In Situ Stress and Tectonic Evolution of Brunei: Implications for Shale Dike Emplacement

Mark Tingay, Heidelberg Academy of Sciences, World Stress Map Project, Geophysics Institute, University Karlsruhe, Hertzstrasse 16, Karlsruhe, 76187, Germany, phone: +49 721 608 4427, fax: +49 721 71173, mark.tingay@gpi.uni-karlsruhe.de, Richard Hillis, National Centre for Petroleum Geology and Geophysics, University of Adelaide, Thebarton Campus, Adelaide, SA, 5152, Australia, Chris Morley, Department of Petroleum Geoscience, University Brunei Darussalam, Jalan Tungku Link, Bandar Seri Begawan, BE 1410, Brunei, Richard Swarbrick, Department of Earth Sciences, University of Durham, South Road, Durham, DH1 3LE, England, and Steve Drake, Exploration, Brunei Shell Petroleum, Seria, Brunei.

Shale dykes, mud volcanoes and shale diapirs are common in the onshore and offshore regions of the rapidly-prograding Tertiary Baram Delta province, Brunei. Over 50 shale dykes crop out in the Jerudong anticline, displaying two major phases and modes of emplacement: Middle Miocene dykes injected predominately along NE-SW striking faults and Middle-Late Miocene dykes injected primarily along NW-SE striking tensile fractures.

The present-day state of stress in Baram Delta Province reveals unique insights into the mechanics of the Jerudong Anticline shale dyke emplacement. Borehole breakouts in 19 wells reveal a margin-parallel (NE-SW) deltaic maximum horizontal stress orientation in the outer shelf and a margin-perpendicular (NW-SE) basement-associated orientation in the inner shelf. The spatial rotation of the present-day stress field (in comparison with structural styles) reveals that the ‘deltaic' stress field, and associated tectonics, have shifted basinwards over time as the delta prograded, resulting in the rotation in shale dyke orientation. Minifracture and Repeat Formation tests suggest a pore pressure-stress coupling ratio of 0.59. This coupling ratio is used with the stress rotation to explain the observed change in the mode of shale dyke emplacement, and is supported by evidence from offshore blowout fractures.

Improved Understanding of Plays Related to Salt and Shale Tectonics I
2005 AAPG International Conference and Exhibition (September 11-14, 2005) Technical Program