The present-day stress field and paleo tectonics in offshore Brunei Darussalam

Present-day maximum horizontal stress orientation is one of the fundamental inputs for well design and stability analysis. Since the previous stress orientation map in Brunei Darussalam published in 2005, more borehole image data have been acquired to evaluate both reservoir geology and hole damages of different causes. The image-based breakout orientations in recent wells not only filled critical data gaps but also replaced the low-confident stress orientations from dipmeter data on the previous map. Integration with seismic interpretation also gives new insights into the present-day stress variations in relations to the tectonic evolution. The new map suggests three main stress domains transitioning to each other. Domain A in the deepwater to shelf slope region is compressional, characterized by northwest-southeast maximum horizontal stress, consistently perpendicular to the associated delta toe thrust structures near the well locations. The shelf to onshore region is divided into domain B in the east and domain C in the west as result of different evolutions and interactions between shallow deltaic and deep plate tectonics. This contrasts with the 2005 stress map, which divided the region into inversed inner shelf and extensional outer shelf provinces. The domain B is possibly in strike slip state with margin normal maximum horizontal stress, which is a reversal from the paleo extensions expressed by collapse faults and a large counter regional normal fault. The counter regional normal fault is interpreted as the result of rapid sediment load over thick ductile shales prior to the toe thrust system. The domain C in the west exhibits extension with east-west maximum horizontal stress, parallel to the normal faults but oblique to the thrust faults and the coastline. It is reversed from the latest compressional episode. Restoration of the structural deformation history suggests that tectonic reversals occurred several times in the domain C. Between the two shelf domains, where two sets of structural lineaments intersect, the maximum horizontal stress rotates across the two systems.