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- Volume 10, Issue 2, 2004
Petroleum Geoscience - Volume 10, Issue 2, 2004
Volume 10, Issue 2, 2004
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Kela-2: a major gas field in the Tarim Basin of west China
Authors Xu Shilin, Lu Xiuxiang, Sun Zhonghua, Pi Xuejun, Liu Luofu, Li Qiming and Xie HuiwenKela-2, a large gas field developed in a sub-thrust anticline within the Kuqa depression of the Tarim Basin and with reserves of 7.55×1012 SCF, represents the largest unsegmented gas pool in China. Detailed petrographic, geochemical, structural and fluid inclusion studies show that the field is a result of a favourable interrelationship of geological events, involving several periods of compressive structuring, multiple periods of oil and gas migration and a complex diagenetic history. The Kela-2 discovery well encountered abundant oil and gas shows over a 448 m gas column within Cretaceous to Lower Tertiary Bashijiqike and Baxigai Formation sands. Gas is trapped in a large, heavily faulted, northeast-east striking fold with a décollement horizon developed along a gypsiferous horizon. The structure, with a closure of 55 km2, was initiated in the Late Tertiary compressive phase and was accentuated during Quaternary Xiyu compression.
The source of the gas is Triassic and Jurassic coals, which communicate with the reservoir vertically through faults. It is possible through fluid inclusion studies to identify two periods of secondary oil migration and two periods of re-migration within the reservoir. The reservoirs were deposited in a continental environment, within which secondary porosity has been developed by leaching of carbonate cement that infilled original primary pore spaces. The gypsiferous mudstones of the Lower Tertiary provide a common seal to both reservoirs, which are abnormally pressured.
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Modelling of the hydrocarbon generation history and volumetric considerations of the coal-sourced Lulita Field, Danish North Sea
Authors A. D. Carr and H. I. PetersenThe hydrocarbon generation history of the Middle Jurassic Lulita Field in the Søgne Basin in the northeastern part of the Danish Central Graben has been predicted by modelling the Lulita-1Xc well located on-structure and a pseudowell located in the kitchen area. The Lulita Field consists of east-dipping Middle–Upper Jurassic sediments in a structural trap. Waxy oil, sourced from Middle Jurassic coals and type III kerogen (and possibly type I kerogen) shales, occurs in Middle Jurassic sandstone reservoirs sealed by Upper Jurassic marine shales. Maturity modelling (vitrinite reflectance) was carried out by using the PresRo® kinetic model and a heat flow history calibrated from both the tectonic-subsidence history and maturity data. An effective oil window (‘oil expulsion window’) between 0.85% and 1.8%Ro for the Middle Jurassic coal and type III kerogen source rocks was applied. Timing for onset of oil generation differs between the Lulita-1Xc well and the pseudowell. The top of the source rocks becomes mature during the Late Cretaceous in the kitchen area and during the Oligocene in the Lulita-1Xc well. However, oil expulsion from the entire Middle Jurassic source rocks is modelled to have started from the Paleocene in the kitchen area and appears to have continued through to the present day. No, or little, expulsion is modelled in the Lulita-1Xc well, indicating that the Lulita Field was charged by updip migration of hydrocarbons from the Søgne Basin kitchen to the east. Hydrocarbon charging pre-dates the time of potential diagenetic modification of the reservoir, thus limiting reduction in reservoir porosity and permeability. Based on the known coal source rock volume and quality, calculated maximum generated and expelled hydrocarbons cannot account for the estimated in-place volumes of oil. The additional hydrocarbons were probably generated from coaly shales and shaly coals that form the lateral equivalent of the Middle Jurassic coals in the kitchen area, possibly with contributions from lacustrine to brackish type I kerogen shales.
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An example study using conventional 3D seismic data to delineate shallow gas drilling hazards from the West Delta Deep Marine Concession, offshore Nile Delta, Egypt
Authors Andy Sharp and Andy SamuelThis paper details a study that was completed at the end of 1999 to investigate the suitability of using existing, high quality, conventionally processed 3D seismic data (hereafter 3D data) to replace specifically commissioned 2D seismic site survey data (hereafter 2D site data). For this study, three wells from an existing dataset of six in the West Delta Deep Marine Concession, offshore Egypt (WDDM) were investigated. A prescriptive workflow was developed using the 3D data and the results compared to those obtained from a conventional previously commissioned 2D digital site survey dataset. For this dataset from WDDM, the results obtained from the 3D data and the 2D data were similar, the main difference being that more numerous, smaller probable hazards were prognosed by the 3D data. After undertaking this investigation and following an independent review of the results, a recommendation was made to operate the 2000 WDDM drilling campaign using hazard prognosis from the conventional 3D data, i.e. not to acquire well-specific, new digital site surveys. This recommendation was followed and five exploration wells were drilled in 2000 with no specific, digital site survey data being acquired.No safety, operational or environmental implications were encountered as a result of this replacement and significant operational savings accrued to the operating partnership.
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From seismic to bed: surface–subsurface correlations within the turbiditic Cellino Formation (central Italy)
Authors S. Carruba, R. Casnedi and F. FellettiThe area east of the Gran Sasso Chain in central Italy has been explored in detail because it holds a hydrocarbon field (Cellino Field), located in the Lower Pliocene foredeep turbidites of the Cellino Formation. Correlation has been made between the hydrocarbon-bearing sedimentary bodies and the same stratigraphic intervals cropping out only few kilometres to the west. The area, thus, offers a rare example where comparison can be made between lithofacies – observed in the field, electrofacies – displayed by the electric logs of the wells, and seismofacies – revealed by seismic survey.
This surface–subsurface integrated study of the Cellino Formation has revealed the presence of different turbiditic facies associations, their related electrical expressions and the possibility of a seismostratigraphic subdivision of the unit. These correlations exemplify the different resolution of these three complementary methods of investigation.
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Quantitative assessment of mudstone lithology using geophysical wireline logs and artificial neural networks
Authors Yunlai Yang, Andrew C. Aplin and Steve R. LarterUsing a dataset of 530 analysed mudstone samples from 19 North Sea and 9 Gulf of Mexico wells, back propagation artificial neural networks (ANNs) have been trained to estimate the clay content (proportion of particles smaller than 2 μm diameter), grain density and total organic content (TOC) of mudstones from standard wireline log data (gamma, resistivity, sonic, density, calliper). ANNs have also been trained to discriminate carbonates from clastic mudstones and also give a preliminary indication of the extent to which mudstones are lithified or cemented. Results show that for clay content, 85% of predictions are within ±10% of the measured value; for TOC, 92% of predictions are within ±1% of the measured value; for grain density, 91% of predictions are within ±0.07 g cm−3 of the measured value; for the discrimination of carbonates from clastics, 98.3% of carbonate samples and 99.9% of non-carbonate samples are classified correctly. The ANNs work well not only in the areas from where training data were measured, but also (as an example) in offshore West Africa. Potential applications of the technique include (1) the possibility to define the 3D sedimentary architecture of mudstone sequences from wireline data and, because both the porosity–effective stress and porosity–permeability relationships of mudstones are strongly influenced by clay content, (2) more accurate, basin-scale fluid flow modelling.
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Definition and practical application of mudstone porosity–effective stress relationships
Authors Yunlai Yang and Andrew C. AplinThe relationship developed in soil mechanics between void ratio and vertical effective stress is a simple but practical way of describing the one-dimensional, mechanical compaction of fine-grained, clastic mudstones. The compression coefficients (e100 and ß) that define this relationship are strongly influenced by grain size, which can be simply described by the sediment's clay content. In this paper, data both from the soil mechanics literature and from geological samples from the North Sea and Gulf of Mexico are used to construct the relationship between clay content and compression coefficients. The two datasets yield different values for the coefficients, but the authors believe that the coefficients derived from the large geological database should be used to describe geological compaction. Regression of the geological data generates the following relationships between clay content and compression coefficients:
\[e_{100}{\,}={\,}0.3024{\,}+{\,}1.6867clay{\,}+{\,}1.9505clay^{2}\] \[{\beta}{\,}={\,}0.0407{\,}+{\,}0.2479clay{\,}+{\,}0.3684clay^{2}\] There is excellent agreement between true porosities and porosities calculated using the standard soil mechanics compaction equation with these coefficients. This indicates that the mechanical compaction of natural mudstones to 40 MPa can be adequately described using the soil mechanics approach. Practical application of the work is a two-stage process which first involves the evaluation of clay content and, thus, compression coefficients directly from wireline log data. These data can then be used to (a) help define effective stress–porosity inputs for basin models and (b) estimate mudstone pore pressures. Examples of pore pressure estimates are shown from west Africa, the North Sea and the Gulf of Mexico.
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Organic geochemical characteristics of the bituminous Eocene units in the Çorum–Suluova Basin, Central Anatolia, Turkey
Authors M. Altunsoy, O. Özçelik and N. Yalçin ErikIn the Çorum–Suluova Basin (central Anatolia) the Eocene Çeltek and Armutlu formations consist of conglomerate, sandstone, siltstone, marl, mudstone, shale, limestone, clayey limestone, bituminous shale and coal. Total organic carbon (TOC) values in the Çeltek Formation range between 0.1 and 8%. Rock-Eval analyses on core samples with the highest TOC values give hydrogen index (HI) values from 45–846 mgHC g−1 TOC and oxygen index (OI) values from 9–64 mgCO2 g−1 TOC. The organic matter can be classified as Type I and Type II kerogen on the modified van Krevelen diagram. T max values vary between 428° and 440°C, with an average of 436°C, indicating the catagenesis stage. Based on the microscopic studies, organic matter is composed of predominantly autochthonous algal and amorphous material, with a minor contribution of terrestrial material. The Çeltek Formation can be regarded as a source rock for hydrocarbons. The TOC content of the Armutlu Formation is poor. Samples consist of terrestrial and reworked organic matter. Vitrinite reflectance values are between 0.27% and 0.43%, with an average of 0.34%. T max values are between 433°C and 436°C, with an average of 435°C. The HI value is between 29 mgHC g−1 TOC and 39 mgHC g−1 TOC. Considering the data, the Armutlu Formation cannot be the source rock.
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Upscaled models of flow and transport in faulted sandstone: boundary condition effects and explicit fracture modelling
Authors Eric A. Flodin, Louis J. Durlofsky and Atilla AydinFaults formed by shearing of joint zones in sandstone contain fine-scale features that cannot be represented explicitly in large-scale flow simulations. Upscaled models are, therefore, required for reservoir engineering computations. These models attempt to capture fine-scale effects through equivalent permeabilities that are computed from the underlying fine-scale characterization. In this paper the impact of several different local boundary conditions on the calculated equivalent permeability is assessed. Pressure–no-flow, periodic and mirror-periodic boundary specifications are considered. The resulting coarse-scale permeability tensors are shown to be highly dependent on the local boundary conditions used in the models. In cases with through-going high-permeability features, pressure–no-flow and mirror-periodic boundary conditions provide upscaled permeabilities that correctly capture global flow characteristics. Periodic boundary conditions, by contrast, are more suitable for systems lacking through-going high-permeability features. This sensitivity to boundary conditions calls into question the robustness of the equivalent permeability for the general case and suggests that dominant through-going features would best be modelled explicitly. In addition, due to the very small thickness and high permeability of some through-going structural features (e.g. slip surfaces), globally upscaled models are inadequate for the modelling of transport. To address these issues, a ‘partial upscaling’ method – removing the through-going high-permeability features from the fine model, upscaling to a coarse grid and then reintroducing the high-permeability features back into the coarsened model – is adopted. This procedure is shown to provide coarse models that give accurate predictions for both flow and transport.
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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)