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.
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