Abstract:

Shale is at the forefront of oil and gas geological research and a hotspot for exploration; however, research has mainly focused on marine and lacustrine shale systems, while studies on shale within transitional coal-measure strata are relatively limited. The Carboniferous-Permian coal-measure strata in the Bohai Bay Basin are well-developed, characterized by widely distributed, regionally stable, and thick shale layers. These strata represent excellent source rocks and reservoirs, indicating significant potential for oil and gas exploration and development. This study investigated the coal-measure shale of the Carboniferous-Permian Taiyuan and Shanxi Formations in the Huanghua Depression of the Bohai Bay Basin. Using data from core analysis, thin sections, well logging, organic carbon content, Rock-Eval pyrolysis, and vitrinite reflectance (R₀), this study examined the depositional environment types of coal-measure shale, the vertical evolution of the depositional environments, and the organic geochemical properties of the shale from different depositional environments. This research aims to provide a theoretical basis for oil and gas exploration in the Carboniferous-Permian coal-measure strata of the Bohai Bay Basin. The Carboniferous-Permian coal-measure strata in the Huanghua Depression were divided into the Taiyuan Formation and the Shanxi Formation. The Taiyuan Formation was mainly characterized by barrier coastal facies, while the Shanxi Formation was dominated by deltaic facies. The shale of the Taiyuan Formation was primarily deposited in lagoon and tidal flat environments of the barrier coastal system, whereas the shale of the Shanxi Formation was mainly deposited in subaqueous distributary channels and interdistributary bay environments of the deltaic system. The lithological and logging characteristics of shale from different sedimentary facies were identified. Lagoon shale was gray-black, with well-developed horizontal laminations. Under the microscope, felsic material was visible, with fine particle sizes generally at the silt grade. Brownish-red siderite concretions were common, often exhibiting irregular ellipsoidal shapes with their long axes typically aligned parallel to the bedding planes. Lagoon shale exhibited distinct logging responses, characterized by high natural gamma and high resistivity on conventional logs, and bright yellow to bright red backgrounds with faint lamination structures on image logs. Tidal flat shale was mainly deposited in tidal flat environments. It was predominantly gray to black or dark gray. In core samples, well-developed felsic bands with a thickness of approximately 1 mm were visible. These felsic bands were laterally discontinuous and tapered off within the core samples. The particles within the bands were fine-grained, mainly silt-sized. Compared to lagoon shale, the tidal flat shale exhibited significantly lower resistivity. In imaging logs, the color appeared noticeably darker. The low response was attributed to the development of felsic bands within the tidal flat shale. The interbedding of thin sand and mud layers resulted in individual shale layers that were thinner than the vertical resolution of resistivity logging tools, leading to the measured apparent resistivity values being lower than the true formation resistivity. Consequently, the resistivity of tidal flat shale in the study area was significantly lower than that of the lagoon shale. Shale in subaqueous distributary channels was dark gray to gray-black and contained abundant siderite concretions occurring in banded and irregularly massive forms. These concretions mainly consisted of microcrystalline siderite grains, with minor felsic detrital particles, and were commonly associated with carbonaceous debris. Carbon and oxygen isotope analyses indicated 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 pore water to form siderite. The water coverage in the delta front also provided favorable conditions for siderite development. The abundant siderite in the shale reduced the formation conductivity and radioactive element content, resulting in low resistivity, uranium, and thorium readings on logs. Conversely, the high photoelectric absorption cross-section (P_e) of siderite increased the P_e value of the formation. Shale in interdistributary bays exhibited diverse colors, including dark gray, gray, and variegated colors, indicating strong water-level fluctuations during deposition and the presence of both subaqueous and emergent environments. Siderite was less developed in the interdistributary bay shale. Consequently, its resistivity and radioactive element content were significantly higher, and its P_e value was significantly lower than those of the subaqueous distributary channel shale. The depositional evolution of the Taiyuan and Shanxi Formations recorded a transition from the peak of the Late Paleozoic marine transgression to subsequent regression. Consequently, the depositional environments of shale transitioned from barrier coastal to deltaic facies, with shale sequentially developing in lagoon, tidal flat, delta front, and delta plain subfacies from bottom to top. The measured total organic carbon content of the shale varied among depositional environments: lagoon shale (0.11%~19.30%, avg. 3.81%), tidal flat shale (0.70%~17.99%, avg. 4.18%), subaqueous distributary channel shale (0.29%~5.91%, avg. 2.45%), and interdistributary bay shale (0.03%~7.36%, avg. 2.21%). A comparison showed that the tidal flat shale had the highest average total organic carbon abundance, followed by lagoon shale, subaqueous distributary channel shale, and interdistributary bay shale. Overall, the organic matter abundance of shale from barrier coastal facies was higher than that from deltaic facies. The organic matter types of shales from different depositional environments were similar, primarily Type III kerogen with some Type II₂, indicating a mixed input of terrestrial higher plants and aquatic lower organisms, with terrestrial higher plants being the dominant source. The measured R₀ values ranged from 0.60% to 1.12%, indicating that the organic matter was generally in a low-maturity to mature stage. The total organic carbon abundance of tidal flat shale (avg. 4.18%) was slightly higher than that of lagoon shale and significantly higher than that of deltaic shales, making it favorable for shale gas generation. The higher content of felsic particles in tidal flat shale enhanced the development of macropores and micropores, which were beneficial for shale gas storage. Meanwhile, the felsic particles increased the brittle mineral content, thereby enhancing the stimulation potential of the shale. Gas logging data also indicated gas-rich intervals within the shale. Overall, the Taiyuan Formation exhibited stronger gas logging responses than the Shanxi Formation, and tidal flat shale outperformed lagoon shale. These characteristics indicated that the tidal flat shale in the upper Taiyuan Formation was the most promising gas-rich interval. During the Early Permian deposition of the upper Taiyuan Formation, the marine transgression in North China mainly originated from the southeast. 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. Tidal flats were primarily distributed in the western part of the Huanghua Depression, with a northeast-southwest trend. Within this trend, the Cangxian uplift, Dongguang, Wumaying, Kongdian, Beidagang, Qibei, and Qinan buried hills were identified as favorable areas for shale gas exploration.

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