Geochemistry and origins of natural gases in the Yinggehai and Qiongdongnan basins, offshore South China Sea

Abstract

Four gas fields and a number of gas-bearing structures have been discovered in the Yinggehai and Qiongdongnan basins offshore South China Sea. Chemical and isotopic data indicate the presence of at least three genetic groups of gases in these basins: biogenic gas, thermogenic gas, and gases with mixed origin. Thermogenic gases produced from the Yacheng field are characterized by relatively high contents of benzene and toluene, relatively low δ 13C values of toluene, and high abundance of bicadinanes and oleanane in the associated condensates, showing good correlation with the coal-bearing sequence of the Yacheng Formation in the Qiongdongnan Basin. In contrast, the gases from the DF1-1 and LD gas fields contain high amounts of N 2 and CO 2, low to moderate amounts of benzene and toluene, with relatively high δ 13C values of toluene. These characteristics correlate well with the Miocene neritic shales in the Yinggehai Basin. Analyses on potential source rock samples indicate excellent gas source potential for both the coal-bearing sequence in the Yacheng Formation and the Miocene neritic shales containing type III∼II2 kerogens. As the result of recent rapid subsidence and sedimentation, high temperature and overpressure systems are well developed in the Yinggehai and Qiongdongnan basins. The rapid heating resulted in advanced maturation of organic matter deposited in normally pressured and overpressured strata, whereas the strongly overpressured systems led to retarded organic matter maturation, postponing the time when the source rocks reached peak gas generation. Results of 1-D modeling indicate that the coal-bearing Yacheng Formation in the Yacheng Sag reached peak gas generation during mid Miocene-Pliocene time, after the deposition of upper Oligocene to lower Miocene reservoir rocks. The lower-mid Miocene marine shales reached their peak gas generation stage during the Pliocene-Quaternary in the central Yinggehai basin, supplying abundant gases to charge the Pliocene reservoirs. More importantly, focused-episodic gas migration significantly increased the hydrocarbon expulsion efficiencies of source rocks, and thus provided favorable conditions for the accumulation of large amounts of gases in the diapiric structures in a very short geological time. The gas–source relationship for the Yacheng field suggests dominantly short–distance gas migration and thus strong source facies controls on the geographic distribution of gas fields within the Qiongdongnan Basin.