Abstract
In the Benxi Formation of the Carboniferous system of the Upper Paleozoic in the Ordos Basin, there are many sets of coal measures dark organic-rich shale, being marine continental transitional facies, with significant unconventional natural gas potential. Previous studies are only limited to the evaluation of tight sandstone reservoir in this set of strata, with no sufficient study on gas bearing and geological characteristics of organic-rich shale, restricting the exploration and evaluation of shale gas resources. In this study, analysis has been conducted on the organic carbon content, the major elements, the trace elements, and the mineral composition of core samples from the Benxi Formation in key drilling sections. In addition, qualitative and quantitative pore observation and characterization of core samples have been conducted. The sedimentary environments and reservoir characteristics of the shale of the Benxi Formation have been analyzed. Combined with the gas content analyzing the results of the field coring samples, the shale gas resource potentials of the Benxi Formation have been studied, and the geological characteristics of the Benxi Formation shale gas in the eastern Ordos Basin have been made clear, to provide a theoretical basis for shale gas resource evaluation of the Benxi Formation in the Ordos Basin. The results show that (1) in the Hutian Member, Pangou Member, and Jinci Member of the Benxi Formation, organic-rich shale is well developed, with the characteristics of seawater input as a whole. There is a slight difference in sedimentary redox index, which shows that the reducibility increases gradually from bottom to top. (2) There is an evident difference in the mineral characteristics of shale in these three members. The Hutian Member is rich in clay minerals, while the Jinci Member is high in quartz minerals. (3) The pores are mainly inorganic mineral intergranular pores, clay interlayer fractures, and micro fractures, and organic matter pores are developed on the surface of local organic matter. (4) The mud shale in the Jinci Member has a large cumulative thickness, has relatively high gas-bearing property, and is rich in brittle minerals. The Jinci Member is a favorable section for shale gas exploration of the Benxi Formation.
Highlights
Petroleum and natural gas are a national strategic resource
Three types of organic-rich shale are widely developed in China, including marine shale dominated by the Early Paleozoic, marine-continental transitional shale dominated by the Carboniferous-Permian, and continental shale dominated by the Mesozoic and the Cenozoic [1,2]
The aluminous mudstone in Hutian Member is low in organic matter abundance, and The organic carbon content (TOC) ranges from 0.15% to 0.59%, with an average of 0.4%
Summary
Petroleum and natural gas are a national strategic resource. Shale (mudrock) is a kind of traditional source rock, and a petroliferous system can be developed in shale by self-generation, self-storage, and self-sealing, with huge exploration potential. The marine-continental transitional facies shale in China covers a hugely large area, with a resource potential of about 19.8 × 1012 m3, showing great prospects for exploration and development [1,10]. Commercial gas flow has been obtained from the Permian Longtan Formation in the Eastern Sichuan Basin and the Permian Shanxi Formation in the Ordos Basin in the key horizontal well, showing the exploration and development potential of marine-continental transitional facies shale gas [10]. Because the geological conditions of marine-continental transitional shale gas accumulation not been made clear, and the geological theory of shale gas enrichment has not yet been established, the exploration and development of marine-continental transitional shale gas been restricted to a certain extent.
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