The platform margin belts of the Dengying Formation flanking the Deyang-Anyue rift trough represent a critical frontier for natural gas exploration and development in the Sichuan Basin. While giant gas fields, such as Anyue and Penglai, have been discovered along the eastern margin, the sedimentary characteristics of the western margin and the architectural disparities between the two flanks remain poorly understood, thereby constraining further exploration efforts in the region. By integrating core descriptions, well logs, and 3D seismic data, we systematically characterize the development of the Dengying Formation platform margins on both sides of the trough. Lithofacies paleogeographic maps were reconstructed for key intervals, and refined sedimentary facies models were established for the dual-flank system. As indicated by the research results (1) During the Late Sinian deposition of the Dengying Formation, the platform margins on both flanks primarily consisted of microbial mound, shoal, and mound-shoal complex facies, along with inter-mound/shoal subfacies, whereas the trough interior was dominated by slope-to-basin facies. (2) Distinct heterogeneities exist in the development and evolution of the platform margins. The eastern margin, controlled by basement faults, exhibits a steeply dipping, vertically stacked architecture with minimal lateral migration. In contrast, the western margin is characterized by multi-stage, gently sloping geometries with significant lateral migration distances. (3) Lithofacies paleogeographic mapping reveals substantial spatial variations in platform margin distribution between the second (Deng 2) and fourth (Deng 4) members of the Dengying Formation. During the Deng-2 period, the platform margin followed a quasi-circular distribution along the Chengdu-Suining-Zizhong trend. During the Deng-4 period, the platform margins expanded significantly, with the western margin trending NS along the Hongya-Jingyan-Zigong area and the eastern margin extending NS along the YantingSuining-Moxi area. Overall, the Deyang-Anyue rift trough is characterized by a “steep-east vs. gentle-west” and “single-stage-east vs. multi-stage-west” sedimentary facies model.

Presently, large-scale oil and gas fields discovered in deep carbonate rocks are predominantly distributed within porous dolomite reservoirs and fractured-vuggy karst limestone reservoirs. However, recent discoveries in wells Pengshen 12 and Nanchong 2 in the Sichuan Basin confirm the presence of porous limestone reservoirs in the 2nd member of the deep Maokou Formation (also referred to as the Mao 2 Member). This finding challenges the traditional geological understanding of reservoirs, establishing the porous limestone reservoirs as a hot research topic. In this study, geological insights are gained using core and thin section observations, geochemical analysis of reservoirs, and reservoir identification and tracking through well-tied seismic interpretation. The results indicate that porous bioclastic limestone reservoirs are present in the Mao 2 Member. The dominant storage spaces include intergranular pores, biological cavity pores, moldic pores, and dissolved pores, which are formed in sedimentary and supergene environments. Specifically, the reservoir encountered in drilling well Pengshen 12 exhibits a cumulative thickness of 24.80 m, an average measured porosity of 5.1%, and an average measured permeability of 0.05 × 10^-3 μm². Early hydrocarbon charging and the presence of anomalous overpressure (overpressured compartments) are identified as key factors contributing to the deep preservation of pores formed during the sedimentary and early supergene stages. The intermittently distributed porous bioclastic limestones are wrapped by relatively tight micritic limestones, resulting in the formation of local anomalous overpressure within bioclastic shoals under the influence of ultra-high temperatures at great depths. Reservoirs in the Mao 2 Member are sandwiched by the floor of tight micritic limestones of the Mao 1 Member and the roof of tight mudstones interbedded with limestones from the Wujiaping Formation. This configuration facilitates the formation of regional anomalous overpressure in the Mao 2 Member. The mechanisms underlying pore formation and preservation suggest that the large-scale porous limestone reservoirs in the Mao 2 Member are developed under a combination of favorable conditions: intermittently distributed porous bioclastic shoals, early hydrocarbon charging, local anomalous overpressure within individual shoals, and regional anomalous overpressure. These reservoirs exhibit a laterally intermittent distribution. Based on the identification of bioclastic shoals, the roof and floor of the member, it is predicted through well-tied seismic interpretation that favorable bioclastic limestone shoal reservoirs cover an area of 9.5 × 10⁴ km². These insights provide a theoretical foundation for the occurrence of porous limestone reservoirs in deep parts, expanding the exploration targets of deep limestone reservoirs in the Sichuan Basin. Additionally, this study offers a valuable reference for the exploration of deep limestone reservoirs in other basins.

To investigate the exploration potential of deep coalbed methane (CBM) in the Longtan Formation, Sichuan Basin, we conduct coring of coal seams in risk exploratory well NT1H. Through experiments and tests on the coal quality and the physical properties, adsorption, and gas-bearing properties of coal reservoirs in the Longtan Formation, we examine the geological characteristics and exploration potential of deep CBM in the formation. The results indicate that the deep coal seams in the Longtan Formation exhibit coals with intact structure and well-developed cleats, vitrinite reflectance (R_o) ranging from 2.70 % to 3.13 %, average vitrinite, inertinite, and inorganic component contents of 60.6 %, 23.4 %, and 16.0 %, respectively, and minerals dominated by clay. These coal seams feature high total porosity, with a porosity varying from 4.83 % to 10.13 % and effective porosity from 2.82 % to 9.66 % (average: 6.53 %). Their pores, with strong structural heterogeneity, are dominated by micropores with sizes less than 2 nm, followed by macropores or fractures with sizes greater than 10 μm, while pores with sizes ranging from 2 to 50 nm are relatively underdeveloped. The coals in the Longtan Formation display Langmuir volume generally exceeding 25 m³/t and average Langmuir pressure of 3.24 MPa at 120 ℃ . Pressure coring reveals that the coal seams manifest gas content varying from 26.52 to 31.24 m³/t, gas saturation from 123 % to 146 %, free gas content from approximately 5 to 10 m³/t, and low in-situ water saturation from 18.2 % to 48.6 %. Their total gas content increases with burial depth. In all, the comprehensive investigation suggests that the deep coal reservoirs in the Longtan Formation exhibit conditions favorable for gas-bearing properties. Therefore, the deep reservoirs with supersaturated CBM in the Upper Permian Longtan Formation in the Sichuan Basin have great exploration potential. For these reservoirs, it is recommended to target closely spaced coal seams for integrated volume fracturing and three-dimentional reconstruction for CBM recovery.