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The drilling data and test results of a large number of shale samples collected from Upper Triassic Xujiahe Formation, in the Sichuan Basin were used to find out the distribution characteristics, geochemical characteristics, reservoir characteristics and gas-bearing property of the continental shale in Xujiahe Formation, to figure out its resource prospects. The shale reservoir of the Xujiahe Formation was high in organic matter content (with a TOC content of 0.06% to 14.8%, 2.36% on average), with an organic matter of type III kerogen, Ro% of 0.8% to 3.0% (1.62% on average). In the high maturity stage of large quantity gas generation, it is rich in brittle mineral (>40%), and high in clay mineral content (average of 49.5%) which is mainly composed of mixed illite/smectite layer and illite. The shale is 1%–3% in porosity (2.73% on average). The pores are mainly nanoscale matrix ones (2 to 50 nm in size), and organic pores in local areas. The gas content of Xujiahe shale was measured at 3.01 m3/t, on average. This study suggests that Xujiahe shale has some gas resource potential; but, there are still a number of challenges and difficulties which need to be resolved in its exploration and development in the future.

In order to better decipher the paleoenvironment of the Early Silurian black shale in the southern Sichuan Basin and constrain their shale gas potential, organic carbon and stable C-O isotopic studies have been performed on the Longmaxi Formation from Well N203. The organic carbon isotopic studies show a relative low δ13Corg value at the lower part of the Longmaxi Formation, corresponding to the negative stable carbon isotope (δ13C) excursion and positive oxygen isotope (δ18O) excursion. These isotopic features indicate an anoxic environment at the lower part of the Longmaxi Formation, which might be related to the high atmosphere temperature and sea-level rise. The anoxic environment is also proved by framboidal pyrites studies, which present diameter of most framboids in the range of 3–5 μm. The anoxic environment promoted the deposition of organic carbon, leading to the high TOC content at the lower part. This anoxic environment only lasted for a short period, with a gradually evolution to oxic environment at the middle and upper part of the Longmaxi Formation, evidenced by very low TOC content, increasing δ13Corg values, positive δ13C values and negative δ18O values. Our studies have further constrained the best source of rocks for shale gas at the lower part of the Longmaxi Formation.

China extracted shale gas of 23, 500∼24, 600 m3/d from the Lower Cambrian Qiongzhusi Shale Formation in Weiyuan gas field of Sichuan Basin in 1966. This shale interval, however, was considered less prospective because the Sinian Doushantuo Formation dolomite reservoir was defined as the target layer for drilling at that time. Forty years later, this black shale interval became attractive as triggered by the “Shale Gas Evolution” in North America. In 2005, by learning from shale gas mode of the United States, PetroChina Research Institute of Petroleum Exploration and Development (RIPED) reviewed the existing well data of the region where the Weiyuan and Longchang gas fields and the Changning structure were located within the Southern Sichuan Basin. In order to re-examine the gas shows and geochemical property and to evaluate the gas potential of shale strata within the Upper Ordovician Wufeng Formation-Lower Silurian Longmaxi Formation and the Lower Cambrian Qiongzhusi Formation. On that basis, for the entire Sichuan Basin and adjoining regions, PetroChina has carried out a geologic survey on organic-rich shales, drilled shallow shale gas appraisal wells, evaluated the shale gas resource potential, and defined some favorable targets for shale gas exploration and development by 2008. W201, the first shale gas appraisal well of China, drilled by PetroChina in 2009, initially produced 17, 000 m3/day from the Wufeng-Longmaxi Formation shale and 10, 800 m3/day from the Qiongzhusi Formation shale, marking the commencement of shale gas exploration and development activities in China. Thus far, China has received shale gas flow from marine, transitional-limnetic coaly and lacustrine shales in many regions, such as Sichuan, Chongqing, Guizhou, etc. The commercialization of marine shale gas has been achieved in Sichuan and Chongqing totaling 0.2 billion cubic meters of shale gas production in 2013. Through an overall review on the progress of shale gas exploration and development in China, we concluded that the geological conditions for shale gas in China are extremely complex. First, the most sedimentary basins in China belong to superimposed basins, all of them have experienced the depositional evolution from marine to terrestrial facies, and three types of shales were well developed including marine, transitional, and terrestrial organic-rich shales. Second, the conditions of shale gas formation and accumulation in China are favorable and shale gas resource has a great potential for further development in China. Third, like North America, the marine organic-rich shales of China have the best and most realistic shale gas exploration and development prospects. But it faces technical, environmental, regulatory, infrastructural and other challenges that are likely to delay rapid shale gas development. Fourth, until now, although the exploration and exploitation of shale gas in China are relatively low, very good signs have been emerged to prove that China has good prospects in the future.

Well head gas samples from nine shale gas wells in Xiasiwan area were collected to examine their geochemical characteristics through analyzing their composition, carbon and hydrogen isotopes, and light hydrocarbon. The study shows the shale gas from Yanchang Formation is wet gas from primary cracking of sapropelic kerogen, and the shale system, still a closed system, has expelled only small amount of gas, and thermal maturity is the paramount factor restricting shale gas exploration; the shale gas from Shanxi Formation is dry coal-derived gas from secondary cracking, characterized by isotope reversal and δ13C2 of less than −29‰, which proves that the ethane's carbon isotope of coal-derived gas can be very low in high or over mature stage. A similar isotope “rollover” phenomenon in shale gas has also been observed in conventional gas of the Ordos Basin, so it is concluded that the gas in Lower Paleozoic is coal-derived gas and the 13C light ethane is the result of secondary cracking instead of mixing with oil-associated gas. Analysis of some indexes (such as δ13C1, δ13C2–δ13C1 and light hydrocarbon) shows that the recoverable resources of shale gas in the mature stage are overestimated.

Rock brittleness is now one of the significant parameters for geological evaluation of shale gas reservoirs due to the wide application of hydraulic fracturing in shale gas development. Currently, the evaluation of shale brittleness is generally based on mineral composition or elastic parameters, which both derive from the exploration experiences in North America. Due to the easier access of mineral composition data through x-ray diffraction method than rock mechanical data through array sonic logging method, the mineral-based brittleness indices have stronger practicability. However, there is not a unified definition of brittle minerals yet, which now usually refers to quartz or quartz and carbonate minerals. Considering the specific conditions of Wufeng–Longmaxi Formation in Southern Sichuan Basin; depositional environment of calcareous shelf; and higher content of calcite, dolomite, and pyrite, we redefined brittle minerals according to the elastic parameters of each mineral, taking quartz, dolomite, and pyrite which best satisfy the standard of “Young’s modulus>30 GPa and Poisson’s ratio

In recent years, a lot of work on the development of oil shale has been done with regard to the Chinese oil shales. However, the research is mainly with respect to the processing aspects of oil shale. The pyrolysis mechanism of oil shale has been limited, to some extent, by instrumentation and analytical techniques. In this paper, pyrolysis kinetics characteristics of oil shale samples obtained from Mudanjiang were investigated by Rock-Eval analysis. The weight loss data, pyrolysis mechanism, and properties of oil shale were analyzed at different heating rates (10, 15, 20, 25, 30℃/min) in the temperature range from 200 to 600℃. In this paper, a distribution function of apparent activation energy and a relationship equation of apparent energy with frequency factor have been obtained. A special kinetic model was developed, fully suitable for the pyrolysis characteristics of oil shale. This model was also verified by experiment. Throughout the analysis of pyrolysis kinetics, it was observed that the pyrolysis reaction mechanism at different temperatures could be explained. The results may provide the theoretical basis for further effective and economical exploitation of oil shale.