Hydrocarbon habitat of the offshore Bohai Basin, China

The Bohai Basin is a major continental petroliferous basin located in eastern China, of which the offshore part of the basin contains more than one-third of the total petroleum resources in the basin. Although the basin has been explored for nearly 40 years, the offshore part of the basin is just reaching a moderately mature stage of exploration, and its petroleum geological features are gradually becoming known. The subduction of the Pacific Plate to the Eurasia Plate resulted in the convective upwelling of the mantle in the Bohai area and triggered the development of NNE and NE striking extensional faults along the Mesozoic structures in the Late Paleocene, causing the formation of the rift basin. During the Oligocene, the depocenters and subsidence centers of the greater Bohai Basin migrated from west to east and from south to north and finally concentrated in the offshore part of the basin. Since the Oligocene, especially during the Neogene, due to the indentation of the India Block to the Eurasia Plate, the right-lateral strike-slip movement of the Tan-Lu fault was relatively intense in the offshore part of the basin. These characteristics led to the differences between the offshore and onshore sectors in sedimentation, structural features, and petroleum geology. During the deposition of the Dongying Formation, the offshore sector became the depocenter and subsidence center of the basin, resulting in the deposition of the lower Dongying Formation mudstone source rocks; while these mudstones are immature in the onshore part of the basin. In the offshore Bohai, most major discoveries have been made in the drape anticlines on the rises in the Upper Tertiary reservoirs; while the Lower Tertiary reservoirs within the half-grabens only serve as secondary reservoirs. As the Neogene depocenter of the basin, the offshore part accommodated some fine-grained shallow lacustrine clastics of the lower Minghuazhen Formation, serving as important regional seals for the Neogene reservoirs. A set of shallow faults and a central faulted anticlinal belt in the shallow Upper Tertiary sequences were formed during the Neogene in the offshore sector under the influence of right-lateral strike-slip faulting of the Tan-Lu fault. These shallow faults networked with deep extensional faults, serving as hydrocarbon conduits. The central faulted anticline belt, along the Tan-Lu fault system, also provides significant traps. (C) 2004 Elsevier Ltd. All rights reserved.