The delineation of sedimentary organic facies is crucial for understanding the oil and gas generation in hydrocarbon source rocks. The Fushun Basin, characterized by abundant dark-colored sedimentary rocks rich in organic matter, serves as a prototypical region for establishing organic facies. This study employs maceral analysis on a comprehensive set of organic geochemical data to investigate the origins of organic matter-rich mudstones, utilizing the perspective of sedimentary organic facies. The organic matter types in the Fushun Basin transitioned from the Guchengzi Formation to the Xiloutian Formation, evolving from type II2 and I to type II1 and I, and subsequently to type II1. Organic matter in the coal seams of the Guchengzi Formation mainly originates from terrestrial vascular plants, while in the oil shales of both the Jijuntun Formation and Xiloutian Formation, it primarily derives from lake algae. The sedimentary environment shifted from a peat swamp to a semi-deep lake, then to a deep lake, and ultimately to a shallow lake. Depending on the lake’s openness or closure, the organic facies are classified into wetland-forest (swamp), open-basin lake-alginite, and closed-basin lake-alginite facies. This suggests that the rapid accumulation of abundant higher plants in the shallow-water environment has facilitated coal seam deposition, while the extensive enrichment of algae in the deep-water environment has contributed to the formation of thick oil shale layers in the Fushun Basin.
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