Geological characteristics of source rocks of Mesoproterozoic Changchengian System in Shujigou area of the northern margin of Ordos Basin

The development of effective source rocks is the key to Mesoproterozoic hydrocarbon accumulation in Ordos area. In this paper, the geological characteristics of the Agulugou Formation source rocks in Shujigou area were studied by means of field geological investigation, microscopic section identification and geochemical analysis. Researches show that the source rocks of Agulugou Formation are carbonaceous slate and phyllite, and the original rocks are black sand-bearing shale. The contents of organic carbon in the source rocks are high, and the value ranges from 0.34% to 9.56%, with an average of 3.3%. The types of organic matter are Ⅰ and Ⅱ1 kerogen. The value of R_o ranges from 3.57% to 4.68%, and the T_max value is from 404 ℃ to 562 ℃. These results show that the thermal evolution of source rocks is high, and is in the over-mature stage. The distribution pattern of rare earth elements is a slightly right-leaning pattern with slight enrichment of LREE and no obvious differentiation of LREE and HREE. The δCe values range from 0.74 to 0.92, showing a negative Ce anomaly, and the δEu range is relatively large, ranging from 0.95 to 4.83, that indicates that the deposit of the source rocks formed in a transitional environment from reduction to oxidation. The content of trace elements indicates that the Agulugou Formation deposited in a back-plateform basin with a water depth of less than 250 m, and the sedimentary facies were fluvial-shore sands, shallow water carbonaceous-argillaceous deposits of the barrier island. Combined with the above research, it shows that the Mesoproterozoic source rocks in the study area are mainly good-excellent source rocks formed in the reduction-oxidation environment, and the hydrocarbon generation geological conditions are favorable.