Characteristics of the Coal-Associated Shale and Its Paleoenvironmental Analysis of the Upper Triassic Tumengela Formation in the Central Part of the Qiangtang Basin

The Upper Triassic is an important target interval for oil and gas exploration in the Qiangtang Basin and contains well-developed dark shales. Among these units, the Tumengela Formation coal-associated shale exposed in the Central Uplift Zone and its two sides is the most extensive. In this study, we selected the Tumengela Formation coal-associated shale in the Woruoshan and Chaiduochaka areas as the key research objects and systematically evaluated the quality and hydrocarbon generation potential of the coal-associated shale in the central part of the Qiangtang Basin based on previous achievements. Meanwhile, the microscopic characteristics, mineralogy, and elemental geochemistry of the typical shale in the Woruoshan area were conducted to understand its reservoir properties, adsorption properties, and formation mechanisms. The results show that the coal-associated shale in the central part of the Qiangtang Basin is primarily medium to poor source rocks. Among them, its quality in the Woruoshan-Dopulenai-Chalangla areas is relatively good, and well-developed source rocks are located locally. Organic matter is dominated by high to overmature type II₂-III kerogen. In the Woruoshan area, coal-associated shale is characterized by high clay mineral content, medium brittle mineral contents, and low carbonate mineral contents, while the coal-associated shale in the Chaiduochaka area is characterized by high brittle mineral contents, medium clay mineral contents, and low carbonate mineral contents. In these shales, reservoir space is characterized by microscopic interlayer cracks, structural cracks, intergranular pores, erosion pores, and organic pores. The above coal-associated shale is formed in a warm-humid climate, in a low-salinity, oxidized water column, and associated with medium to high primary productivity level. High primary productivity, rapid sedimentation, and clay-mineral adsorption chiefly controlled the formation of these shales. Based on the organic matter abundance, type, maturity, thickness of the coal-associated shale in the central part of the Qiangtang Basin, and the paleogeographic pattern, we infer that the depression between Woruoshan and Jiangaidarina likely represents a favorable hydrocarbon area and a significant target for future oil and gas exploration. These findings provide valuable insights into the hydrocarbon potential and organic matter enrichment mechanism of coal-associated shale in a marine-terrestrial transitional environment.