Effects of Source and Distribution of Organic Matter on Methane in Offshore Sediments of Zhoushan Islands

Hangzhou Bay, located at the junction of land and sea in the northeast of Zhejiang Province, is a primary depositional site for terrigenous detrital material carried by the Yangtze River and Qiantang River to the sea, and biogenic shallow gas is widely distributed in the coastal sediments of this area. In this study, the relationship between the distribution of organic matter and methane gas in the sediments of Hangzhou Bay was analyzed using four columnar sediment boreholes of YS3, YS4, YS6, and YS7. It is concluded that the organic matter in the sediments of methane-bearing gas layers in each borehole is a mixture of terrigenous and Marine sources. According to the calculation of terrigenous (CT) and Marine (CM) contributions of sediment organic matter, the upper part of the YS4 and YS7 holes is dominated by terrigenous input. In contrast, the lower part of the YS3 and YS6 holes is dominated by Marine input. The organic matter content of sediments showed a trend of increasing gradually from top to bottom, and the organic carbon content (SOC) of sediments in methane-bearing reservoirs was higher than that in non-gas pools. The methane-bearing gas reservoirs in the pore sediments mainly occur in the silt and clay sediments, with an average porosity of about 50%. The larger the porosity of the sediments, the higher the gas content, and the methane gas in the sediments did not have an extensive range of longitudinal migration.