Prediction of effective source rocks in deep-water basins via integrated geochemical, sedimentary, and seismic analyses: A case study in the Changchang Sag, Qiongdongnan Basin, South China Sea

The Changchang Sag, located in the Qiongdongnan Basin in the South China Sea, is a particularly promising area for deep-water hydrocarbon exploration. The spatial distributions and geochemical features of the source rocks in the sag remain unconfirmed, owing to the limited availability of rock samples and corresponding geochemical data resulting from the sparsity of boreholes. This study combines geochemical data with seismic attributes to preliminarily reveal the thickness and distribution of the mudstones of the 2 nd Member of the Oligocene Lingshui Formation (LS2) and the 2 nd Member of the Oligocene Yacheng Formation (YC2). Analysis of 47 source rock samples from the LS2 and YC2 members reveals the Changchang Sag mudstones as potential effective source rocks, with average total organic carbon ( TOC ) contents of 0.5 wt.% and 0.6 wt.%, respectively. The thermal maturity of the source rocks in the Changchang Sag was calculated using the vitrinite reflectance ( R o )-porosity model and representative thermal history curves of individual wells, with R o values all exceeding 1.2%. The areal distribution of the effective source rocks was therefore determined by combining the measured geochemical data with the thickness, thermal maturity distribution, and organic facies of the mudstones. The most favorable source rocks in the LS2 and YC2 members occur in the eastern and central parts of the sag, with maximum thicknesses of up to 800 m and 500 m, respectively. The method provides a practical approach to typical source rock prediction for petroliferous basins with limited drilling and geochemical data.