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1 School of Geosciences, The Queen's University Belfast, Belfast BT7 1NN, United Kingdom; r.worden{at}qub.ac.uk
2 BP-Amoco, Sunbury Research and Engineering, Chertsey Road, Sunbury on Thames, Middlesex TW16 7LN, United Kingdom
3 BP-Amoco, Sunbury Research and Engineering, Chertsey Road, Sunbury on Thames, Middlesex TW16 7LN, United Kingdom; present address: BP-Amoco, Sherwood House, Holton Heath Trading Park, Poole, Dorset BH16 6LS, United Kingdom
Richard Worden has a geology and geochemistry degree from Manchester University (1984) and a Ph.D. in geology from Manchester University (1988). Following postdoctoral research at Edinburgh University, he joined BP working as a sedimentary geochemist in 1989. He left BP in 1994 and took up a lectureship in geology at Queen's University in Belfast where he now works on several geochemical topics relevant to the oil industry, including clastic and carbonate diagenesis, production geochemistry, and nonhydrocarbon gas geochemistry.Mike Mayall has a geology degree obtained at Cardiff University (1975), an M.Sc. degree in sedimentology (1976), and a Ph.D. in sedimentology completed in 1979 at the University of Reading. He joined BP immediately afterward and worked in BP International sedimentology groups, including projects in the North Sea, Europe, southeast Asia, Australia, the Gulf of Mexico, and Alaska. He is currently manager of integrated reservoir description groups in London and Houston and heavily involved in deep-water exploration and development projects.
Jonathan Evans has an M.A. degree in geological sciences from Cambridge University and a Ph.D. from PRIS (Reading University), where he studied mudrock sedimentology and geochemistry. After a year as a BP-funded postdoctoral worker at the Centre de Geochimie de la Surface (CNRS) in Strasbourg, he joined the BP Research Centre in 1990. There, he worked on drilling problems and reservoir quality prediction. After spells as an exploration geologist and a petrophysicist/rock properties specialist working on Vietnam and Azerbaijan, he moved to his current job of senior geologist at BP Amoco's Wytch Farm field in southern England.
Ductile, clay-rich sand grains control porosity and permeability in the fluvio-deltaic Oligocene and lower Miocene sandstones of the South China Sea. Ductile grains account for between approximately 5 and 50% of the original sand grain population. There is a pronounced loss of porosity with increasing burial depth in the basin. At depths of less than 3000 m this is due solely to ductile grain compaction where the rate of porosity loss with depth increases with increasing ductile grain content. At depths greater than 3000 m, the steep porosity loss with depth is due to combined ductile grain compaction and quartz cementation. The amount of quartz cement increases with increasing burial depth; however, cleaner sandstones tend to have greater amounts of quartz cement at any given depth below 3000 m. This leads to convergence of porosity evolution for the clean and ductile-rich sandstones below 3000 m. There is a rapid loss of permeability with decreasing porosity because compaction of ductile grains smears them between rigid quartzose grains leading to blocked pore throats. A consequence of this process is that the lowest permeabilities are found in sandstones with the highest ductile grain contents. Quartz cement does not have a clear and discernible control on permeability. The pronounced loss of porosity with increasing depth and permeability with decreasing porosity leads to low permeability at relatively shallow burial depths. Reservoir quality is thus controlled by the nature of the primary sand and depth of burial. The sediment supply system led to systematic changes in ductile grain content across the basin with ductile content increasing into the more distal part of the sediment system. The consequence is that depth of economic basement (in terms of porosity or permeability) can be predicted as a function of ductile grain content and burial depth for prospects across the basin.
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