Research status and prospects of marine-continental transitional shale reservoirs

The Carboniferous-Permian transitional organic-rich shales are widely distributed in China, and the development of shale gas in them is of great significance to alleviate the huge energy and environmental pressure.The key properties of representative transitional shales in different regions and layers are summarized from the aspects of sedimentary environments, geochemistry, mineralogy, and petrophysics, and compared with typical marine and continental shales.The results show that for the transitional shales, the swamp facies in the delta system and the lagoon and swamp facies in the barrier-lagoon system have superior conditions for shale gas accumulation.The transitional shales are frequently interbedded with coal and sandstone layers, and the vertical change of lithofacies is extremely complex, implying that they are highly heterogeneous.At the same time, the vertical variation of TOC content is large, and the average TOC content is generally higher than that of marine and continental shales, which means that it has great potential for shale gas exploration and development.The organic matter type of marine-continental transitional shales are mainly Type Ⅲ, supplemented by Type Ⅱ, humic maceral accounts for about 70%.The degree of thermal evolution is generally in the stage of maturity to high maturity, with a few reaching over-maturity, and generally between continental and marine shales.The high clay mineral content of transitional shales poses a great challenge to fracturing development.The reservoir space is dominated by intergranular pores and intragranular pores, and a large number of invisible organic matter pores prevalent in organic matter, but they are not theoretically the dominant pore type in transitional shales due to the type and maturity of the organic matter.High clay content may result in the lowest porosity and low permeability of marine-continental transitional shales.The methane adsorption capacity of transition shales are distributed in a wide range, and there are shales with strong methane adsorption capacity, which may be influenced by coal seams.At present, many important research results have been achieved in the study of marine-continental transitional shale gas reservoirs.However, the imperfection of resource evaluation methods, the immaturity of "sweet spot" recognition and prediction technology, the lack of in-depth study on favorable reservoir formation environment, and the uncertainty of the feasibility of the co-mining of various unconventional natural gas resources are the main problems in the exploration and development of transitional shale gas.