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- Volume 2, Issue 1, 1996
Petroleum Geoscience - Volume 2, Issue 1, 1996
Volume 2, Issue 1, 1996
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Predicting well productivity from grain size analysis and logging while drilling
Authors Andrew J. C. Hogg, Alan W. Mitchell and Susan YoungGrain size analysis of drill cuttings and Logging While Drilling (LWD) are used to predict permeability-height (KH) in high-angle wells. In the Sherwood Sandstone reservoir, Wytch Farm Field, permeability is controlled by porosity and grain size. These can be measured using LWD density logs and sieve analysis of cuttings. KH and Productivity Index (PI) are estimated using permeability-bulk density transforms for each grain size class. A PI of 60 ( - 35 / + 120 ) STB/day/psi was predicted for the first extended reach well on Wytch Farm. The initial PI was 100 STB/day/psi. This test result calibrated the model for subsequent wells. The predicted PI of the second well was 42 ( + or - 15 ) STB/day/psi. The initial PI was 40 STB/ day/psi. The KH plot is used to geosteer the well; optimize perforation intervals, and to estimate PI in wells plugged-back for water shut-off.
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Hydrocarbon generation in relation to thrusting in the Sub Andean zone from 18 to 22 degrees S, Bolivia
Authors I. Moretti, P. Baby, E. Mendez and D. ZubietaFormation of the Bolivian eastern cordillera started at the end of the Oligocene and continues to the present day with thrusting affecting the Sub Andean Zone since late Miocene. The 'classical' source rock is the Devonian, but the Silurian interval has some potential. Their initial original potential is not high but they are thick (700 m, Los Monos Fm) and may be thicker still due to stacking in thrust duplexes. The three phases of sedimentation during the Tertiary each start with conglomeratic deposits and are discordant over the previous one. Seismic data allow us to correlate these steps with three periods of tectonic accretion: Late Miocene (Tariquia Fm), Pliocene (Guandacay Fm) and Plio-Quaternary (Emborozu Fm). Modelling demonstrates two phases of hydrocarbon generation. A first phase from Devonian to Carboniferous is due to the deepening of the Palaeozoic basin. The second phase of maturation is due to the Tertiary deposits in the foreland and under the piggyback basins. The central area (Santa Cruz) has never been more deeply buried than during the Palaeozoic and has been a high during the Tertiary.
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A tectono-stratigraphic framework for Yemen and its implications for hydrocarbon potential
Authors A. C. Ellis, H. M. Kerr, C. P. Cornwell and D. O. WilliamsThe Late Jurassic Marib Al Jawf Basin is part of a rift system extending southeastwards into the less well known Wadi Hajar Basin. This rift system may be one arm of a Late Jurassic failed triple rift which also included the Masilah Basin to the east, and the Sir Basin to the north. Late Jurassic rifting is shown to be diachronous, younging eastwards. Renewed rifting during the Early to Mid Cretaceous was accompanied by deposition of the newly recognized Furt Formation. This second rift phase initiated movement on the N-S striking 47 degrees E Fault, which probably extends southwards into Somalia. Extension in the Jeza-Howarime Rift System may also have commenced at this time. Oman Mountain orogenesis led to renewed subsidence in the east, and was followed by polyphase Tertiary rifting, inversion and localized gravity-driven extension on the continental margins. As a result of these events, a number of distinct petroleum systems developed.
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The development of the North Morecambe gas field, East Irish Sea Basin, UK
By Greig CowanThe North Morecambe Gas Field in the East Irish Sea Basin was discovered by well 110/2-3 in 1976 and contains reserves of over 1TCF. The structure is fault-closed on three sides, and dip-closed to the north. Development is by 10 conventionally deviated high-angle wells, from a not-normally-manned platform, through a dedicated pipeline to a new terminal at Barrow. The Triassic Sherwood Sandstone Group reservoir is composed of sandstones deposited in a semi-arid,fluvial and aeolian setting. Thin, aeolian sandstones dominate flow into the well-bore. Platy illite reduces the permeability by 2-3 orders of magnitude in the lower illite-affected zone of the reservoir. RFT measurements from the first development well proved that the free water level was 25 ft higher than expected, giving a maximum gas column of 975 ft. Re-mapping after drilling has shown that 56% of the GIIP is contained in the high permeability illite-free zone.
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The Zechstein Stassfurt carbonate hydrocarbon system of the Thuringian Basin, Germany
Authors W. D. Karnin, E. Idiz, D. Merkel and E. RuprechtThe Thuringian Basin is the oldest hydrocarbon province in Eastern Germany: wet gas and oil accumulations have been discovered since 1930. After a long inactive period of about twenty years, exploration for hydrocarbons now continues with improved methods. Carbonates and evaporites of the Permian Zechstein comprise the source, reservoir and seal for the main gas and oil play in the area. Today, they are buried less than 1000 m deep over the major part of the basin. Modelling of the burial history of the basin indicates that up to 1500 m of Mesozoic strata were eroded during a Late Cretaceous inversion event. Peak hydrocarbon generation occurred just prior to this inversion, implying that hydrocarbons have been effectively trapped in the reservoirs since that time. In addition, dry gas and nitrogen appear to have been sourced from pre-Zechstein intervals, which, with respect to nitrogen, poses a severe exploration risk.
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Thorium distribution in some North Sea sandstones; implications for petrophysical evaluation
Authors Andrew Hurst and Anthony MilodowskiSeparation of minerals from reservoir sandstones into grain size classes, and chemical and mineralogical analysis, allows accurate description of relations between Th occurrence and mineralogy. Detrital heavy minerals, and particularly monazite, are found to account for the majority of the Th. Some heavy minerals occur as inclusions within detrital quartz which, because of its abundance, may account for a substantial proportion of the Th content in individual samples. Minor concentrations of Th are identified in diagenetic rare earth element (REE) phosphates. Kaolinite is not associated with Th-enrichment. Determination of Th content is not recommended as a guide to the kaolinite content of sandstones. Th content may be useful for identification of monazite and lithostratigraphic applications.
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Gravity drainage during gas injection
Authors P. Naylor, N. C. Sargent, A. J. Crosbie, A. P. Tilsed and S. G. GoodyearThe drainage of oil under gravitational forces has been an important mechanism in the production of many oil fields. In order to extend the economic implementation of gas injection into more marginal fields, a reduction in the uncertainties associated with gravity drainage is required. This paper describes a series of three tertiary, nitrogen experiments which investigated the effect of permeability on gravity drainage. The experiments were conducted at low pressure using long, vertical, water-wet sandstone cores and decane in the presence of connate brine. The residual oil saturation following 62 days of nitrogen injection in a 0.37 mu m 2 core was 0.26, following 53 days of nitrogen injection in a 1.5 /mu m 2 core was 0.10 and following 63 days of nitrogen injection in a 2.0 /mu m 2 core was 0.10. The variation of the oil and brine saturations were determined as a function of space and time for each experiment using a radioactive tracer technique. This independent measurement of both oil and brine in situ saturations is a new development and enables core artefacts to be identified and relative permeabilities to be derived. Detailed analysis of the 2 mu m 2 experiment showed that the oil relative permeability was independent of position and was only a function of oil saturation. The relative permeabilities are characterized by a zero asymptotic residual oil saturation and a Corey exponent of approximately four, which is higher than the value of three proposed from theoretical models of film drainage. A numerical simulation of this experiment gave a good match to the production and in situ saturation data. Determination of oil and brine relative permeabilities under a flow regime are presented which are representative of gravity drainage during gas injection. The method outlined gives added confidence when assessing field development options.
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Early discordant diagenesis in the Brent Group, Murchison Field, UK North Sea, detected in high values of seismic-derived acoustic impedance
Authors W. A. Ashcroft and M. S. RidgwayAcoustic impedance inversion of seismic data in the Murchison Field, UK North Sea has revealed a zone of high acoustic impedance within the Rannoch Formation of the Brent Group which is discordant to stratigraphy. The high values of acoustic impedance are caused by the diagenetic growth of siderite and pyrite early in the burial history of the rocks, probably while semi-consolidated at a depth of less than 1000 ft. The zone is approximately parallel to the overlying Base Cretaceous unconformity and its formation may have been controlled by a temperature gradient and/or groundwater chemical boundary that was parallel to ground surface at that time.
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Using 3D seismic to understand the structural evolution of the UK central North Sea
Authors J. W. Eggink, D. E. Riegstra and P. SuzanneInterpretation of regional 3D seismic data suggests that the structural evolution of the Central North Sea occurred in three successive structural regimes. Two successive extensional tectonic regimes from Late Palaeozoic to Early Cretaceous were followed by a predominantly compressive tectonic regime from the Late Cretaceous into the Tertiary. Tectonic movements were controlled by the relative orientation of the stress fields with respect to the fault strikes of older major Variscan fault trends. The amount and direction of strike-slip movement was controlled by a gradual clockwise rotation of the minimum effective stress in the horizontal plane from approximately NE-SW to E-W in the first two phases. Within this framework, halokinesis is only of local importance, and serves to amplify the tectonically controlled structuration. The structural model developed here explains the observed distribution of fields and structures in the Central Graben, as well as enabling prediction of structural development in its less well explored portions.
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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)