranean Sea it appears that the kabylean basement terrain cropping out along the Algerian coast and forming part of Calabro-Peloritani-Kabylian zone to be the parental source of the Numidian of both Sicily and Tunisia. Additionally, petrographical study indicates that the reservoir properties of the Numidian Flysch are a direct function of grain size and textural maturity.
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A Sequence Stratigraphic Model for Ordovician Glacial Deposits, Illizi Basin, Algeria
Authors J. Lang, R.J. Dixon, D.P. Le Heron and J. WinsemannWell data from the Cambrian to Silurian succession of the Illizi Basin, Algeria, were interpreted to derive a depositional model and correlate the wells in a sequence stratigraphic framework. The succession contains thick glacimarine deposits of the Late Ordovician glaciation. The preglacial deposits show a retrogradational stacking pattern of terrestrial and shallow marine deposits. The synglacial succession is dominated by subaqueous fan deposits. Glacimarine depositional sequences were controlled by advance and retreat cycles of the glacier. Two glacial advance and retreat can be correlated across the study area. The postglacial succession was deposited on the shelf, infilling the residual glacial topography.
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Triassic of Meseta Oranaise its Bordering Areas, North Algeria
By Z. AmiaThe ‘Meseta Oranaise’ is located in the north-western part of Algeria. This area is between the Tellian basin to the North and the Atlasic basin in the South. The northern border is affected by faults network and folds. The southern border is characterized by a major fault “ Northen Atlasic Fault”. This area was structured as a horst since the Palaeozoïc. Stratigraphically, this area is characterized by reduced sedimentary layers, which consist of clastic and carbonate deposits(Silurian to Tertiary). The Triassic is mostly developed in the north-eastern part of the study area. In the ‘Meseta’, the Triassic reservoirs are shally sandstone, These sandstones are generally sourced from erosion of Palaeozoïc highs. These formations are continental depositional system, they are often associated with basalt, deposited in fluvial environment, with tidal influence. These reservoirs consist of fine to medium grained sandstone with low angle cross stratifications, coarse sandstone to conglomerate. The reservoir quality is relatively medium (porosity 2 to 5 %, permeability is about 2 md). The diagenetic effects are mainly represented by calcite and anhydrite cement in fractures. In regard to petroleum aspect, the Triassic reservoirs have revealed encouraging gas shows in some wells (CED-1 and CC-1).
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In Search for Optimal Seismic Data Acquisition through Illumination
Authors R. Alai and J.W. ThorbeckeIn this paper a strategy is outlined to analyze illuminating beams for optimally locating target zones, and optimally acquiring all necessary seismic data. Illumination analysis of wave propagation through the earth’s subsurface plays a very important role, in locating shadow zones and target prospects, to optimize the success of exploration wells. Using illuminating beams for optimal data acquisition design and data processing provides a unique integrated and cascaded tool for insight and interpretation of target prospects. The quality and accuracy of seismic images is being calculated from weighted energy distributions along areas that are dominantly contributing to potential oil and gas reservoirs. During illumination analysis, migration parameters are being determined to locate target images in shortest time. The a priori integration of seismic data acquisition design and optimal imaging of target zones minimizes the overall expenses for seismic data acquisition, data processing, and high costs of exploration wells.
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Low-frequency (LF) Passive Seismic Imaging – A New Technique for Reservoir Wealth and Health
More LessThe existence of coherent patterns relating to oil and gas reservoirs in the low frequency domain has been established in many parts of the world. The underlying physical mechanisms of these so-called Direct Hydrocarbon Indicators (DHI) is a key focus of our research. Two mechanisms that can generate DHI in the background spectrum are of special interest: resonant amplification and resonant scattering. Resonant amplification effects of ambient seismic waves are likely candidates for hydrocarbon micro-scale tremor signals. Resonant amplification effects behave like a secondary source within the reservoir. They are supported by the following observations: An often narrow frequency range of the anomaly, The mean absolute power of hydrocarbon tremors depends on the level of environmental noise, Signal power is proportional to hydrocarbon-bearing layer thickness of the reservoir, Three component recordings show a trough instead of a peak in the H/V-ratio, Investigations of the wave-field propagation directions using directionally-sensitive sensor arrays show that the signals causing the anomaly originate from the reservoir direction, Time reverse modeling indicates that the origin of the tremor signals is located within the reservoir
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Is the Extremely Prolific Sirt Basin Onshore Libya Continuing into Deep Water Areas?
The onshore part of Sirt Basin is the most prolific petroleum province in whole Africa. It is a large Mesozoic-Cenozoic rift basin. The offshore part has been limited explored without any big success. The main objectives of this presentation are to discuss the possibility that the onshore petroleum systems extends offshore, based on improved understanding of the regional structural and sedimentological settings. 2D seismic interpretation, modelling of gravity and magnetic anomalies, source maturity modelling, structural and sedimentological descriptions have been carried out. Bathymetry and gravity and magnetic anomalies indicate that some areas have been heavily influenced by different volcanic activities (seamounts and sills). The marine Upper Cretaceous Sirt Fm. is expected to be the most probable hydrocarbon source. The thick overburden (2000-7000 meters) in the offshore extension of Ajdabiya Trough indicates that the source, if present, will be gas and/or oil mature in different parts of the area. Lower Cretaceous Nubian sandstones (expected main reservoir) have been penetrated but are heavily calcite and /or quartzite cemented. This study gives a better understanding of the Mesozoic and Cenozoic basin evolution of offshore Sirt Basin. The hydrocarbon potential is good. Reservoir quality is considered as the main risk factor.
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Fracture Characterization – A Fully Integrated Study, from Borehole Imagery, Cores and Seismic Data to Production Logs
Authors M. Panien, E. Portier, F. Marcy, L. Ghilardini, T. Dehaeck and M. BoulahiaA fully integrated study has been carried out to analyse the fracture network of an Ordovician reservoir of Sbaa Basin in Algeria. Multi-scale information relevant of fractures such as core descriptions, interpreted borehole imagery, standard logs, seismic attributes and dynamic data are used to build the 3D model and to perform dynamic reservoir simulations.
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Reservoir Rock Classification by Comprehensive Petrophysical Data Integration – Acacus Formation, Jenein-Sud, Tunisia
By J. SteckhanPetrophysical properties in the Acacus formation are quite variable and inhomogeneous since the reservoir shows substantial diagenetic overprint and layer heterogeneity. To build a sound property model on such conditions it is imperative to establish a petrophysical rock classification concept first by comprehensive integration of log, core and test data. Due to variable diagenetic effects (mainly chloritization and siderite cementation) there is obviously no simple poro-perm relationship in the Acacus sands. Therefore an empirical rock classification approach based on poro-perm to pore-throat relationship has been applied (Winland-R35). Preliminary results of the Winland-R35 quick-look on core data allow the discrimination of four main petrophysical rock types. Mercury intrusion tests (HPMI) carried out on representative samples show rock class specific shapes of capillary pressure curves which endorse the initial classification by Winland-R35. To apply this rock typing concept to the entire reservoir section the correlation of core permeability to irreducible water saturation from NMR log data has been used to derive a permeability estimate which was finally scaled to the permeability thickness derived from DST. The petrophysical rock classification and its specific parameter sets were used to initialize the geo-model as well as to control the spatial distribution of properties.
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Characterization and Modelling of the Ashtart Fracture Network Using an Integrated DFN Approach
Authors P. Neff, R. Bischoff, H. Granser E and P GmbHThe goal of this new study was to define an improved and agreed Shared Model between partners. Due to the long production history of the field, the model has to integrate all available data and previously realized studies, but as well be entirely consistent from geology to reservoir dynamic simulation. The present study is focused on the characterization and modelling of the existing fracture network. The applied DFN approach allows integrating within a consistent sequential workflow all the fracture information available for Ashtart. Also it delivers a static model consistent between geological and dynamic understanding.
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The Impact of Pickett's Plot for Developing the Petrophysical Model of Abu Roash Reservoir in Ras Qattara Area, Western
By T.F. ShazlyThe present study deals with Pickett's plot to develop the reservoir petrophysical parameters of Abu Roash Formation in Ras Qattara area, Western Desert, Egypt. These parameters include capillary pressure, pore throat aperture radii, height above the free-water table and bulk volume of water. The method involves the use of log-log plots of the effective porosity vs. resistivity combined with empirical equations for calculating the for-mentioned reservoir characteristics of the considered rock unit. Plot of log Rt vs. log Ф can be drawn in a straight line with a slope controlled by the cementation factor (m). This plot results in a straight line for intervals with constant capillary pressure and constant pore throat aperture radii, where the capillary pressure at any point on the Pickett's plot is consistent with porosity, permeability and water saturation at that particular point. The value of Rw is not known in advance, provided that the reservoir contains some water-bearing intervals. However the integration of permeability, capillary pressures and pore-size classes of the pores on a log-log graph of porosity vs. resistivity gives the importance for Pickett's plot to be used in reservoir interpretation.
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Reservoir Rock-types Determination Based on Wire-line Logs and 3D Petrophysical Modeling in Ahnet Basin Field
Authors N. Filali and M.S. TighiltThe vertical and lateral reservoir properties distribution over field scale is a critical factor that requires investigation for the appraisal wells drilling success. This paper provides an approach based on a technical classification of rock-group, the so-called rock-typing technique. Each rock-type has specific sedimentological characteristic and homogeneous petrophysical properties. These rock-types are used in a 3D Static geocellular modeling. This technique is used advantageously in Ahnet field uncored wells, where the interval studied is a layer-cake reservoir type and in which the lower part is generally producing gas. An electrofacies characterization is performed for all wells with complete logs suites. The cores porosity (PHIE) and permeability (KH) are generally used in addition to the logs, to give a petrophysical sense. Because of the low permeability values, these properties are not used and the core porosities are replaced by estimated porosities (PIGE). Six (6) electrofacies are defined, simultaneously calibrated with estimated porosities PIGE and ranked progressively from clean sandstone (Rock-Type 1) with the best porosities to shale rock type (Rock-Type 6).
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The Distribution of Saturation Exponent and Cementation Factor in the Nubian Sandstone Formation, Sirt Basin, Libya Using Global Hydraulic Element App
Authors N.I.A. Mousa and P. CorbettThe main controls on hydraulic properties and hence the fluid (oil-water) distribution in the porous reservoir media is of major importance for reservoir description. The controls on porosity and permeability in the Lower Cretaceous (Nubian sandstone formation) of study area Sirt Basin, Libya are considered in this study. The primary depositional texture, quartz overgrowth and clay minerals are shown to be the key control to the hydraulic properties. Primary depositional texture results in existence of distinct rock types with similar pore throat attributes described using Global Hydraulic Elements (GHE's) approach.
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Understanding Subsidence Mechanisms of the Intracontinental Palaeozoic Basins of North Africa
Authors P.J. Holt, M.B. Allen, J. van Hunen and H.M. BjørnsethThere are a number of large Palaeozoic basins situated on the accretionary crust of North Africa where the subsidence mechanism is unknown and poorly studied. The present theories within the literature explaining the subsidence tend not to explain the duration or areal extent of the subsidence seen. This talk proposes a new theory for the subsidence mechanism which created these basins. This theory is based on the idea that accretionary crust lacks a thick mantle lithosphere when it accretes because it formed through collision of island arcs. The lithosphere thickens as it cools which causes subsidence. This has been tested by comparing the tectonic subsidence curves generated from backstripping wells in the Ghadames and Al Kufrah basins with those generated using forward modelling. The modelling produces a good fit to the magnitude of the subsidence and acts over the same time period. However, there is a discrepancy in the shape of the curves with the forward model predicting higher initial subsidence rates and slower subsidence rates later. This may be because the lithosphere growth is slowed by higher heat production in the crust or greater insulation from the crust than used in the forward modelling.
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Combining Regional Geology and Gravity Data – Input to Different Exploration Phases and Scales in North Africa
Authors J. Salvadores Janssen, C. Gram, J. Scotchmer, M. Slinde, N. McDowell, H. Daleq, A. Van Bockstaele and B. MarkussenUnderstanding the regional events that configured present-day North Africa is a basic step in any hydrocarbon exploration programme. The generalised term of “cratonic basins”, as these basins are often referred to, masks a vast geological history with a sedimentary record that goes back to the Neoproterozoic, i.e. 1000 m.y. ago. Since then, dramatic geological events have alternated with long periods of quiescence that have resulted in basin architecture and hydrocarbon occurrence at present day. Given three outstanding peak periods of this geological history such as the Pan-African Orogeny (1), the Hercynian to Tethys transitional events (2) & the Alpine cycle (3); surface geology, potential data and regional sections can shed light on key exploration issues at different stages of basin exploration.
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Tectonic Heat Flow Modelling for Basin Maturation – Implications for Frontier Areas in the Mediterranean
Authors J.D. Van Wees, D. Bonte and S. NelskampBasement heat flow is one of the most influential parameters on basin maturity. Although rapid progress has been made in the development of tectonic models capable of modelling the thermal consequences of basin formation, these models are hardly used in basin modelling. To better predict heat flows we have developed a multi-1D probabilistic tectonic heat-flow model, incorporating a variety of tectonic scenarios (including rifting, underplating and mantle upwelling). The model has been applied for a range of basin settings, and predicted temporal and spatial heat flow variations are well in accordance with observations on heat flow for specific tectonic settings. In the Mediterranean observed heat flow is markedly heterogenous, ranging from values from as low as 30mW m-2 up to 150 mW m-2, having major implications for basin temperature and maturation levels. Tectonic models for heat allow to asses geodynamic causes for these varations and aid considerably in constrining spatial and temporal variations in heat flow. In particular modelling results indicated that the interplay of rifting, underplating, back-arc extension, has resulted in much stronger temporal and spatial tectonic heat-flow variations than hitherto assumed.
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Stress and Fracture Prediction with Geomechanical Reservoir Models – Concepts and Case Study, Eastern Sirte Basin
Authors A. Henk and A. FrischbutterThe study explores the potential of numerical models to predict tectonic stresses and fracture networks on a reservoir-scale. A workflow is presented to build such geomechanical models on the basis of finite element techniques. Input data required are the reservoir geometry, mechanical material properties and boundary conditions. Modeling results are calibrated against observed data, e.g., in-situ stress measurements and fractures from cores and logs. This validated model is then used to predict stresses and fracture properties in the inter-well space. To assess its practical value this workflow for geomechanical modeling is applied to data from a reservoir in the eastern Sirte Basin/Libya.
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Basin Evolution of the Al Kufrah Basin in Southeast Libya
Authors H.M. Bjørnseth, L. Gindre, S. Higgins, A. Schimanski, H. Gröger, C. Vandré, B. Bonnier and M. GeigerIn the Early Palaeozoic the Al Kufrah Basin was part of a very wide, low relief platform, gently dipping to the north. Thick units of fluvial and shallow marine sandstones were deposited on this platform in Cambro-Ordovician times. A major glaciation in Late Ordovician led to a complex history of deep glacial erosion and deposition of high quality shallow marine sandstone reservoirs. The Tanezzuft Fm. was deposited during a rapid transgression in Early Silurian. Elevated Uranium contents within the Tanezzuft Fm. have recently been recorded in the Jabal Azbah region to the east. These “hot shales” represent the first potential source rock unit encountered in the Al Kufrah Basin. The “Top Silurian” and “Hercynian” uplift events led to significant erosion in the central and southern part of the basin. However, the most dramatic event in the basin evolution occurred in Mid - Late Cretaceous times when regional uplift resulted in around three kilometres of erosion in the Jabal Az Zalmah region to the north.
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Structural Study and Impact on the Hydrocarbons Exploration, Chott El Jerid Area – Central Tunisia
Authors G. Chaari and P. TremolièresThe careful study of the seismic data shows at the edge of Chott Jerid a flexural fault, dipping to the north, with a general E-W orientation associated with a high amplitude tectonic inversion. This inversion implies the presence of a graben situated between this E-W normal fault and a second major fault, dipping to the south. An N-S extension during Permian and Triassic times has most probably initiated the opening of the graben. Later, during Upper Jurassic and Early Cretaceous, subsidence was very active. The NW-SE compressions, from uppermost Cretaceous to Upper Eocene initially then during Late Miocene, induced the tectonic inversion. Later, an intensive compression phase affected the Mio-Pliocene series. The E-W faults played like reverse faults accompanied by important folding. This is the N-S Villafranchian (Quaternary) compression. Consequently, the northern area affected by Quaternary compression is now considered as of secondary importance to hydrocarbon exploration. Furthermore, the location of the structures identified on both sides of the southern flexural fault is critical for the structures feeding and for the enhancement of the petrophysical properties of the targeted reservoirs.
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Petroleum Geological Atlas of the Southern Permian Basin Area - overview SPB-Atlas Project – Organisation and Results
Authors J.C. Doornenbal, O.A. Abbink, H.J.M. Pagnier and J.D. van WeesThe Southern Permian Basin (also referred to as Central European Basin) is Europe’s largest sedimentary basin. It is a typical intracontinental basin that evolved from latest Carboniferous to recent times and extends from eastern England to the Belarussian-Polish border and from Denmark to South Germany. The Southern Permian Basin Atlas (SPBA) project is a joint project between the Geological Surveys of Belgium, Denmark, Germany, The Netherlands, Poland, and the UK and is being supported by a wide range of E&P companies, government licensing authorities, universities and research institutes. The aim of the Atlas is to present an overview of the results of over 150 years of petroleum exploration and research in this basin area. As this gas and oil province continues to mature and with field sizes inevitably decreasing, more and more careful data integration and geoscientific effort is required to discover new reserves. The subsurface characterization provided in the Atlas will also be of great value to governments, researchers and other individuals interested in the deep subsurface.
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Controlled-source Seismic Interferometry by Multi-dimensional Deconvolution Applied to a Laterally Varying Elastic Model
Authors J.R. van der Neut, K. Wapenaar and M.K. SenIn controlled-source seismic interferometry (SI), one typical aim is to redatum an array of sources to a receiver location. In cross-correlation (CC) based SI this goal is achieved by cross-correlating registrations at two receiver locations and summation over sources. Although a closed boundary of multi-component sources surrounding the receiver array is required, applications generally involve one-sided illumination and single source types only, which can result in spurious artifacts, erroneous kinematics and incorrect amplitudes. Therefore it can be helpful in some cases to replace cross-correlation by multi-dimensional deconvolution (MDD). In MDD based SI the reflection response is retrieved by inverting a general integral equation, which we implement in a least-squares sense. Additional advantages include improved radiation characteristics of the retrieved (virtual) sources and a relaxation of some assumptions, including the absence of loss terms and knowledge of the source wavelet. Disadvantages include the need for accurate wavefield decomposition and instabilities that might occur in the matrix inversion that forms the core of MDD. We apply SI by both MDD and CC to an elastic model with lateral variations and report slight improvements of MDD compared to CC.
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Sub-salt Imaging through Pre-stack Depth Migration – A Case Study from Offshore Egypt
Authors H. El Meligy, M. Sheath, J. Høyen, D. Renshaw, Ø. Skinnemoen, A. Issa, W. Badr, M. Abdelaty, E. Shady, M. Hårde and C. JebsenThe Middle East hosts a wide range of different geological setting, a large number of which exhibit structural complexity and hence can benefit from depth imaging. These structurally complex areas range from over thrust zones on the U.A.E - Oman border to salt structures in the Gulf of Suez and fault shadow problems in the Western Desert of Egypt in addition to offshore Egypt
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Seismic Interferometry by Multi-dimensional Deconvolution for Passive Transient Sources
Authors J.R. van der Neut, K. Wapenaar and E.N. RuigrokIt has been shown that cross-correlation of passive seismic recordings at the earth surface can yield a Green's function as if both sources and receivers were at the surface. One of the important underlaying assumptions is that the passive sources are uniformly distributed throughout the subsurface. This assumption is generally not fulfilled. We present an alternative approach that can be applied to transient passive seismic recordings registered by an array of receivers, where Cross-Correlation (CC) is replaced by Multi-Dimensional Deconvolution (MDD). We show that MDD can correct implicitly for a non-uniform source distribution and demonstrate this with a synthetic example. The main drawback of our methodology is that individual transient events need to be identified and isolated in the shot records.
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The Attenuation of a Complex Multiple Wavefield Using 3D General Surface Multiple Prediction (GSMP) – A Case Study
Authors H.E. El-Meligy, M. Sheath, J. Høyen, D. Renshaw, Ø. Skinnemoen, M. Hårde, M. Abdelaty, E. Shady, A. Cooke, T. El Melegy and B. BroussardIn this case study, we show how a new implementation of 3D surface-related multiple elimination (3D SRME) can be used to successfully predict and attenuate complex multiples, including diffracted multiples, on a dataset from Egypt’s Mediterranean Sea.
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Flow Unit Definition and 3D Petrophysical Modeling in a Complex Carbonate Reservoir, SW Iran
Authors M. Homaie and B. AlizadehAt first, Flow unit study helped to interpret Petrophysical potential of complex reservoir behavior. Core analysis such as Porosity, phi, Permeability, k, and permeability/porosity ratio (computed r35 port size in microns) and Winland r35 plot were used for the this progress. Lorenz and stratigraphic modified Lorenz plot indicated that five rock/pore units were indictable, each with an individual flow behavior called Flow Unit. They are defined as, disperse vugs (b), touching vugs (c), meso (m) and micro port size (e) and fracture (a) units. The flow units (c) and (m) constitute 55% and 20% reservoir flow respectively. In other side they are contained only 25% reservoir storage capacity. But flow unit (b) imparts 22% reservoir flow and 76% storage capacity. Different digenetic processes such as solution, cementation, dolomitization and anhydrite nodules precipitation affecting from Pyrnean disconformity have caused that flow units were irregularly distributed within whole reservoir. At second step porosity and water saturation were 3D modeled. Its results support complexity of reservoir situation depending to various Petrophysical factors. Lower reservoir, Jahrum fm, although contains better Petrophysical properties but production is mostly focused in upper reservoir or Asmari fm, where flow units (c) and (a) are generated.
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Detecting Remaining Gas Reserves Based on Coupled Modelling of Subsidence and Reservoir Depletion Processes
Authors O.A. Abbink, P.A. Fokker, A.G. Muntendam-Bos, B. Orlic and K. van Thienen-VisserDepletion of large volumes of natural gas, oil and water from hydrocarbon reservoirs may lead to movements of the Earths surface (tilt or subsidence). Observations of these movements (Interferometric Synthetic Aperture Radar (InSAR), leveling measurements) can provide a better insight on subsurface processes like reservoir compaction or the aquifer strength. A simple inversion approach does not usually provide a sensible solution due to the non-uniqueness of the inverse problem and the sensitivity of the inverse problem to small fluctuations in the data. This necessitates the use of all available prior information (geologic model, reservoir model) in the inversion procedure to better constrain the subsurface parameters. A time-dependent inversion scheme is adopted for resolving the spatial and temporal reservoir pressure drop from the surface subsidence observations. The innovative inversion method is effectively applied to one synthetic and two field cases (one from The Netherlands, one from North Africa). The results suggest that incremental gas reserves due to reservoir compartmentalization can be detected successfully.
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Hydrocarbon Production and Reservoir Management – Recent Advances in Closed-loop Optimization Technology
Authors O.A. Abbink, R.G. Hanea, E.D. Nennie and R.C.A.M. PetersPetroleum production is a relatively inefficient process. For oil production, it is, generally, less than 60 % effective on a macro scale and less than 60 % effective on a micro scale. This results, commonly, in an actual oil recovery of less than 35 %. Optimization of the production process will, therefore, have a significant impact on the supply of energy. At various locations in the world consortia have been set up to develop new technology that would help optimize the production process. In 2005 Shell, Delft University of Technology, and Netherlands Organization for Applied Research TNO have started the ISAPP knowledge centre. ISAPP aims to develop innovative solutions for petroleum production based on a closed loop integrated systems approach. The projects in ISAPP address both reservoir characterization issues and control issues. Apart from a short overview, this paper presents two cases from the ISAPP project portfolio: an optimal control example that shows how well head choke control can be used to prevent wax deposition during production; a reservoir characterization example that deals with production history matching and prediction of production from new wells.
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Stratigraphy, Sedimentology and Depositional Environments of the Late Ordovician Succession in Al Kufrah Basin, SE Libya
Authors C.R. Wilson, D.P. Le Heron, H.A. Armstrong and L. GindreOutcrops on the northern and south-eastern flanks of Al Kufrah Basin, Libya (Jabal Azbah and Jabal az Zalmah) help to improve understanding of the End-Ordovician (Hirnantian) glacial event in Gondwana. Several glacial erosion surfaces are identified, attributed to waxing and waning of ice sheets extending into the marine environment, and the sedimentary succession reveals an extensive glacial outwash fan apron deposited on a broad, shallow continental shelf. During deglaciation, fluvial and tidal cross-bedded sandstones were deposited in the southeast of Al Kufrah Basin and the entire region was overlain by Tanezzuft shales, siltstones and fine sandstones. Evidence from Al Kufrah Basin strongly suggests an extremely broad continental shelf setting during the Lower Palaeozoic, with implications for the morphology and dynamics of the ice sheet margin, and the availability of accommodation space during deglaciation. These implications must be considered when developing and refining depositional models for Lower Palaeozoic petroleum plays in the basin and elsewhere in Libya.
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Reservoir Characterisation of Deepwater Massive Sandstones – Case Studies from the Numidian Flysch and Mediterranean
Authors D.A.V. Stow, S. Riahi, M. Soussi, C. Fildes, U. Patel, S. Marsh Company LLC and M. Johansson OilfieldThe deep-water turbidite play is currently one of the most prolific and important in hydrocarbon exploration and production. Within this play, the best reservoirs are those that occur in thick sequences of deep-water massive sandstones (DWMS). These are very thick (1- >50m) sand beds or units that are devoid of primary sedimentary structures, deposited by high-density turbidity currents and/or sandy debris flows. They are commonly associated with thin-bedded turbidites, hemipelagites and other deep-water sediments, and display excellent and uniform reservoir properties and geometries. Typically, they also give rise to sand injection structures within the superjacent and laterally adjacent sediments, and display a variety of feather-edge relationships with the associated facies. The Oligo-Miocene Numidian Flysch system from both northern Tunisia and Sicily includes excellent examples of DWMS, which are the subject of ongoing investigations. This contribution aims to present an overview of what we know about DWMS, with particular focus on their reservoir characteristics, to introduce some new data on examples from the Mediterranean region, and to place the Numidian system within the context of DWMS globally.
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Giant Sandwaves from the Late Ordovician of the Tassili N'Ager, Algeria
Authors R.J. Dixon, T.L. Patton and J.P.P. HirstGiant sandwaves are described from the Ordovician outcrops of the Tassili N’Ager (SE Algeria). The sandwaves have been examined in the field and mapped using remote sensing techniques. They occur at the top of a Late Ordovician glacial sequence and unconformably overlie a sequence of pro – glacial sediments. They are in turn overlain by shelfal sediments of Lower Silurian age, mostly organic – rich, graptolitic shales that are a major petroleum source rock. The sandwaves are developed in medium to coarse grained sand and are characterised by large scale, cross sets up to 4m in height with dips of 10 – 15 degrees, dips are unidirectional with palaeo flow toward the east. The coarse grained nature and large size of the bedforms suggests the existence of strong unidirectional currents in the eastern Tassili N’Ager at the end of the Ordovician. The interaction of tidal currents and post – glacial topography in the early stages of the Silurian transgression seems the most likely explanation of these spectacular features.
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Geochemical Characterisation and Correlation of Cenomanian Turonian Sediments in Central-eastern Tunisia
Authors A. Mabrouk, H. Khaldi and I. JarvisGeochemical analysis of 254 core and cuttings samples from three petroleum exploration and development wells (Well 1, 2, and 3) in the Sfax area of eastern Tunisia, has successfully characterised the Cenomanian Fahdene Formation, and the Turonian Annaba, Bireno and Douleb members of the Aleg Formation. The cored mid-Turonian Bireno Member in the Well 3 is subdivided into two lithostratigraphic units: (i) Unit 1, attributed to a shallow subtidal environment; (ii) Unit 2, believed to represent an intertidal to supratidal sabkha environment. Vertical geochemical profiles were established for each well for 11 major elements and 7 trace elements. Chemostratigraphic correlation based on marker geochemical shifts that reflect member boundaries and the two Bireno Member sub-units has been successfully established between the three wells. Porosity seems to be strongly correlated with Ca and Mg, reflecting the importance of peri-reefal limestones and sabkha dolostones as important reservoir horizons, while clay minerals provide the main source of gamma ray response, and clay-rich beds typically represent relatively tight intervals. Chemostratigraphic studies of shallow-marine sediments such as the Bireno Member, offer considerable potential for the characterisation, subdivision and correlation of strata in petroleum basins.
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The Control of Geomechanics and Lithofacies on Fracture and Fault Permeability in Carbonates – Examples from Maiella
Authors P.A. Gillespie and B.G. WallPermeability in faulted carbonates is a key problem in exploration and production, and yet there is no existing rule base for its prediction. Fault zone properties are affected both by mechanical processes such as cataclasis and also by diagenetic processes, including precipitation and pressure solution. We have studied faults and associated fractures in a variety of limestone lithologies in the Maiella Mountain, Abruzzo, Italy and have used rock mechanical testing in order to better define the conditions of development of the faults. Combining this information with subsurface data from fractured carbonates, we propose a conceptual model for the development of carbonate fault rock permeability, based on the pore type and the loading history.
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Ability of FMS in Detecting Fractures and Other Geological Features of Asmari Fractured Carbonate Reservoir
Authors F. Khoshbakht, H. Memarian, M. Azizzadeh, G. Nourozi and A. MoallemiAsmari is a heterogeneous carbonate reservoir, which fractures play main role in hydrocarbon production. The main goal of present study is evaluation of FMS’s ability in detecting fractures and other geological features of Asmari formation. More than 380 m whole core that cut from Asmari described in order to extract structural features. FMS of this interval has also been processed and interpreted. Then, in order to evaluate and compare structural features on both core materials and image logs, the frequency logs (as frequency per 0.5 m) (for fractures, compactional features, beddings and anhydrite),orientation plots (for fractures and beddings) and aperture log (For open fractures) have been generated. Results checked by fractures data of seven wells (five image logs and two cores) of studied field and one well of neighbor field that obtained from core and image logs. Comparison of core and FMS revealed that they have excellent correlation for open fracture. FMS rarely detects filled fractures and faults of Asmari in studied well. It also is average in detecting bedding plains. Most of the 940 compaction features observed in core could not be recognized in FMS. Anhydrites in Asmari determined by FMS had good correlation with core.
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Predicting Carbonate Reservoirs, Offshore Sirt Basin, Libya
Authors K.J. Whidden, T.D. Mason, J. Iliffe and T. BevanThis paper presents a model for predicting shallow water carbonate facies in the offshore Sirt Basin. The transition between the onshore and offshore has an overall ramp geometry, cut by Cretaceous rifting, so that the offshore is deeper than the onshore. Shallow water carbonate facies are predicted to occur offshore when the onshore is non-marine to very shallow marine.
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Fractured Basement Exploration on the Algerian Sahara Platform Applied New Play Concept on the Ramade Horst
By M. BellikOn the Ramade Horst, the fractured granitic basement at a depth of 1900m has been an active tectonic zone during the Austrian phase. The Silurian is unconformably overlain by the Albian resulting in 2000m of uplift which resulted in the Silurian hot shales being downthrown along the Ramade fault and juxtaposed with the Cambrian and the Basement. The weathered material, the low permeability of the Cambrian/R3 and the Ante-Cambrian shales would be a potential top seal separating the Basement from the Cambrian. Exploration of this unconventional target in the existing fields is recommended as a mean to increase the production and extend the lifetime of the facilities. Also in the Ramade this play can be bundled with the Cambrian.
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Sequence Stratigraphy and Reservoir Characterization of the Uppermost Devonian-carboniferous in Southwest Algeria
Authors H. Abdallah, J.M. Jauregui, S. Adrianto, J. Gerard, F. Djettou, R. Afrazmanech, E. Figari, K. Nouar and N. IbrihenThe Upper Devonian-Carboniferous reservoirs correspond to new hydrocarbon plays in south western Algeria, highlighted during the recent Repsol’s exploration drilling campaign (2003-2007). Strunian and Tournaisian sandstone units produced potential gas flows from several wells essentially located in the western trend of Reggane basin. They correspond to prograding fluvial-deltaic cycles from South to North, alternating with transgressive events, prior to the global marine environment during the Upper Carboniferous. Tidal currents and storm action were the main processes which controlled sediment deposition. Reservoir characterization is the main objective of this integrated study using chronostratigraphy and 3D modelling. Biostratigraphy and field work will complete the data base in this wide area. In terms of reservoir properties, main regional quality variations seem to be related to facies changes and diagenesis effects.
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Sequence Stratigraphy and Reservoir Characterization of the Lower Devonian in the Southwest of Algeria
Authors E. Figari, J.M. Jáuregui, J. Gerard, S. Adrianto, H. Abdallah, F. Djettou, R. Afrazmanech, T. Castro and C. SakherLower Devonian reservoirs in Southwest Algeria are included in a stratigraphic interval between the so called “Caledonian Unconformity”, at the Silurian/Devonian (Lochkovian) limit, and a major regional flooding surface at the Emsian top. Three lithostratigraphic units are identified and correlated over hundreds of kilometers in outcrops and through Reggane basin. The Lochkovian/Gedinnian A & B reservoir units have been respectively deposited in tidal and foreshore environment. The Pragian/Siegenian C & D and Emsian reservoir units correspond to shoreface sandstones separated by offshore shales. In terms of sequence stratigraphy framework, they are equivalent to five 3rd Order Sequences (Vail et al., 1978), called De 1, 2, 3 4 and 5. A progressive decrease of the sand-shale ratio is expected from the SE to the NW of Reggane basin in every reservoir unit according to the overall progradation of the system and increasing marine influences. In spite of the extremely good correlation of these units around the whole area, internal heterogeneities exist due to facies associations and additional diagenetic processes.
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The Silurian Acacus Outcrop in Southern Libya – An Analogue to Jenein Sud?
Authors M. Scherer, D. Radies and R. KoehazyOutcropping cliffs of the Silurian Acacus Formation in Southern Libya were laser-scanned, and based on field sections mapped for reservoir bodies to resolve reservoir architecture. Pseudo-wells were constructed along these sections based on Jenein Sud well logs. Utilising seismic wavelet extractions accounting for frequencies and hence resolution, synthetic seismic sections were constructed to test the limits of seismic resolution for mapping Acacus reservoirs.
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Reservoir Characterization and Modeling of the Upper Ordovician Sandstones Unit in Southwestern Sahara, Algeria
More LessIn the southwestern part of the Algerian Sahara platform, the production of dry gas from Ordovician reservoir is related not only to lateral facies changes but also to the spatial distribution of natural fractures throughout the reservoir. In order to check out these assumptions, an integrated approach using a large data set, and combining several methods is used to simulate separately porosity, subseismic fault and fractures models. The main data integrated, derived from core, outcrop, wire-line logs, seismic interpretation, seismic attributes and PVT. The different concepts and approaches are related to sedimentological and structural aspects, geostatistics, sequence stratigraphy, seismic stratigraphy, and attributes generation. This leads to generate acoustic impedance which is integrated in the porosity simulation workflow as a secondary propriety, by using deterministic methods. The subseismic fault model is carried out by the means of a specific workflow, smoothing, variance and antracking attributes generations. The analysis of distribution of these subseismic faults throughout the main study field shows a closer relationship between fractures density, the curvature axes of the folds and the productivity. All these results have an impact on the planning of the future delineation wells and in the exploration-production strategy.
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Airborne Gas Flux Mapping for Frontier Exploration
By W.J.S HirstThe deployment of LightTouch in Tunisia in 2008 will be described and sample data and results presented. The principles of operation will be described as well as the ways in which the findings can support exploration in a frontier setting.
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An Analysis of the Near Surface Using Remote Sensing for the Prediction of Logistics and Data Quality Risk
More LessRemote sensing offers the unique ability to view the earth’s surface without actually being in contact with it. Using multi-spectral satellite data and Digital Elevation Models (DEM) a workflow is presented to build a topography and a lithology based classification of the near surface. This enables the creation of logistics and data quality (surface scatter and surface velocity) risk maps. The paper draws on data from the Western Desert of Egypt including surface geological sampling and correlation with seismic data. The results demonstrate that the interpretation of remote sensing data allow the prediction of risks associated with land seismic acquisition.
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Seismic Acquisition without Cables – Choices, Benefits and Field Experience
By R.G. HeathLand seismic instrumentation is changing very rapidly and new technology can solve many of the problems associated with traditional equipment. This paper reviews the latest technology and explains how its use, alone or side-by-side existing hardware, can benefit many aspects of exploration in North Africa.
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Interferometric Rayparameter Estimation and Applications
Authors L. Velasco, R. Ferber and L. WestModern seismic surveys accumulate large amounts of data due to the increased channel and shot counts (see Rached et al., 2006). Therefore we need to move away from the manual estimation of the first breaks, needed for the production of refraction based static corrections, and instead use more automatic methods. The novel technique described here is based on the theory of seismic interferometry to the estimation of rayparameter values as function of source and receiver offset. Seismic interferometry is the process of creating secondary seismic traces (interferograms) from pairs of genuine seismograms by cross-correlation (or spectral division). One major benefit of dealing with interferograms is that of estimating delay times by picking maxima of interferograms as opposed to picking first-break travel times from the actual seismograms. One application of the estimated rayparameter is to enhance automatic first break picking. The rayparameter is estimated in the region of the first breaks and is subsequently used in the stacking of adjacent traces. The stacked traces, with an improved signal-to-noise ratio, are analyzed in place of the original traces to provide better estimates of the first break times.
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The Use of Organic Matter Study for the Reconstitution of Basin Paleogeography and the Record of Global Climate Change
Authors N. Ouertani, R. Hamouda and H. BelayouniFourteen cores have been executed across the southern part of the lake of Tunis, to trace the physico-chemical evolution of the sedimentary environment. The vertical distribution of TOC along cores, attests the existence from bottom to top, of a positive organic sequence illustrating a progressive enrichment of the sedimentary column in organic matter. From the marine stage up to the present lagoon stage, the lake of Tunis, acted as an organic matter trap. The maximum organic matter accumulation is recorded in the upper black sediments due to the development of highly anoxic conditions amplified by a marked anthropic influence. The paleo-morphology of the lagoon deduced from the organic evolution profiles interpretation of the sediment cores, indicates the existence, of two relatively deep anoxic marine areas (one at the West and one at the East) separated by a shallow marine less anoxic central area which remained constantly under the influence of strong hydrodynamic activity. The distribution of S2 (pyrolysable organic matter) is quasi-similar to that of the TOC contents. Such result clearly indicates that the organic matter associated with these sediments is homogeneous and exhibit the same origin (Tissot et Welte, 1984; Bélayouni, 1990; Disnar, 2003).
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Abundance and Diversity Patterns of Nummulite Species of Libya as Indictors of Climatic and Tectonic Changes
Authors K.J. Whidden, R.W. Jones and T. AfiffiThe purpose of this study is to look at well data from Libya and adjacent areas of North Africa to study changes in the abundance and diversity of nummulites from the Late Paleocene to Early Oligocene, in order to further evaluate tectonic vs. climatic controls on the timing of nummulite speciation.
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Quantifying Undiscovered Resource Potential
By M. ZaterThe main critical issue in the exploration is the risk associated to the play elements . Mapping the risk of the different play element and using the best basin analogues can provide a good approach to estimate the remaining hydrocarbon potential. Lack of data is a barrier. indeed the regional geology understanding and the basin exploration history of a basin is a key factor to help compensate the lacking data.Two methods of Geox estmation are used to quantify the remaining potential: the area yield method in the emerging basins and field size distribution in area with more data on a play level.
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Asset Lifecycle OPEX Modelling with Montecarlo Simulation to Reduce Uncertainties and to Improve Field Exploitation
Authors F. Verre, A. Giubileo and C. CadegianiBe cost effective at early stages of field development and during mature production it is one of the most critical steps for improving risk and uncertainties management and optimization of field exploitation during the asset lifecycle. This study shows the methodology adopted by EniTunisia to evaluate the operating expenditures (OPEX) for development projects and producing assets. This methodology adopts the concept of activity based cost (ABC) and Monte Carlo simulations to improve OPEX estimates, to influence the design optimising costs, and to reduce uncertainties throughout the exploration and production (E&P) phases from evaluation phase through mature operations up to decommissioning. This methodology has been successfully applied for both development projects and mature production. In particular when used during design phases it provides an improved OPEX cost estimates compared to percentage of CAPEX methods, which enables project and operations teams to better evaluate the feasibility and operability of a project. In addition a risk analysis will be carried out for the cost model in order to mitigate the risks and uncertainties associated with the different assumptions.
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Validation of a Semi-automatic Interpretation of Image Logs Using Two Wells from a North Africa Sandstone Reservoir
Authors R. Di Cuia, D. Ferraretti, G. Gamberoni, E. Portier and L. EscaréA new system was developed to extrapolate the maximum amount of information from the image logs by considering not only the surfaces that cut the borehole but also the textural features of the images. The main objective of developing this system was to reduce the subjectivity and the time of interpretation tasks by largely automating the log interpretation, although some level of human interaction and correction is still necessary. This approach exploits image processing algorithms to analyze borehole images and artificial intelligence techniques to classify them. The resulting implemented system produces a semi-automatic interpretation of the image logs. This software was used over the FMI logs of four wells from the north African region in order to test the validity of the results.
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Rock Type-based Characterization and Petrophysical Analysis of the Silurian Acacus Sandstones, Ghadames Basin, Tunisia
Authors T.C. Axtmann, D. Baldwin, R. Peveraro and R. VessellCareful petrographic examination of sidewall cores from Acacus sands, combined with routine core analysis, XRD and SEM data allowed for identification of 5 discrete rock families. Integration of the petrographic and wireline data, particularly NMR (CMR) data, enabled the identification of the 3 principle reservoir rocks 80+% of the time. Starting at the pore scale and discriminating discrete petrophysical Rock Types with unique petrophysical properties yields a much more robust characterization of the reservoir than can be obtained by conventional evaluation techniques. The integrated approach should reduce logging costs by enhancing the ability to identify rock types capable of storing and producing fluids and through the generation of more accurate fluid saturation profiles. Distinction of Rock Types permits more coherent interpretation of porosity-permeability, capillary pressure and electrical properties data measured in the lab. Applying these data in log analysis allows for better saturation and net pay calculations. Finally, the integration of rock types with depositional environments ultimately leads to better and more meaningful reservoir mapping which can be used to better locate future wells.
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