'Prograding' tectonics in Brunei: Regional implications for fault sealing.

Abstract

The Baram Delta province of Brunei (northwest Borneo) is unusual when compared with most other Tertiary deltas, as it has built up upon an active margin. Hence, the structures observed in Brunei 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 46 wells throughout the basin. Breakouts and DITFs observed in 19 wells suggest that the maximum horizontal stress (σHmax) is oriented margin-normal (NW-SE) in the proximal parts of the basin and margin-parallel (NE-SW) in the distal region. The margin-normal σHmax direction is perpendicular to the strike of Miocene-Pliocene normal growth faults in the delta. Hence, there has been an approximately 90 degree rotation of the σHmax direction over time, in addition to the observed present-day spatial horizontal stress rotation across the delta. The present-day and temporal stress rotations observed from breakouts and DITFs are confirmed by observations of shale dykes, blowout-induced fractures and active growth faulting. Furthermore, the Baram Delta province also shows a very large variation invertical stress gradient from 18.3 MPa/km at the delta front to 24.3 MPa/km in the delta hinterland, equivalent to a change in bulk density from 2.07 to 2.48 g/cm3 across the delta.The present-day and temporal stress rotations and the variation in vertical stress yield a 'snapshot' of a delta that is inverting and self-cannibalising. The proximity of the active margin has resulted in uplift of the hinterland and successive inversion of normal-faults in a basin-ward direction. The region of active deltaic growth faulting is also 'prograding' as demonstrated by the margin-parallel σHmax direction in the distal region of the delta. The present-day stress tensor and 'prograding' tectonic history are primary controls on fault reactivation and subsequent seal breach in the region. Hydrocarbon leakage has occurred along an inverted fault in the Miri Field and only a few small fields have been found in the distal zone of active growth faulting. However, the margin-normal σHmax direction observed in the proximal part of the Baram Delta province is poorly orientated for fault reactivation and seal breach and several giant fault-bound fields are located in this region.