Characteristics and Depositional Model of Source Rocks in Outcrops of the 1st and 2nd Member of Nenjiang Formation in the Southeast Uplift Area of Songliao Basin—Take Yaojiazhan Section of Caiyuanzi Town, Dehui City as an Example

The research results of nenjiang Formation source rocks based on core analysis in Songliao Basin are abundant. However, few people pay attention to the characteristics and sedimentary environment of outcrop source rocks. Based on geochemical methods such as lithologic characterization, whole-rock microscopic examination, rock pyrolysis analysis and constant microanalysis of outcrops, the characteristics and depositional model of mud shale in the Nenjiang Formation of Yaojiazhan in southeastern Songliao Basin were analyzed. The results show that the strata of nenjiang Formation exposed in Yaojiazhan, Songliao Basin have a thickness of 50–70 m, and the lithology is mainly lacustrine gray-green and gray-black mud shale. The foliation of the rock is relatively developed, rich in ostracodes, phyllometes, plant detritus and other biological fossils. The source rocks are mainly planktonic algal bodies, the second is sapropelic indefiniteness. Kerogen is mainly type I, followed by type II1, and its hydrocarbon generation potential is from medium to poor. The pyrolysis parameters of source rocks in the Nenjiang Formation range from 5.4 to 9.58, Tmax from 432 to 437 ℃, S1 from 0.50 to 1.16 mg/g, S2 from 5.8 to 76 mg/g, and RO from 0.38 to 0.54%, which are located near the oil generation window and have moderate to good hydrocarbon generation potential. The paleoclimate parameter w(Sr)/w (Cu) ranges from 1.04 to 9.04, the paleosalinity parameter w (Sr)/w (Ba) ranges from 0.39 to 0.72, and the REDOX environmental parameter w (V/ (V + Ni)) ranges from 0.71 to 0.80. It indicates that the source rocks were formed in a fresh-brackish water reducing environment under warm and humid climate. A slightly deformed lacustrine sedimentary model with dark mud shale as the main framework and dolomite, oolitic limestone and ostracoda as the secondary framework. The research results can effectively guide the exploration and deployment of Gulong shale oil in Songliao Basin.