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1 School of Earth and Environmental Sciences, University of Adelaide, Adelaide, Australia;
present address: Department of Applied Geology, Curtin University of Technology, GPO Box U1987, Perth, Western Australia 6845, Australia;
m.tingay{at}curtin.edu.au
2 Australian School of Petroleum, University of Adelaide, Adelaide, Australia
3 GeoPressure Technology, Durham, United Kingdom
4 PTT Exploration and Production, Bangkok, Thailand
5 Brunei Shell Petroleum, Seria, Negara, Brunei Darussalam
Mark Tingay is currently an Australian postdoctoral fellow at Curtin University, where he works on stress, overpressure, and the tectonic evolution of Southeast Asia. He received his Ph.D. in 2003 from the Australian School of Petroleum. He then became a petroleum geomechanics researcher at the World Stress Map Project, where he worked on projects in 11 countries, including the United States, Egypt, Azerbaijan, and Thailand.
Richard Hillis is the head of the Australian School of Petroleum and a state of South Australia professor of petroleum geology at the University of Adelaide. He received his B.Sc. (hons) degree from Imperial College and Ph.D. from the University of Edinburgh. He is a director of JRS Petroleum Research, an image log and geomechanics consulting company, and of Petratherm, a geothermal exploration company.
Richard commenced his career in 1979 when he joined Mobil with assignments in the United Kingdom and the United States. He joined Durham University in 1989 and was a principal investigator for a multidisciplinary research group funded by 17 oil and gas companies. Over that period, he developed training courses in subsurface pressures and founded the company GeoPressure Technology. He is an honorary professor at Durham University and has been an AAPG member since 1982.
Chris received his Ph.D. in 1983 before working for Amoco and Elf Aquitaine and as a professor at the University of Brunei Darussalam. He is currently working for PTT Exploration and Production as a senior geophysicist. He has worked as an exploration geologist and as a structural geologist in east Africa, Morocco, the Norwegian Caledonides, the Carpathians, northwest Borneo, and Thailand.
Abdul Razak Damit is currently the chief geologist with the National Oil Company of Brunei (PetroleumBRUNEI). He obtained his Ph.D. at Aberdeen University and has 20 years of industry experience, primarily in Shell where he worked on both reservoir and regional evaluation. His main interests are in the geology of northwest Borneo and in raising public awareness of the natural and social history of Brunei.
ABSTRACT
Accurate pore-pressure prediction is critical in hydrocarbon exploration and is especially important in the rapidly deposited Tertiary Baram Delta province where all economic fields exhibit overpressures, commonly of high magnitude and with narrow transition zones. A pore-pressure database was compiled using wireline formation interval tests, drillstem tests, and mud weights from 157 wells in 61 fields throughout Brunei. Overpressures are observed in 54 fields both in the inner-shelf deltaic sequences and in the underlying prodelta shales. Porosity vs. vertical effective stress plots from 31 fields reveal that overpressures are primarily generated by disequilibrium compaction in the prodelta shales but have been generated by fluid expansion in the inner-shelf deltaic sequences. However, the geology of Brunei precludes overpressures in the inner-shelf deltaics being generated by any conventional fluid expansion mechanism (e.g., kerogen-to-gas maturation), and we propose that these overpressures have been vertically transferred into reservoir units, via faults, from the prodelta shales. Sediments overpressured by disequilibrium compaction exhibit different physical properties to those overpressured by vertical transfer, and hence, different pore-pressure prediction strategies need to be applied in the prodelta shales and inner-shelf deltaic sequences. Sonic and density log data detect overpressures generated by disequilibrium compaction, and pore pressures are accurately predicted using an Eaton exponent of 3.0. Sonic log data detect vertically transferred overpressures even in the absence of a porosity anomaly, and pore pressures are reasonably predicted using an Eaton exponent of 6.5.
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