Geology and geochemistry of large gas fields in the deepwater areas, continental margin basins of northern South China Sea

The Qiongdongnan (QDN) and Pearl River Mouth (PRM) basins are two important Cenozoic petroliferous basins along the northern margin of the South China Sea (SCS). In this study, the geological and geochemical characteristics of the LS17-2 and LW3-1 gas fields, the respective largest gas field in the deepwater areas of the QDN and PRM basins, were investigated. The tectonic evolution of the two basins can be divided into four stages, including rift, rift-depression transition, post-rift thermal subsidence and neotectonic stages. During the early rifting period, strong crustal thinning in the deepwater areas led to the formation of the broad and deep sags where Eocene shallow lake and Oligocene transitional to neritic source rocks were developed whose organic matter was dominated by gas-prone type II2–III kerogens. The gases from the LS17-2 gas field in the QDN Basin are characterized by relatively heavy δ13C1 (−36.8 to −40.1‰) and δ13C2 (−23.5 to −26.2‰) values. The associated condensates contain high abundance of oleanane and low content of bicadinanes, showing close affinity to the Oligocene Yacheng source rocks. The large-scale Huangliu channel sandstones act as reservoirs with a massive mudstone cap of lower Yinggehai Formation. The diapiric faults provide the main pathways for the upward migration of gases into the shallow reservoirs. By contrast, the gases from the LW3-1 gas field in the PRM Basin have δ13C1 values similar to LS17-2 gases, while their δ13C2 values are relatively light (−28.4 to −29.6‰). Like the LS17-2 condensates, LW3-1 condensates also contain abundant oleanane. These features indicate that the gases/condensates could be derived from the lower Oligocene Enping and Eocene Wenchang shallow-lake source rocks. The Zhuhai delta and the Zhujiang submarine fan sandstones and thick marine mudstones of the upper Zhujiang Formation form the favorable “reservoir-cap” assemblage. The effective combination of faults and permeable sandbodies make up the main migration pathway. The anatomy of geological elements for the two large fields suggests that both the source rock facies and reservoirs have controlled the large-scale gas accumulation. This study provides new insights into the hydrocarbon accumulation models in the deepwater areas of two basins, and implies highly promising gas exploration potentials.