Present-day stress orientation in Brunei: a snapshot of ‘prograding tectonics’ in a Tertiary delta

Abstract

The Baram Delta province of NW Borneo is unusual when compared with most other Tertiary deltas, as it has built up upon an active margin. Hence, structures observed in the Baram Delta province are the result of both margin-parallel gravity-driven deltaic tectonics and approximately margin-normal transpressive tectonics associated with the active margin. Image and dipmeter logs have been examined for breakouts and drilling-induced tensile fractures (DITFs) in 47 wells throughout Brunei. Breakouts and DITFs observed in 19 wells suggest that the maximum horizontal stress is oriented margin-normal (NW–SE) in the proximal parts of the basin and margin-parallel (NE–SW) in the outer shelf region. The margin-parallel outer shelf stress field is interpreted as a local ‘deltaic’ stress field caused by the shape of the clastic wedge. The margin-normal maximum horizontal stress in the inner shelf is interpreted to reflect basement stresses associated with the active margin. However, the maximum horizontal stress in the inner shelf is approximately perpendicular to the strike of Miocene–Pliocene normal growth faults, suggesting that maximum horizontal stress in the inner shelf has rotated from margin-parallel (‘deltaic’) to margin-normal (‘basement-associated’) over time. Hence, approximately the same stress rotation has occurred over time in the inner shelf as is currently observed spatially from the outer to inner shelf. The spatial and temporal stress rotations in Brunei are thus interpreted to be the result of ‘deltaic’ and ‘basement-associated’ tectonic regimes that are ‘prograding’ basin-wards. The proximity of the active margin has resulted in progressive uplift and inversion of the hinterland that has ‘forced’ the delta system to prograde rapidly. The zone of active deltaic growth faulting (and margin-parallel maximum horizontal stress) has shifted basin-wards (‘prograded’) as the delta system has rapidly prograded across the shelf. After uplift and delta progradation, the old growth faults of the inner shelf ceased being active and have then been successively reactivated by a similarly ‘prograding’ margin-normal inversion front.