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- Volume 11, Issue 4, 2005
Petroleum Geoscience - Volume 11, Issue 4, 2005
Volume 11, Issue 4, 2005
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Sub-basalt structures east of the Faroe Islands revealed from wide-angle seismic and gravity data
Authors T. Raum, R. Mjelde, A. M. Berge, J. T. Paulsen, P. Digranes, H. Shimamura, H. Shiobara, S. Kodaira, V. B. Larsen, R. Fredsted, D. J. Harrison and M. JohnsonTwo semi-regional wide-angle Ocean Bottom Seismograph (OBS) profiles, acquired east of the Faroe Islands, have been analysed by use of forward and inverse modelling to map the crustal structures. In the present wide-angle data, the Tertiary basalt shows a maximum thickness of 3 km under the Faroe Islands, decreasing towards the Faroe–Shetland Channel where it terminates. Sedimentary rocks are present below the basalts and vary in thickness from 2 km to a maximum of 8 km towards the Faroe–Shetland Channel. These sedimentary rocks appear mainly as a low-velocity zone, and the presence of high-velocity intrusions in these layers generate several step-back features in the wide-angle refraction data. Pre-Cretaceous sedimentary rocks are only inferred north of the Clair fracture zone, while Cretaceous rocks dominate southeast of the Westray fracture zone. The crystalline basement is divided into an upper granitic and a lower granodioritic part. P-wave velocities around 7.0 km s−1 are modelled in the lowermost part of the crust, indicating that magmatic underplating is not present below the Faroe Islands. The depth to the Moho is modelled with a maximum depth of 29–30 km below the northern part of the Faroe Islands, decreasing both southeastwards and southwestwards to 25 km and 17 km, respectively.
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The Deep-Water Architecture Knowledge Base: towards an objective comparison of deep-marine sedimentary systems
Authors Jaco H. Baas, William D. McCaffrey and Robert J. KnipeA quantitative method is proposed for the comparison of deep-marine clastic depositional systems and the analysis of their architectural properties. The method comprises a knowledge base of quantitative, literature-derived information from modern and ancient, surface and subsurface deep-water systems, implemented as a relational database management system and referred to as the Deep-Water Architecture Knowledge Base (DWAKB). The types of information contained within the knowledge base include: (1) internal and external parameters controlling system architecture, such as tectonic setting, grain size of available sediment, degree of basin confinement and number and distribution of sediment input points; (2) the dimensions of architectural elements, such as channels, levees, lobes, open slope and basin plain; (3) the spatial organization of the architectural elements; and (4) the bed thickness distribution and proportion of different lithologies within the architectural elements. The potential value of the DWAKB for comparative studies of deep-marine clastic systems is considerably higher than that of classification schemes and system analogue concepts presently available. Thus, in contrast to classification schemes, the knowledge base is not limited in the number of controlling parameters and it does not have a limited time span because of the flexibility to update existing records and add new datasets. Moreover, system analogues can be selected more objectively through the use of statistical methods, and the knowledge base allows unsurpassed integration of large-scale architectural data with bed- and facies-scale data. The expected value of the DWAKB is illustrated with several examples of quantitative data analysis. These include the determination of the frequency of occurrence of levees in systems of different grain size, the calculation of the dimensions of submarine channels as a function of grain size and proximity to the source area, and the construction of idealized models for sand-rich and mixed mud–sand systems based on probabilities of spatial transitions between architectural elements.
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Ekofisk Field: fracture permeability evaluation and implementation in the flow model
Authors A. Toublanc, S. Renaud, J. E. Sylte, C. K. Clausen, T. Eiben and G. NådlandThe Ekofisk Field is a naturally fractured chalk reservoir located in the Norwegian sector of the North Sea. The natural fractures clearly control the permeability distribution, as the effective permeability can reach 50 mD whereas the matrix permeability only ranges between 0.1 mD and 10 mD. Permeability mapping in this field has been challenging due to the structural, stratigraphic and mineralogical complexity, tectonic history and non-negligible matrix permeability. A detailed fault interpretation has resulted in a complex fault pattern. A fault intensity (P21) parameter calculated from the fault pattern has proved to be the critical component for permeability mapping. Correlations were found between the fault intensity (P21) values and (1) the fracture distributions from cores and logs at individual wells, and (2) the fracture component of the well test permeability (K frac – total permeability less matrix permeability). These relationships allowed field-wide fracture permeability maps to be developed based on the P21 results. Total permeabilities were obtained by summing the matrix permeability and the calculated fracture permeability. These permeability maps were introduced in the Ekofisk Flow Model 2002 and refined to match the rate performance of the 50 initial wells run in prediction mode (well head pressure constraint). The runs in prediction mode have proved to be very effective for calibrating the permeability distributions, based on initial well performance above the bubble point. This simulation technique was extended to cover all producers (262 wells) during the entire history of the field to refine the maps further. After calibration with the performance data, a satisfactory history match was obtained by making minor changes to permeability and other dynamic parameters. Additionally, the running of the model of the mature Ekofisk Field in prediction mode for its full field life has provided a robust tool for calibrating field performance.
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North Sea Quaternary morphology from seismic and magnetic data: indications for gas hydrates during glaciation?
Authors Christine Fichler, Sverre Henriksen, Haakon Rueslaatten and Martin HovlandBuried circular depressions and channels in Quaternary strata have been investigated by 3D seismic data and, less commonly, by high-resolution aeromagnetic data. Sub-glacial melt-water drainage channels of various dimensions are the most distinct morphological features. In the same strata, numerous crater-shaped depressions were found, some coinciding with the initiation of channels. The diameter ranges from 500 m to 3000 m and the depth from 20 to 300 m – appreciably larger than common pockmarks. It is argued here that the craters were generated by gas expulsion from melted gas hydrates, combined with melt-water expulsion and erosion. This is supported by: (1) a similarity with published seafloor craters that originate from melted gas hydrates; (2) appropriate physical conditions for the formation and melting of gas hydrates during glacial and interglacial periods; (3) correlation with shallow gas occurrences and seismic gas indications. An increased number of craters occurs above a major fault and some Tertiary hydrocarbon discoveries, which may indicate thermogenic gas. At such locations, an increase in high-frequency magnetic anomalies can be explained by deposition of sediments with contrasting magnetic susceptibilities. This is an alternative explanation for the occasionally increased shallow magnetic anomalies above hydrocarbon fields, otherwise attributed to secondary changes in the magnetic mineralogy.
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Unstructured 3D gridding and upscaling for coarse modelling of geometrically complex reservoirs
Authors Mathieu Prévost, François Lepage, Louis J. Durlofsky and Jean-Laurent MalletThe generation of accurate and reliable unstructured 3D models for reservoir simulation remains a challenge. In this paper, new developments for grid generation, upscaling and streamline simulation for such models are described. In combination, these techniques provide a prototype workflow for the construction of unstructured simulation models. The grid generation framework described here allows the incorporation of both geometrical constraints and grid-resolution targets. Flow adaptation of the unstructured grid (i.e. higher grid density in key regions) is accomplished through the use of single-phase flow calculations on the underlying geocellular grid, which are used to generate target grid resolution maps for the unstructured coarse model. A novel transmissibility upscaling procedure is introduced to capture the effects of fine-scale heterogeneity. A new method for streamline simulation on unstructured grids is also introduced. This technique provides an efficient flow-based diagnostic for the assessment of the coarse simulation model in terms of flow response. The performance of the various components of the methodology is demonstrated using several examples.
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Geothermal regime and hydrocarbon generation in the Albanides
More LessFollowing a summary of the geological structure of the Albanides and of the oil and gas reservoirs in Albania, the parameters controlling the distribution of the present-day geothermal field are analysed in detail: temperature, geothermal gradient and heat flow density in the centre of the Albanian Sedimentary Basin. The paper analyses the palaeothermal regime of the External Albanides for the Upper Triassic–Eocene carbonates and the Middle–Upper Miocene and Pliocene molasse. It looks in detail at the burial and thermal history of the External Albanides and discusses the implications for hydrocarbon generation.
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Effects of hydrocarbon generation, basal heat flow and sediment compaction on overpressure development: a numerical study
Authors Jennifer Hansom and Ming-Kuo LeeThis study outlines numerical experiments to investigate the effects of hydrocarbon generation, basal heat flow and sediment compaction on overpressure development in evolving sedimentary basins. The model integrates predicted groundwater flow and temperature and pressure distribution with thermal maturation simulations. The programme uses the Arrhenius kinetic model to simulate the kerogen–oil or oil–gas conversion processes. Such conversion processes result in an increase in fluid volume and overpressure development since oil and gas generated are less dense than their precursors. The model integrates an equation of state to calculate gas densities for the CH4–CO2–H2O system over a wide temperature–pressure (T–P) range expected in sedimentary basins; this approach allows for prediction of the rate of pore volume increases and fluid pressure changes due to gas generation. Sample calculations of compaction of kerogen-rich shales in the Delaware Basin shed light on the magnitudes of overpressures created by hydrocarbon generation from the Late Pennsylvanian to Middle Permian. Oil generation can cause excess pore pressure (c. 425 atm) up to c. 40% of that generated by compaction only (c. 300 atm). Oil and CH4 gas generation together yield the maximum excess pressure (c. 750 atm) up to about 150% of that generated by compaction only. There is much greater pore pressure build-up from oil to CH4 conversion (c. 325 atm) than oil to CO2 conversion (c. 75 atm) because density of CH4 gas is less than that of CO2 under the same P and T conditions. Sensitivity analyses also show that lower activation energy and higher pre-exponential factor lead to faster thermal cracking that allows oil or gas to reach peak generation earlier. Moreover, a basin experiencing a high heat flow throughout the burial history reaches hydrocarbon generation and overpressure development earlier. Calculation results also show that the oil and gas windows become deeper as the sedimentation rate increases. Thus, a basin experiencing high sedimentation rates would exhibit higher levels of thermal maturity and excess pore pressure over the deeper section. This also implies that greater overpressure may be expected at shallower depths in a basin with relatively low sedimentation rates. The modelling results demonstrate that kinetic parameters, basal heat flow and sedimentation rates all influence the timing, duration and depth of oil and gas generation, which in turn, profoundly affects the spatial and temporal distribution of overpressure.
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Tectono-thermal evolution of the Junggar Basin, NW China: constraints from Ro and apatite fission track modelling
Authors Qiu Nansheng, Zha Ming, Wang Xulong and Yang HaiboThe thermal evolution of the Junggar Basin, northwest China, was evaluated based on the thermal modelling results of 59 wells by using vitrinite reflectance (Ro) and apatite fission track (AFT) data. The thermal history indicates a cooling process of the basin since the Permian, but some differences in thermal evolution existed among the six structural units of the basin due to tectonic movements. The Junggar Basin was a ‘hot basin’ during the Permian, after which a cooling process with normal heat flow values occurred during the Mesozoic. Then the basin became a ‘cool basin’ from the beginning of the Tertiary. The average heat flow of the whole basin was 80 mW m−2 at the beginning of the Permian, then it decreased to 68 mW m−2 at the end of the Permian, to 63 mW m−2 at the end of the Triassic, 55 mW m−2 at the end of the Jurassic, 50 mW m−2 at the end of the Cretaceous and 42 mW m−2 at the present day.
The heat flow distribution of the basin at different geological times also shows the thermal evolution characteristics of the Junggar Basin. At the beginning of the Permian, the highest heat flow, 85 mW m−2, occurred in the central basin and the eastern part of the basin, but the lowest heat flow was distributed along the southern and western basin margins, down to 70 mW m−2. The heat flow values were between 45 mW m−2 and 65 mW m−2 at the end of the Jurassic, with the lower value of 45 mW m−2 at the southern basin margin. The highest heat flow value again occurred at the southern end of the Luliang Uplift, at the northern part of the Central Depression and at the Eastern Uplift area during that period. At the end of the Cretaceous, it was down to 40–55 mW m−2. The lowest heat flow occurred at the Southern Margin and in the Wulungu Depression, and the highest value in the Eastern Uplift area.
The tectonic subsidence also supports this thermal evolution of the basin. The rapid decrease of heat flow during the Tertiary in the Southern Margin of the basin may be caused by the uplift of the Tianshan Mountain. These heat flow data can provide useful parameters for the study of the Junggar Basin. Palaeoheat flow data are the critical parameter for hydrocarbon generation calculations. The results of this study provide a foundation for hydrocarbon generation history modelling and petroleum resource assessment in the Junggar Basin, which are important factors in the exploration of the Wulungu Depression and the study of stratigraphic and subtle traps in the Central Depression.
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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)