Developmental characteristics of source rock and gas source analysis in Qiongdongnan Basin, Northern South China Sea

The Cenozoic Qingdongnan Basin hosts a very thick Tertiary strata including three sets of source rocks from bottom to top: the Eocene, the Oligocene (containing Lingshui-Yacheng Formation), and the Miocene. The Oligocene is the key source rock with high abundance of organic matter widespread in basin and coal-bearing strata developed locally. The Miocene is thick neritic-bathyal facies that distributed stably in the central depression zone with high hydrocarbon conversion rate and is potential type of source rock. The Eocene is a set of important hydrocarbon source rocks and has not been drilled through at present. However, seismology study shows that the northern depression belt developed source rocks of semi-deep lacustrine deposits. Study of elemental geochemistry shows that the central depression zone has the highest paleoproductivity; the central uplift belt is in the second place; and the northern depression belt has the lowest paleoproductivity. The paleoclimate of the early Oligocene and early Miocene were warm and favorable to the development of source rocks. Based on the above information, three developmental modes was put forward, i.e., the transitional facies, the semi-enclosed marine facies, and the marine facies. On the basis of modes above, the most advantageous zones that contain high quality source rock is the central depression belt. The northern depression belt and the central uplift belt are in the second place. In addition, geochemistry data, the composition and carbon isotope characteristics of gas indicate that the gas-bearing structure in the northern depression belt is characterized by oil type gas, which may be contributed by the Eocene. The gas fields in the central depression mainly come from the Oligocene Yacheng Formation, featured coal type gas.