High quality source rocks in trough facies of Upper Permian Dalong Formation, Sichuan Basin

Source rocks in trough facies of the Upper Permian Dalong Formation of the Sichuan Basin were assessed by field observations and measurements in outcrop areas as well as from logging and core/cutting analysis from drilling sites. The results show that in the Dalong Formation, the black shale and sapropelite facies have an average TOC of 5.86%, with the maximum value up to 24.31%, making them high quality hydrocarbon source rocks. Dark cryptite, with a TOC of 0.77% (on average) is also a good hydrocarbon source rock. A dark silicate, with TOC of only 0.13% on average, has no potential to generate hydrocarbons. Since the source rocks are much evolved, the content of chloroform bitumen “A” extracts tend not to be high. The (S1+S2) value of source rocks in the Dalong Formation are low, with only 4.36 mg/g on average. The major macerals of Kerogen of the Dalong Formation are sapropels, averaging 71.5%. Kerogen is mainly Type II1, and a little belongs to II2. The flourishing of pelagic organisms during deposition of the Late Permian Changxing Formation, slow deposition rate of the Dalong Formation and an anoxic and calm deep water depositional environment are the key controlling factors for accumulation, preservation and generation of organic matter in this part of the basin.