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

Tingay, Mark¹, Richard Hillis², Chris Morley³, Richard Swarbrick⁴, Steve Drake⁵ (1) World Stress Map Project, Karlsruhe, Germany (2) University of Adelaide, Adelaide, SA, Australia (3) University Brunei Darussalam, Bandar Seri Begawan, Brunei (4) University of Durham, Durham, England (5) 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.