An Insight into the Tectonic Framework and Structural Evolution of a Frontier Area in Sarawak Offshore Basin, Malaysia

Abstract

Tectonic framework and structural evolution controls the sedimentation of petroleum system elements in a basin. The present study integrates the results of offset well analysis, regional seismic mapping, and gravity modeling, to bring out a better understanding of the tectonic framework and structural evolution of a frontier area in offshore Sarawak basin. Regional depth structure and isochron maps based on reprocessed 2D seismic data were analyzed to characterize the structural fabric of the study area. Depth structure map at basement top reveals a strong overprint of the reactivated, NW-SE to N-S younger trend, over a less distinct NE-SW older trend, especially in the eastern half of the study area. The NW-SE trend is consistent with the regional lineation and the basement high features, interpreted on regional Bouger gravity anomaly maps. The basement map, output from 3D inversion of free air gravity data over the study area, also corroborates these structural elements. Four distinct structural high axes, designated Trend1 to 4, aligned NW-SE to N-S, have been identified in the eastern half of the study area, on depth structure maps at Middle Miocene Unconformity (MMU), and Base Pleistocene levels. Trend 1 towards west, is aligned along the West Balingian line, a regional strike slip zone recognized in the shallower part of this basin. West of this zone, no significant tectonic elements have been observed, other than the NE-SW trending faults, resulting from an earlier phase of extension. The other three structural high axes identified are located between Trend 1 to the west, and the West Baram line to the east. The alignment of the structural high axes along older basement trends, suggests basement involved structuring related to wrench tectonics. Isochrons of post MMU sequences indicate a progressive shift in the timing of wrench activity, from Late Early Miocene (?) towards west at Trend 1, to Recent towards east at Trend 4. Further studies are required to explain the causative mechanism. The study has helped in understanding the timing of the structures with respect to hydrocarbon migration, and has a direct bearing on the hydrocarbon prospectivity of the study area.