TECTONIC EVOLUTION AND ITS CONSTRAINTS ON THE FORMATION OF DEEPWATER GIANT GAS FIELD IN LINGSHUI SAG, QIONGDONGNAN BASIN

ecently, a deepwater giant gas field has been discovered in the Lingshui sag of the Qiongdongnan basin. As the result, Cenozoic structures and their constraining effects on the formation of deepwater giant gas field has gained great attention from researchers. In this paper, well data and seismic data are applied to study the Cenozoic structural evolution of the sag and their controlling effects over the formation of deepwater giant gas field from viewpoints of sequence stratigraghy and structural analysis. Compared with previous studies, the coupling of structure evolution, tectonic mechanism, sedimentary infilling and natural gas accumulation are emphasized. It is revealed that the Cenozoic structure evolution of the sag consisted of stages of Paleocene-Eocene rifting, Oligocene depressing-rifting, Early Miocene rifting-depressing and Middle-Miocene to Quaternary depressing. Data suggests that: a. Oligocene depressing-rifting controlled the distribution of source rocks of marine-terrestrial transitional facies and marine facies, and the depression from Middle-Miocene to Quaternary expanded the gas generation window. b. Oligocene depressing-rifting provided the opportunity for the formation of fan-delta reservoir, and the depression from Middle-Miocene to Pliocene caused the formation of confined and non-confined deepwater clastic reservoirs and reef carbonate reservoirs. c. Oligocene depressing-rifting later on provided the opportunities for the formation of various structural traps, such as the strike-slip-extention structures, fault-noses, fault-anticlines, and various lithological traps such as deepwater confined channel sandstone of gravity flow, non-confined basinal lithological traps and reef stratigraphic traps. d. overpressure in the stages of Oligocene depressing-rifting and Middle-Miocene depressing stages formed the new faults/fissures as the paths for migration.