Micro seepage of hydrocarbon gas and its significance for oil-gas exploration in the Northern South China Sea

Abstract Submarine hydrocarbon gas seepage is a widespread natural phenomenon, resulting in hydrocarbon anomalies of shallow surface sediment. Seepage includes macro seepage and micro seepage, a large number of macro seepage studies have investigated in South China Sea, relatively little is known about micro seepage, and very few studies of micro seepage 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 key to successful exploration. Gas chromatography-mass spectrometry is used for qualitative and quantitative analysis of the abundance and structural anomalies of light and heavy hydrocarbons, and Kriging method is used to determine the areal distribution characteristics of light and heavy hydrocarbons and the carbon isotope values δ13C (‰) of methane. As a result, 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 petroleum reservoirs are identified as gas reservoirs, and the seismic imaged DS-A structure is predicted to be a gas-rich structure. By correlating the seismic data and geochemical anomalies, it is clarified that micro-fractures are the main controlling factors for the occurrence of the seabed geochemical anomalies, and a composite formation mechanism of “lower generation, upper accumulation and micro fractures leaking” has been proposed.