Constraints on strike-slip motion from seismic and gravity data along the Vietnam margin offshore Da Nang: implications for hydrocarbon prospectivity and opening of the East Vietnam Sea

Abstract

Abstract The Vietnamese continental margin records the tectonic processes which have controlled opening of the East Vietnam Sea basin. The geometry and chronology of Cenozoic structures linked to the opening of this marginal basin have been studied offshore Da Nang (central Vietnam), based on unpublished data acquired by BP Exploration from 1989 to 1991. These data demonstrate that a N160° E fault trend is the main structural orientation recognised on both gravity and seismic data. Important crustal extension on shallow-dipping fault planes striking N060° E and connected to the N160° E faults is observed also. This horse-tail map-pattern is compatible with transtensional dextral motion along the N160° E faults. The discrepancy between the amount of motion observed onshore and offshore suggests the presence of a major shear zone which trends subparallel to the east Vietnam margin. This shear zone is located in the region of the offshore Qui Nhon ridge, which is blanketed by carbonate sediments. These carbonates have been drilled and dated as Middle Miocene. Dextral motion along this shear zone diminished during the late Early Miocene (around 20 Ma.), as evidenced by the onset of carbonate deposition on top of the ridge. This is broadly synchronous with propagation of the East Vietnam Sea basin towards the southwest. This major shear zone has been a fundamental part of the fault network which developed during opening of the East Vietnam Sea. The structural development of the offshore Da Nang region has resulted in the development of contrasting sub-basin geometries; those within the major shear zone appear to be relatively restricted laterally whereas those to the east display geometries more typical of extensional sub-basins. Both sub-basins show a variety of potential hydrocarbon trapping styles and by analogy with comparable areas in SE Asia have the potential to contain important oil reserves, in addition to the considerable volume of gas already proven by recent drilling.