Lithofacies types and assemblage features of continental shale strata and their implications for shale gas exploration: A case study of the Middle and Lower Jurassic strata in the Sichuan Basin

Fine identification and division of lithofacies types of continental shale strata is an important basis for the evaluation of shale gas exploration and development potential. At present, however, there is no consensus on the identification standard and division scheme of shale lithofacies. Taking the continental shale strata of the Middle-Lower Jurassic in the Sichuan Basin as an example, this paper established a lithofacies division method by means of core observation, whole-rock mineral X-ray diffraction analysis, thin section analysis, total organic carbon (TOC) measurement and helium porosity measurement after analyzing whole-rock mineral composition and shale characteristics. Then lithofacies types of shale strata were identified and divided, and characteristics of lithofacies assemblages in different scales were investigated. Finally, their significance for shale gas exploration was discussed. The following research results were obtained. First, 20 shale lithofacies types of 6 categories are totally identified in this continental shale strata using the newly established three-step lithofacies division method (whole-rock mineral composition partition- TOC classification-correction and improvement of mineral texture and sedimentary structure). Among them, medium-high TOC clay shale lithofacies, lamina-thin layer clay shale lithofacies and low-medium TOC silty shale lithofacies are dominant, followed by low-medium TOC shell limy clay shale lithofacies, and TOC bearing and low TOC silty clay shale lithofacies. Second, the average TOC and the average porosity of clay shale lithofacies and shell limestone clay shale lithofacies are higher than those of silty and silty clay shale lithofacies. It is indicated that mineral composition and lithofacies types of shale have a certain impact on gas source and reservoir performance. Third, three types of assemblages are identified in the continental shale strata, including mudstone-limestone assemblage, mudstone-sandstone assemblage and mudstone-limestone-sandstone mixed assemblage, which reflect the sedimentary characteristics of distal region, proximal region and transitional region in the lacustrine environment, respectively; and that the characterization of different lithofacies assemblages is conducive to recognizing the differences between different shale sedimentary environments. Fourth, fine identification and statistic of the number and frequency of limy shell laminae and thin layers in the terrestrial organic-rich shale with high clay mineral content can provide a basis for the fracturability evaluation of gas-rich zones and the optimization of optimum exploration and development intervals. (C) 2020 Sichuan Petroleum Administration. Publishing services by Elsevier B.V. on behalf of KeAi Communication Co. Ltd.