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- Volume 15, Issue 1, 2009
Petroleum Geoscience - Volume 15, Issue 1, 2009
Volume 15, Issue 1, 2009
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Structural consequences of fluid overpressure and seepage forces in the outer thrust belt of the Niger Delta
Authors Peter R. Cobbold, Benjamin J. Clarke and Helge LøsethABSTRACTIn the Niger Delta, which has prograded onto the African continental shelf and neighbouring oceanic crust since the Eocene, abundant thin-skinned structures provide evidence for slope instability. The structures have detached on shale of the Akata Fm. At the toe-of-slope, the outer thrust belts contain compressional structures. The surface slopes and apical angles of the thrust wedges are small. Also, fore-thrusts alternate with back-thrusts. This is evidence for a small resistance to basal slip and it implies high values of fluid overpressure. Thrust faults are planar through the overburden, but strongly curved beneath it. By analogy with physical models, we take this as evidence for normal fluid pressures through the overburden, and for a pressure ramp, inducing seepage forces, beneath it. The seismic velocity of the Akata Fm. beneath the thrust belts is abnormally slow. This probably results from overpressure. Fluid vents are abundant within hanging-wall anticlines. A large one at a culmination communicates from a fore-thrust to the surface. The contents are seismically transparent, whereas the host rock is well stratified. We attribute the vent to fluidization. A nearby deep well encountered normal fluid pressures throughout the Agbada Fm., but a strong overpressure ramp beneath it.
We argue that the most likely mechanism for explaining the observations is chemical compaction as a result of hydrocarbon generation, and it is suggested that the thrust belts have advanced seaward by a feedback mechanism.
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The effect of water pressure on hydrocarbon generation reactions: some inferences from laboratory experiments
Authors A. D. Carr, C. E. Snape, W. Meredith, C. Uguna, I. C. Scotchman and R. C. DavisABSTRACTFor the last twenty-five years most petroleum geochemists and basin modellers have produced and used models for maturation and hydrocarbon generation reactions in geological basins that do not consider pressure as a primary control. These conclusions are based on extensive laboratory investigations mainly using pyrolysis. Chemical theory, however, indicates that endothermic volume expansion reactions, such as maturation and hydrocarbon generation, are controlled by both the system pressure and temperature, and geochemists and basin modellers may need to reconsider the importance of pressure on maturation and hydrocarbon generation reactions in geological basins. Unusual earlier studies (at least in terms of petroleum geochemical pyrolysis research) used a vessel in which the pressure is entirely derived from liquid water rather than both liquid water and vapour, as in hydrous pyrolysis experimentation. Results from these experiments showed that both total organic carbon (TOC) and hydrogen index (HI) were elevated in the pyrolysed kerogen residue, suggesting that hydrocarbon generation was being retarded by the effect of water pressure. This paper presents the results of an experimental investigation into the effects of water pressure and phase on hydrocarbon generation and expulsion from the Kimmeridge Clay Formation (KCF) in the temperature range 310–350°C and in the pressure range 0–500 bar, and presents quantitative results both for the amounts of gas and bitumen generated and the composition of the generated gas. The experimental results show that the water pressure retards both bitumen and gas generation, with gas generation being retarded more severely than bitumen generation.
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Mapping palaeostructure and palaeobathymetry along the Norwegian Atlantic continental margin: Møre and Vøring basins
ABSTRACTThe post-Jurassic bathymetric and subsidence history of the Mid-Norwegian Atlantic margin (Møre and Vøring basins) has been investigated using the technique of 3D-flexural backstripping. Eleven mapped horizons from seabed to the Base Cretaceous have been sequentially backstripped to produce a suite of palaeobathymetry and palaeostructure maps for all horizons at all time stages. Backstripping has incorporated thermal subsidence in response to (1) an initial rift event at c. 140 Ma and (2) a subsequent rift-to-break-up event in the latest Cretaceous/early Tertiary. Transient dynamic uplift of the margin during the Paleocene is also incorporated, having a particular impact on palaeobathymetric predictions at this time.
In an exploration context, the palaeobathymetric maps were used predictively to investigate sediment dispersal and deposition through time; the subsurface palaeostructure maps were used for maturation modelling and identification of hydrocarbon-migration pathways, while all of the maps contribute to an understanding of hydrocarbon-trap development through time. The backstripped maps yield considerable information on the regional structural evolution of the margin. After regionally distributed extension of both the Møre and Vøring basins during the Late Jurassic/Early Cretaceous, map restorations of the two basins show contrasting structural histories. There is no younger fault-controlled overprint on the Møre Basin, whereas the inner Vøring Basin was extended again in the mid-Cretaceous and the outer Vøring Basin was extended in the latest Cretaceous/Paleocene.
Three-dimensional backstripping also confirms an active compressional origin, rather than a wholly passive loading origin, for the post-break-up Tertiary domes located along the margin. This is the first time that a temporal analysis of the domes' formation has been possible in 3D map form rather than 2D section form. The technique of 3D flexural backstripping has wide application to the study of rifts and continental margins elsewhere.
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Dynamic investigation of the effect of a relay ramp on simulated fluid flow: geocellular modelling of the Delicate Arch Ramp, Utah
Authors A. Rotevatn, J. Tveranger, J. A. Howell and H. FossenABSTRACTA fluid flow simulation study was performed to investigate potential contrasts in reservoir performance between models displaying a soft-linked relay ramp vs. models with a continuous fault. The relay ramp model is based on a well-exposed outcrop analogue – the Delicate Arch Ramp, Arches National Park, Utah. In outcrop, the relay ramp exhibits a pervasive system of cataclastic deformation bands, which were mapped and incorporated into the reservoir simulation models. Several models were simulated, using deformation-band permeability as the main variable tested. Results show that when compared to flow across continuous faults, the presence of soft-linked relay ramps enhances net flow across the fault zone in all tested scenarios. Comparing models with deformation bands to the model that includes the ramp but no deformation bands illustrates that, although having an impact on flow tortuosity and sweep efficiency, deformation bands must have a very low permeability and be numerous before having a negative impact on recovery. Deformation bands with midrange permeabilities were shown to have a positive effect on recovery in the relay models, as they increase flow tortuosity and enhance sweep, causing later water breakthrough and prolonging production. Using very low-permeable deformation bands in the same models caused extremely poor pressure communication between the fault-breached compartments, despite the geometric connectivity provided by the sub-continuous relay beds.
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Flow processes and pressure evolution in aquifers during the injection of supercritical CO2 as a greenhouse gas mitigation measure
Authors R. A. Chadwick, D. J. Noy and S. HollowayABSTRACTRegional saline aquifers offer the greatest potential for very large-scale underground CO2 storage as a means of mitigating greenhouse gas emissions. Their dynamic storage capacity, in terms of induced increases in formation pressure, will limit the rate at which CO2 can be injected and may ultimately limit the amount of CO2 that can be stored. Generic flow models were generated to examine the effects on pressure evolution of various reservoir parameters (dimensions, permeability, porosity, presence and nature of flow barriers). CO2 injection involves dominantly hydrogeological (single-phase flow) processes in much of the reservoir and surrounding adjacent strata, with additional two-phase flow effects around the CO2 plume itself. Large, thick aquifers with no significant flow barriers can accept high injection rates (c. 10 million tonnes of CO2 per year) without undue pressure effects. However, flow barriers, such as faults, increase induced pressures considerably; for reservoirs with such features, careful site characterization and operational planning will be required for large storage projects. The principles established from the generic modelling were applied to a real aquifer storage operation at Sleipner in the North Sea. Here, CO2 is being injected into the Utsira Sand, a large relatively homogeneous reservoir. Modelling indicates that pressure increase should be negligible. In fact, observed wellhead pressures do show a small rise, but this can be attributed to temperature changes in the fluid column in the wellbore. Pressure changes in the reservoir are likely to be very small.
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Predicting brittle cap-seal failure of petroleum traps: an application of 2D and 3D distinct element method
Authors Bronwyn A. Camac, Suzanne P. Hunt and Peter J. BoultABSTRACTPrevious research shows the importance of understanding the relationship between fault geometry and current applied tectonic stresses in the prediction of critically stressed faults and their propensity for fluid flow via generated fracture networks along and/or around the fault plane. Data collected from the Penola Trough, onshore Otway Basin, South Australia shows that a more complex 3D failure mechanism may be active, whereby the cap-seal may fracture preferentially to fault failure.
An emerging application for a two- and three-dimensional distinct element numerical modelling technique, at the play and prospect level, respectively, which assists in assessing pre-drill seal integrity, is presented by way of two case studies from the Penola Trough. Sensitivity studies at the prospect scale show how (1) fault rock strength, (2) fault zone width and (3) the interaction of two fault sets generates local perturbations in the regional stress field. At the play scale, the depth to which a younger active fault set propagates can be explained by the distribution of stress within the rock mass generated by the present-day far-field stress acting on older regionally significant faults. This work offers a workflow and an additional technique to predict cap-seal integrity prior to drilling.
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Vorticity as a measure of heterogeneity for improving coarse grid generation
Authors Hassan Mahani, Ann H. Muggeridge and Mohammad Ali AshjariABSTRACTThis paper presents a novel coarse grid generation technique based on using vorticity as a measure of the impact of heterogeneity on flow. Vorticity is a maximum when the total flow is high and perpendicular to a large permeability gradient. Maps of vorticity were generated from single-phase flow simulations and used to generate coarse simulation grids from finely gridded geological models. The resulting grid was more refined in areas of high vorticity and coarser in areas of low vorticity. The method is first demonstrated on a simple five-layered model before being applied to three, 2D models of geologically realistic heterogeneity. The homogenized model generated from vorticity maps shows an improved match to the finely gridded simulation in all cases compared to simulation results using a regularly gridded homogenization.
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Volumes & issues
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Volume 30 (2024)
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Volume 29 (2023)
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Volume 28 (2022)
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Volume 27 (2021)
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Volume 26 (2020)
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Volume 25 (2019)
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Volume 24 (2018)
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Volume 23 (2017)
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Volume 22 (2016)
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Volume 21 (2015)
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Volume 20 (2014)
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Volume 19 (2013)
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Volume 18 (2012)
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Volume 17 (2011)
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Volume 16 (2010)
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Volume 15 (2009)
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Volume 14 (2008)
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Volume 13 (2007)
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Volume 12 (2006)
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Volume 11 (2005)
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Volume 10 (2004)
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Volume 9 (2003)
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Volume 8 (2002)
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Volume 7 (2001)
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Volume 6 (2000)
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Volume 5 (1999)
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Volume 4 (1998)
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Volume 3 (1997)
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Volume 2 (1996)
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Volume 1 (1995)