Leading effect of the Seventh Member high-quality source rock of Yanchang Formation in Ordos Basin during the enrichment of low-penetrating oil-gas accumulation: Geology and geochemistry

A suit of high-quality source rock of semi-deep to deep lacustrine facies, which developed during the age of the Seventh Member of Yanchang Formation (Chang 7 for short) at Late Triassic in the Ordos Basin, has the most abundant content of organic matter in Yanchang Formation. On the area of 5×104 km2, its thickness of a single layer is commonly 5～25 m and the accumulative one is mainly from 10 m to 50 m, with a maximum of 80 m. The analyses of the detailed geochemical data show that the Chang 7 high-quality source rock was formed under the reductive sedimentary condition with deep water body, low salinity, and indistinct delaminating water body; the main organic matters were originated from alga, a low form of lacustrine organism poor in 13C; the kerogen is of type I-II1; the organic matter abundance is high and the residual TOC value is commonly 6%-14%, with a maximum value of 30%; and the organic matter has been in maturation and high maturation evolution stage. It is indicated that the source rock underwent violent hydrocarbon expulsion by low content of S1+S2 and chloroform bitumen A, and low ratio of saturated hydrocarbon to aromatic hydrocarbon. The precise delimitation of this suite of high-quality source rock is of great importance in oil exploration and deep understanding of petroleum deposit mechanism of low-penetrating reservoir in this region.