Genesis and sources of natural gas in eastern Qiongdongnan Basin, South China Sea

The genesis and sources of natural gas in the Songdong and Baodao sags in the eastern Qiongdongnan Basin were analyzed through gas geochemistry and pool-forming research methods based on sample tests and seismic data. The natural gas in the study area is mainly the oil type. In the BF13 and SF24 regions on the northern slope of the Songdong Sag, mature natural gas of the oil type was found in the Miocene. In the BF19-2-3 region in the fault belt no.2 in the northern Baodao Sag, high-maturity and over-mature natural gas of the oil type (sometimes mixed with coal gas) was found in the Upper Oligocene Lingshui Formation. The findings differed from former reports about the coal-type gas in the Yinggehai and Qiongdongnan basins. The discovered oil type natural gas mainly was sourced from lacustrine deposits in the Eocene, and partially from the coal formations in the Yacheng Formation. It was deduced that there was another set of important gas source rocks besides the coal formations in the Yacheng Formation. The northwestern slope of the Songdong Sag and the fault belt no. 2 in the northern Baodao Sag are important exploration targets for natural gas.