Microseepage of Hydrocarbon Gas and its Significance for Oil–Gas Exploration in the Northern South China Sea

The microseepage activity in Chaoshan Depression is obvious, and the relationship between microseepage and oil–gas reservoirs is a matter of great concern. Submarine hydrocarbon gas seepage includes macroseepage and microseepage; a large number of macroseepage studies have been investigated in the South China Sea, but relatively little is known about microseepage, and very few studies of microseepage in oil–gas exploration have been carried out. Chaoshan Depression is a Mesozoic and Cenozoic superimposed depression in the northern South China Sea; it has good potential for oil–gas exploration, but oil and gas have not been discovered through drilling. Effective identification and prediction of favorable exploration areas and reservoirs are the keys to successful exploration. In this paper, gas chromatography–mass spectrometry was used for qualitative and quantitative analysis of the seafloor geochemical anomaly; two comprehensive abnormal zones for oil–gas exploration were delineated in the northeast and southwest parts of the Chaoshan Depression, respectively. By analyzing the differentiation and fabric characteristics of hydrocarbon gases, as well as methane δ¹³C (‰), the underlying reservoirs are identified as gas reservoirs, and DS-A is predicted to be a gas-rich structure. Microfractures are the main controlling factors for the occurrence of microseepage, and gas microseepage causes the submarine geochemical anomaly on the top of oil and gas reservoirs. Submarine geochemical anomaly and underlying reservoir have a good associative relationship; it can be used to predict the oil and gas reservoirs accurately.