Relationship between the geochemical characteristics and sedimentary environment of Permian hydrocarbon source rocks in the Ordos basin

Abstract：Permian source rocks are important and potential source rocks in the Ordos basin. A systematic analysis of the trace element and REE geochemistry and organic geochemistry of argillaceous carbonate rocks and mudstones has been carried out mainly according to the geochemical data of the well core samples. The LREE abundance is high and the HREE abundance is low and Ce/Ce* has a relatively pronounced positive anomaly, indicating that the source rocks were deposited in a reducing environment. The contents of trace elements in the source rocks are significantly different from their clarke values in similar rocks in the Ordos basin. Study shows that grayish black mudstone source rocks deposited in semi-deep lake-deep lake facies zones are strongly reduced and rich in organic matter, where organic matter could be buried timely, and grayish black mudstone source rocks deposited in a fluvial-swampy environment have a low organic matter abundance. The strongly reduced depositional facies zone with a fairly deep water body was an ideal locus for deposition of primary organic matter, where the organic matter could be buried and preserved timely; whereas organic matter abundance was obviously low in argillaceous carbonate source rocks deposited where the climate was dry with little precipitation and terrigenous clastic material supply was inadequate or in a long-term oxidizing environment.