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Ever since a breakthrough of marine shales in China, lacustrine shales have been attracting by the policy makers and scientists. Organic-rich shales of the Middle Jurassic strata are widely distributed in the Yuqia Coalfield of northern Qaidam Basin. In this paper, a total of 42 shale samples with a burial depth ranging from 475.5 m to 658.5 m were collected from the Shimengou Formation in the YQ-1 shale gas borehole of the study area, including 16 samples from the Lower Member and 26 samples from the Upper Member. Geochemistry, reservoir characteristics and hydrocarbon generation potential of the lacustrine shales in YQ-1 well were preliminarily investigated using the experiments of vitrinite reflectance measurement, maceral identification, mineralogical composition, carbon stable isotope, low-temperature nitrogen adsorption, methane isothermal adsorption and rock eval pyrolysis. The results show that the Shimengou shales have rich organic carbon (averaged 3.83%), which belong to a low thermal maturity stage with a mean vitrinite reflectance ( R o ) of 0.49% and an average pyrolytic temperature of the generated maximum remaining hydrocarbon ( T max ) of 432.8 °C. Relative to marine shales, the lacustrine shales show low brittleness index (averaged 34.9) but high clay contents (averaged 55.1%), high total porosities (averaged 13.71%) and great Langmuir volumes (averaged 4.73 cm −3 g). Unlike the marine and marine-transitional shales, the quartz contents and brittleness index (BI) values of the lacustrine shales first decrease then increase with the rising TOC contents. The kerogens from the Upper Member shales are dominant by the oil-prone types, whereas the kerogens from the Lower Member shales by the gas-prone types. The sedimentary environment of the shales influences the TOC contents, thus has a close connection with the hydrocarbon potential, mineralogical composition, kerogen types and pore structure. Additionally, in terms of the hydrocarbon generation potential, the Upper Member shales are regarded as very good and excellent rocks whereas the Lower Member shales mainly as poor and fair rocks. In overall, the shales in the top of the Upper Member can be explored for shale oil due to the higher free hydrocarbon amount ( S 1 ), whereas the shales in the Lower Member and the Upper Member, with the depths greater than 1000 m, can be suggested to explore shale gas. [ABSTRACT FROM AUTHOR]

Adsorbed gas is prevalent in shale, especially in plays in China. Quantitatively predicting the adsorbed gas content is significant for resource assessment and selection of favorable areas for shale-gas production. This paper focuses on methods to optimize dominant factors and establish a prediction model for shale gas adsorption capacity, considering the Palaeogene Lower Ganchaigou lacustrine shale in the western Qaidam Basin in China as an example. Using total organic carbon (TOC), Rock-Eval, X-ray diffraction, N 2 adsorption measurement, and methane isothermal adsorption assessments, the basic characteristics of a shale reservoir were analyzed. Through the obtained data, the relations among shale gas adsorption capacity and TOC, maturity, mineral composition, temperature, and pressure were determined. Gas adsorption capacity has an evident relationship with TOC and quartz content. However, no apparent correlation was observed in the presence of clay minerals, indicating that clay minerals do not contribute significantly to gas adsorption capacity in the shale samples studied. Gas adsorption capacity also varied with temperature and pressure. Based on these variations, several principles were established for the selection of dominant factors that are easy to obtain, have evident relationships with adsorption capacity, are supported by sufficient data, and accurately reflect regularity. TOC, quartz content, temperature, and pressure were eventually optimized through multiple regression analysis to establish a prediction model. The predicted results were compared with actual measured results, yielding a correlation coefficient as high as 0.9192. Although the geological conditions of the Lower Ganchaigou shale do not completely meet the standards expected from a typical case, the obtained function and research process can be applied to any play to illustrate the impacts of pressure and temperature regimes on the gas adsorption capacity of shales with varying abundances of organic matter and matrix mineral compositions. [ABSTRACT FROM AUTHOR]