Geochemistry and geological significance of the Upper Paleozoic and Mesozoic source rocks in the Lower Yangtze region

Abstract

The Lower Yangtze region is one of the important marine sedimentation areas of oil and gas distribution in southern China, for its favorable source rocks, reservoirs and covers. However, the intense tectonic movements and complex hydrocarbon generation process made it highly impossible to form large-sized oil and gas reservoirs. So it was divided to different hydrocarbon-bearing preservation units in oil-gas exploration. Recent study shows that the Permian and Lower Triassic source rocks in the Lower Yangtze region are complicated in lithology. The hydrocarbon generation potential of limestone there is low while argillaceous source rocks are overall of high abundance with excellent organic types, now in the process of hydrocarbon generation, so differences in high maturity influence the evaluation of organic matter abundance and type. Biomarker characteristics indicate a reductive environment. n-alkanes are marked by a single peak, with no odd-even predominance. The composition and distribution of the carbon numbers of n-alkanes, and the high abundance of long-chain tricyclic terpanes are indicative of marine sedimentation. The high contents of pregnane, homopregnane, rearranged hopane suggest that the source rocks are of high maturity. There is a good linear correlation between methylphenanthrene index and vitrinite reflectance. The correlation of oil-source rocks indicated that the oil of Well HT-3 may come from the Permian Longtan Formation in the Huangqiao area, the oil of Wells Rong-2 and Juping-1 came from the Lower Triassic Qinglong Formation in the Jurong area. The exploration here is promising in those different source rocks which all have great potential in hydrocarbon generating, and oil and gas were produced in the late stage of hydrocarbon generation.