Abstract: Shale is currently at the forefront of oil and gas geological research and a hotspot for exploration, but research efforts have mainly focused on marine shale and lacustrine shale systems, studies on shale within transitional coal-bearing strata are relatively scarce. The Carboniferous-Permian transitional coal-bearing strata in the Bohai Bay Basin are well-developed, featuring thick shale layers with wide distribution and regional stability. These strata represent excellent source rocks and reservoirs, holding significant potential for oil and gas exploration and development. This paper takes the coal-bearing shale in the Carboniferous-Permian strata of the Huanghua Depression in the Bohai Bay Basin as the research object. Using data from core samples, thin sections, well logging, organic carbon content, pyrolysis, and vitrinite reflectance, the study examines the depositional environment types and characteristics of coal-bearing shale, the vertical evolution of the depositional environment, and the organic geochemical properties. The goal is to provide a foundation for the exploration of oil and gas resources in the Carboniferous-Permian coal-bearing strata of the Bohai Bay Basin. The Carboniferous-Permian coal-bearing strata in the Huanghua Depression can be divided into the Taiyuan Formation and the Shanxi Formation. The Taiyuan Formation is mainly characterized by barrier coastal facies, while the Shanxi Formation is dominated by deltaic facies. Shale in the Taiyuan Formation primarily formed in lagoon and tidal flat environments within a barrier coastal setting, whereas shale in the Shanxi Formation was mainly deposited in subaqueous distributary channels and interdistributary bay environments of a deltaic setting. Lagoon shale is gray-black, with well-developed horizontal laminations. Under the microscope, felsic material is visible in the shale, with fine particle sizes, generally at the silt grade. Brownish-red siderite concretions are commonly observed, often with irregular ellipsoidal shapes, and their long axes are usually aligned with the bedding planes. Lagoon shale exhibits distinct logging responses, characterized by high natural gamma and high resistivity in conventional logging, and bright yellow to bright red backgrounds in imaging logging, with faint lamination structures. Tidal flat shale is mainly formed in mudflat environments. It is predominantly gray-black or dark gray shale. In the cores, well-developed felsic bands with a thickness of about 1 mm are visible. These felsic bands are laterally discontinuous, tapering off in the cores. The grain size of the particles within the bands is fine, mainly at the silt grade. Compared to lagoon shale, mudflat shale exhibits significantly lower resistivity. In imaging logging, the color appears noticeably darker. This response is related to the development of felsic bands in the mudflat shale. The interbedding of thin sand and mud layers often results in individual shale layers being thinner than the vertical resolution of resistivity logging tools, causing the measured apparent resistivity values to be much lower than the true resistivity of the formation. Consequently, the resistivity of mudflat shale in the study area is significantly lower than that of lagoon shale. Shale in subaqueous distributary channels is dark gray to gray-black, with abundant siderite concretions, which occur in banded and irregularly massive forms. The siderite concretions mainly consist of microcrystalline siderite grains, with minor felsic detrital particles and are often associated with carbonaceous debris. Carbon and oxygen isotope analyses indicate that the formation of siderite in the delta front was influenced by organic matter and the water chemistry of the depositional environment. After deposition in the delta front, terrestrial carbonaceous debris decomposed, releasing CO₃^2-, which combined with Fe²⁺ in the water to form siderite. The water coverage in the delta front also provided favorable conditions for the development of siderite. The abundant siderite in the shale reduces the conductivity and radioactive element content of the formation, resulting in low resistivity, low uranium, and low thorium characteristics in logging, while the high photoelectric absorption cross-section (Pe) value of siderite increases the Pe value of the formation. Shale in interdistributary bays exhibits diverse colors, including dark gray, gray, and variegated colors, indicating strong fluctuations in water levels during deposition, with both water-covered and exposed environments being present. Siderite is less developed in interdistributary bay shale, and its resistivity and radioactive element content are significantly higher than those of subaqueous distributary channel shale, while its Pe value is significantly lower. The Taiyuan and Shanxi formations in the Huanghua Depression experienced a transformation from the peak of Late Paleozoic marine transgression to regression. Against this background, the depositional environments of shale transitioned from barrier coastal to deltaic settings, with shale sequentially developing in lagoon, tidal flat, delta front, and delta plain subfacies from bottom to top. The organic carbon content of lagoon shale ranges from 0.11% to 19.30%, with an average of 3.81%. The organic carbon content of mudflat shale ranges from 0.70% to 17.99%, with an average of 4.18%. The organic carbon content of subaqueous distributary channel shale ranges from 0.29% to 5.91%, with an average of 2.45%. The organic carbon content of interdistributary bay shale ranges from 0.03% to 7.36%, with an average of 2.21%. Comparing the organic carbon abundance of shale in different environments shows that mudflat shale has the highest average organic carbon abundance, followed by lagoon shale, then subaqueous distributary channel shale, and finally interdistributary bay shale. Overall, the organic matter abundance in shale from barrier coastal facies is higher than that in deltaic facies. The organic matter types of different origin shales are similar, primarily Type III, with some Type II₂, indicating a mixed input of terrestrial higher plants and aquatic lower organisms, with terrestrial higher plants being the dominant input. The Ro values range from 0.60% to 1.12%, indicating that the shale is generally in a low-maturity to mature evolutionary stage. The organic matter abundance in mudflat shale, averaging 4.18%, is slightly higher than that of lagoon shale but significantly higher than that of subaqueous distributary channel shale and interdistributary bay shale, making it favorable for shale gas generation. The higher content of felsic particles in mudflat shale, including felsic bands and scattered felsic grains, enhances the development of macropores and micropores, which is beneficial for shale gas storage. Meanwhile, the development of felsic particles increases the brittle mineral content of the rock, enhancing its reworkability, which is advantageous for hydraulic fracturing during shale gas development. Gas logging also indicates gas-rich intervals in the shale. Overall, the Taiyuan Formation exhibits stronger gas logging responses than the Shanxi Formation, with mudflat shale outperforming lagoon shale. These characteristics indicate that the mudflat shale in the upper Taiyuan Formation is the most promising gas-rich interval. During the Early Permian deposition of the upper Taiyuan Formation, marine transgression in North China mainly originated from the southeast of the basin. Tidal flat deposits were extensively developed across most of the Huanghua Depression, while barrier islands and lagoon deposits were confined to the eastern Chenghai area. Mudflats were primarily distributed in the western part of the Huanghua Depression, trending northeast-southwest. Within this range, the Cangxian Uplift, Dongguang, Wumaying, Kongdian, Beidagang, Qibei, and Qinan buried hills are favorable areas for shale gas exploration.

Key words: Huanghua Depression, Carboniferous-Permian, coal measure shale, sedimentary environment, sedimentary organic matter