Geological characteristics and reservoir-forming potential of shale gas of transitional facies in Linxing area, eastern margin of Ordos Basin

Shale gas of transitional facies is the important replacing area for increase of shale gas reserves and production. On the basis of the experiment and research data of shale of transitional facies of Linxing area in the eastern margin of Ordos Basin, by selecting the strata series of Shanxi Formation, Taiyuan Formation and Benxi Formation as the research object, from the sedimentary environments, shale distribution, organic geochemical characteristics, mineralogy, physical characteristics and gas-bearing characteristics, etc., the geology characteristics of the shale gas transitional facies in the study area were summarized and dissected systematically to analyze its enrichment and reservoir-forming potential. During the Late Carboniferous to the Early Permian, the Ordos Basin was affected by the active tectonic activities in the region, and the water environment changed frequently, resulting in the formation of multiple sedimentary cycles of littoral shallow sea, delta front and littoral shallow lake assemblages, and many sets of organic-rich shales of marine-continental transition facies were deposited. The study area is located in the north-central area of the western Shanxi flexural belt in the eastern part of the Ordos Basin, where the sedimentary environments are stable and organic-rich transitional shale is widely distributed. The rock characteristics of the shale are mainly interbedded gray-light gray fine-grained sandstone and dark mudstone, and the accumulated vertical thickness of the shale is big(60-180 m). The organic matter types of the organic-rich transitional shale in the study area are Ⅱ₂-Ⅲ type kerogen, and the total organic matter carbon content is high, with TOC 3.07% which is in the mature gas generation stage. The mineral composition of the shale is mainly quartz and clay, and the content of feldspar and carbonate minerals is low. Macroscopically, the main types of pores are inorganic pores and organic pores, and the fractures are not developed. Microscopically, the pores are controlled by clay minerals, and the pore morphology is mostly open slit micropores and mesopores. The shale of the transitional facies in the study area has the physical characteristics of low porosity and low permeability, but has good gas-bearing characteristics, the average gas content is 1.15 m³/t. The comprehensive analysis of the shales' characteristics shows that the shale gas in the study area has great potential, providing a theoretical basis for the exploration and development of shale gas in the later stage of the area.