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1 School of Earth and Environmental Sciences, University of Adelaide, Adelaide, South Australia 5005, Australia; present address: Department of Applied Geology, Curtin University of Technology, Perth, Western Australia; M.tingay{at}curtin.edu.au
2 Australian School of Petroleum, University of Adelaide, Adelaide, South Australia 5005, Australia; rhillis{at}asp.adelaide.edu.au
3 PTT Exploration and Production Public Company Limited, 535 Vibhavadi-Rangsit Road Bangkok 10900, Thailand; chrissmorley{at}gmail.com
4 Australian School of Petroleum, University of Adelaide, Adelaide, South Australia 5005, Australia; rking{at}asp.adelaide.edu.au
5 GeoPressure Technology Mountjoy Research Centre, Stockton Road, Durham DH1 3UZ, United Kingdom; r.e.swarbrick{at}geopressure.co.uk
6 Brunei Shell Petroleum, Jalan Utara, Seria KB3534, Brunei; present address: Brunei National Petroleum Company Sendirian Berhad, Unit 2.02, 2nd Floor, Block D, Yayasan Sultan Haji Hassanal Bolkiah Complex, Jalan Pretty Bandar Seri Begawan BS8711, Brunei; razak.damit{at}pb.com.bn
Mark Tingay is currently an Australian postdoctoral fellow at Curtin University, where he works on stress, overpressure, and tectonic evolution of Southeast Asia. He received his Ph.D. in 2003 from the Australian School of Petroleum. He then became the 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 State of South Australia professor of petroleum geology at the University of Adelaide. He received a B.S. degree (hons) from Imperial College and a Ph.D. from the University of Edinburgh. He is a director of image log and a geomechanics consultant of JRS Petroleum Research and of the geothermal exploration company Petratherm.
Chris Morley 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.
Rosalind King is a postdoctoral researcher at the Australian School of Petroleum where she studies the present-day stress and neotectonics of northwest Borneo as well as delta and deep-water fold and thrust belt systems worldwide. She completed her Ph.D. on the structural evolution of the Cape fold belt and southwest Karoo Basin, South Africa.
Richard Swarbrick 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.
Abdul Razak Damit is currently the chief geologist for the National Oil Company of Brunei (PetroleumBRUNEI). He obtained his Ph.D. at Aberdeen University and has 20 years of industry experience, primarily at 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
The present-day state of stress in Tertiary deltas is poorly understood but vital for a range of applications such as wellbore stability and fracture stimulation. The Tertiary Baram Delta province, Brunei, exhibits a range of contemporary stress values that reflect the competing influence of the northwest Borneo active margin (situated underneath the basin) and local stresses generated within the delta. Vertical stress (
v) gradients at 1500-m (4921-ft) depth range from 18.3 MPa/km (0.81 psi/ft) at the shelf edge to 24.3 MPa/km (1.07 psi/ft) in the hinterland, indicating a range in the shallow bulk density across the delta of 2.07–2.48 g/cm3. The maximum horizontal stress (Hmax) orientation rotates from margin parallel (northeast–southwest; deltaic) in the outer shelf to margin normal (northwest–southeast; basement associated) in the inner shelf. Minimum horizontal stress (hmin) gradients in normally pressured sequences range from 13.8 to 17.0 MPa/km (0.61–0.75 psi/ft) with higher gradients observed in older parts of the basin. The variation in contemporary stress across the basin reveals a delta system that is inverting and self-cannibalizing as the delta system rapidly progrades across the margin. The present-day stress in the delta system has implications for a range of exploration and production issues affecting Brunei. Underbalanced wells are more stable if deviated toward the hmin direction, whereas fracture stimulation in mature fields and tight reservoirs can be more easily conducted in wells deviated toward Hmax. Finally, faults near the shelf edge are optimally oriented for reactivation, and hence exploration targets in this region are at a high risk of fault seal breach.
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