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A shale condensate gas reservoir with a high clay content and a high formation pressure was found in the Jurassic shale of the Dongyuemiao Member in the Fuxing area of the eastern Sichuan Basin. Reservoir characteristics and formation pressure have a significant influence on optimal development. The present study investigated the continental shale of the Dongyuemiao Member in Well F. The petrological properties, physical properties, and pore structure of the Dongyuemiao Member were studied using X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), N2 adsorption, and mercury intrusion porosimetry (MIP). The permeability stress sensitivity characteristics of the shale reservoirs are discussed based on the change in shale porosity and permeability under overburden pressure. The tested shale samples yielded total organic carbon (TOC) and S1 + S2 values ranging mainly from 1.0 wt.% to 1.5 wt.% and from 0.39 to 2.28 mg/g, respectively, which was in the high maturity stage of the thermal evolution of organic matter (OM). The shales of the Dongyuemiao Member were found to contain high average clay mineral contents (more than 50%) of calcite and quartz, as well as albite, pyrite, dolomite, and halite. The main developments were identified as silica-rich argillaceous shale lithofacies, argillaceous shale lithofacies, and mixed argillaceous shale lithofacies. The pores were found to mainly be plate-like and flake-like interlayer pores of clay minerals and OM pores with various shapes. The pore size was mainly concentrated below 110 nm, and the pore volume increment increased in flakes with pore diameter. The average porosity and permeability of shale were found to be 4.827% and 0.243 mD, respectively. Clay minerals and quartz are beneficial for improving the porosity and permeability of reservoirs, while carbonate minerals have the opposite effect. The permeability of the shale showed a negative exponential change with increasing effective stress under overburden pressure. When the effective confining pressure was greater than 20 MPa, the decline rate of the shale permeability decreased with increases in the effective stress. The higher the clay mineral and TOC content, the stronger the stress sensitivity of shale permeability. The higher the carbonate mineral content, the weaker the stress sensitivity of shale permeability. The porosity sensitivity exponent indicates that matrix pores and micro-fractures are both developed in the Dongyuemiao Member, and the development of internal fractures is the main factor in the strong stress sensitivity of the shale permeability in the study area.

Ramp-type bioherm is geologically characterized by lateral migration, small size and discrete distribution. The fine bioherm description is of high difficulty and the benefit development of bioherm gas reservoirs can be hardly realized. To solve these difficulties, this paper put forward a new bioherm identification method based on well-seismic combination after cognizing the geological characteristics of ramp-type bioherms deeply. And this new method plays a successful role in guiding the drilling of Well Y012-X16 in the mode of “two bioherms in one well” in the Damaoping Block of eastern Sichuan Basin. High-yield industrial gas flow of 113.65 × 104 m3/d is obtained during the test and great new progress is achieved in the exploration of bioherm gas reservoirs. And the following research results were obtained. First, the degree of slope at the marginal platform slope of ramp-type bioherm is only 2°–4°. The migration and evolution laws of bioherms are mainly controlled by the change of paleogeomorphology and sea level. As a result, small bioherms of multiple rows and multiple periods are formed and the gas-water contact of ramp-type bioherm gas reservoir is not united, presenting the characteristics of “one bioherm, one reservoir”. The gas bearing property is good at the top of the bioherm. The overall gas bearing property is poorer in the early bioherm of the second member of Upper Permian Changxing Formation and better in the late bioherm of the third member of Changxing Formation. Second, a new bioherm identification method with “double-high” integrated seismic processing technology, data driven “box-shaped volume” perspective technology, remaining thickness method based sedimentary paleogeomorphology restoration technology and three-dimensional visualization technology as the core is developed innovatively, which provides the fine description of ramp-type small bioherms in the three dimensional space. Third, based on the application of the new bioherm identification method, the distribution detail of the bioherms in the Damaoping Block is clarified, the development mode of “two bioherms in one well” for long horizontal sections is established and a PetroChina's new record of bioherm reservoir drilling length is set up in Well Y012-X16, which launches a new means to the benefit development of small bioherm group. Fourth, it is predicted that a number of bioherm development areas can be newly increased in the Damaoping Block, which can provide the supplement productivity of 100 × 104 m3/d for the development of gas reservoirs, so their natural gas exploration and development potential is huge. In conclusion, the fine description technologies for ramp-type small bioherms are conducive to the identification of ramp-type small bioherm group and the efficient exploration and development of bioherm gas reservoirs in the Sichuan Basin.

There exists the Wusheng-Shizhu and Chongqing-Qijiang intra-platform depressions in the eastern Sichuan Basin during the Late Permian to the Early Triassic. The clear understanding of the sedimentary pattern and evolution of reef-shoal around the depression-margin has a practical significance for guiding the exploration and development of the Permian–Triassic reef and shoal gas fields. Based on observation, the systematic sampling and laboratory analysis of typical outcrops of the Changxing Formation (i.e. Juandong, Juhuaba, Lengshuixi, and Taiyuan) are located in the eastern Sichuan Basin. Combined with drilling data and previous studies, the study area is dominated by a ramp sedimentary environment, except for the Kaijiang-Liangping Trough in the Early Changxingian. The main sediments are micrite-containing chert nodules and micrite. With the regression that took place during the Middle-Late Changxingian, there is a distinct sedimentary differentiation that began to appear. There is a “two depressions–one platform–one trough” sedimentary pattern from south to north in the study area in the Changxingian. In essence, there exists the Chongqing-Qijiang and the Wusheng-Shizhu intra-platform depressions (two depressions), platform-flat (one platform) between the Wusheng-Shizhu intra-platform depression and the Kaijiang-Liangping Trough, and the Kaijiang-Liangping Trough (one trough). The reef-shoal of the northern depression-margin distributes between the 2nd Member and lower part of the 3rd Member. By comparison, the reef-shoal of the southern depression-margin distributes in the upper part of the 3rd Member; this indicates that the deposition of reef-shoal shows “earlier in the north and later in the south.” It is considered that the sedimentary palaeogeomorphology of the Changxingian is characterized by “higher in the north-lower in the south.” The clear understanding of the sedimentary pattern and evolution of reef-shoal around depression-margin reveal a new field for exploration and development of the Permian Changxing reef-shoal gas fields in the eastern Sichuan Basin.

The exploration of shale gas in Fuling area achieved great success, but the reservoir characteristics and gas content of the lower Jurassic lacustrine in the northern Fuling areas remain unknown. We conducted organic geochemical analyses, Field Emission Scanning Electron Microscope (FE-SEM), X-ray diffraction (XRD) analysis, high-pressure mercury intrusion (MIP) and CH4adsorption experimental methods, as well as NMR logging, to study mineral composition, geochemical, pore structure characteristics of organic-rich shales and their effects on the methane adsorption capacity. The Da’anzhai shale member is generally a set of relatively thick (avg. 75 m) and high carbonate-content (avg. 56.89%) lacustrine sediments with moderate total organic carbon (TOC) (avg. 1.12%) and thermal maturation (Vitrinite reflectance (VR): avg. 1.19%). Five types of lithofacies can be classified: marl (ML), calcareous shale (CS), argillaceous shale (AS), muddy siltstone (MS), and silty shale (SS). CS has good reservoir quality with a high porosity (avg. 4.72%). The small pores with the transverse relaxation time of 0.6–1 ms and 1–3 ms comprised the major part of the porosity in the most lithofacies from Nuclear magnetic resonance (NMR) data, while the large pore (>300 ms) accounts for a small porosity proportion in the CS. The pores mainly constitute of mesopores (avg. 23.2 nm). The clay minerals with a large number of interparticle pores in the SEM contributes most to surface area in the shale lithofacies with a moderate TOC. The adsorption potential of shale samples is huge with an average adsorption capacity of 4.38 mL/g and also has high gas content (avg. 1.04 m3/t). The adsorption capacity of shale samples increases when TOC increases and temperature decreases. Considered reservoir properties and gas properties, CS with the laminated structures in the medium-upper section of the Da’anzhai member is the most advantage lithofacies for shale gas exploitation.