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Abstract

AAPG Bulletin, V. 93, No. 1 (January 2009), P. 75-100.

Copyright copy2009. The American Association of Petroleum Geologists. All rights reserved.

DOI:10.1306/08080808031

Present-day stress and neotectonics of Brunei: Implications for petroleum exploration and production

Previous HitMarkTop R. P. Tingay,1 Richard R. Hillis,2 Chris K. Morley,3 Rosalind C. King,4 Richard E. Swarbrick,5 Abdul Razak Damit6

1School 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; [email protected]
2Australian School of Petroleum, University of Adelaide, Adelaide, South Australia 5005, Australia; [email protected]
3PTT Exploration and Production Public Company Limited, 535 Vibhavadi-Rangsit Road Bangkok 10900, Thailand; [email protected]
4Australian School of Petroleum, University of Adelaide, Adelaide, South Australia 5005, Australia; [email protected]
5GeoPressure Technology Mountjoy Research Centre, Stockton Road, Durham DH1 3UZ, United Kingdom; [email protected]
6Brunei 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; [email protected]

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 (sigmav) 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 (sigmaHmax) 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 (sigmahmin) 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 sigmahmin direction, whereas fracture stimulation in mature fields and tight reservoirs can be more easily conducted in wells deviated toward sigmaHmax. 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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