Source rock geochemistry and gas potential in the deep water area,Qiongdongnan basin

In the deep water area of Qiongdongnan basin,the geochemical data in exploration wells and seismic records show that two sets of source rocks are developed in its central depression,i.e.Eocene lacustrine facies and Oligocene marine-coastal plain facies,in which the coal-bearing source rock of coastal plain facies in Oligocene Yacheng Formation is excellent in gas generation capacity due to its high content of terrigenous organic matter.The source rock of semi-isolated shallow marine facies in Yacheng Formation is mostly between 0.5% and 1.5% in TOC(total organic carbon) and predominated by humic kerogen,with its distribution area and volume quite great.Because of limited drilling data in the deep water area,the tools of depositional facies,seismic facies and organic facies were comprehensively used to predict the distribution of the major gas source rocks in Yacheng Formation,and the results have indicated that the source rocks in Yacheng Formation are more widely developed in the central depression,especially in Baodao,Lingshui,Ledong、Changchang and Beijiao sags.The modeling experiments of gas generation under high temperature and pressure have suggested that the higher geothermal gradient in the central depression could accelerate gas generation,and that the widespread Oligocene overpressure in the depression might restrain early gas generation to a certain degree and promote later gas generation,resulting in the expanding of gas window and gas exploration play.Furthermore,the source rocks of Eocene lacustrine facies could generate oil early and produce oil-cracked gas lately due to their deep burial.Therefore,the main sags in the deep water area are higher in gas generation capacity and good in gas exploration potential.