Lithofacies characteristics and reservoir differences of marine-continental transitional shale in the eastern margin of the Ordos Basin

Marine-continental transitional facies sedimentary rock system is developed in the Carboniferous–Permian along the eastern margin of the Ordos Basin, where exploration breakthroughs and commercial development of various natural gases such as coalbed methane (CBM) and tight gas are realized, so it is an important replacement field of unconventional natural gas. To characterize the differences and distribution patterns of marine–continental transitional shale lithofacies and guide the next oil and gas exploration and development, this paper comprehensively classifies the shale lithofacies types of Carboniferous Benxi Formation and Permian Taiyuan and Shanxi Formations based on field outcrops, cores and whole rock minerals. And combined with gas bearing property, mechanical property, porosity and permeability test data, the controlling effects of different types of lithofacies on reservoir development characteristics are clarified. The following research results are obtained. First, the marine–continental transitional shale is mainly classified into clay, siliceous, and mixed types. The tidal flat facies is dominated by laminated low organic matter-mixed shale, the lagoon facies is dominated by laminated high organic matter-clay rich siliceous shale, and the delta facies is dominated by massive low organic matter-silica rich argillaceous shale. Second, The marine-continental transitional shale has good development potential, which is characterized by medium organic carbon content (averaging about 2.26%), low porosity (averaging about 1.41%), low permeability (about 0.12 mD), low gas content (about 1.23 m3/t), low Poisson's ratio (about 0.2) and high Young's modulus (about 22 GPa). Third, the comprehensive analysis of gas bearing property and physical property shows that the dominant lithofacies mainly include high organic matter-silica rich argillaceous shale and medium organic matter-silica rich argillaceous shale, which are more developed in tidal flat and lagoon facies. In conclusion, marine–continental transitional facies has diverse sedimentary environments, and its lithofacies types are different greatly, so it is necessary to adhere to the exploration and development idea of "dominant environment-dominant intervals-dominant lithofacies" to search for the zones with superimposed geological and engineering sweet spots in exploration deployment. This understanding provides a technical support for the large-scale benefit development of transitional shale gas and multi-layer coal measure gas, and is conducive to the scientific development of unconventional transitional shale gas. © 2023 Natural Gas Industry Journal Agency. All rights reserved.