OCCURRENCE OF UNCONVENTIONAL NATURAL GAS IN COAL-BEARING STRATA IN ORDOS BASIN, CHINA

摘要

The Carboniferous-Permian coal-bearing strata of the Ordos Basin are one of China's major coalbed methane/shale gas and tight sandstone gas reservoirs. Based on the geochemistry of argillaceous shale, coal seam and tight sandstone, this paper systematically analyzes the physical properties of the reservoir, gas content characteristics, gas isotopes, unconventional natural gas and other data to analyze the occurrence characteristics of unconventional natural gas. Studies show that shales rich in coal and organic matter are source rocks of non-traditional natural gas systems, and the organic matter is in a stage of high supersaturation. In terms of reservoir, shale porosity is the lowest and sandstone reservoir has highest permeability, which is an order of magnitude higher than that of mudstones and coal seams. The gas in the mudstone shale occurs mainly in parallel with the formation. In terms of gas content, the average shale gas content is 3.53m3 per ton of formation, sandstone is 5.04m3, and the total gas content per ton in coal seams is 22.39m3. All gas reservoirs in the Taiyuan and Shanxi formations are coal-type gas reservoirs. The isotopic data mainly shows two characteristics: the carbon isotope values of the same type of gas reservoirs are relatively similar while the methane and ethane carbon isotopes of the two gas reservoirs shows a certain difference. In the local scope, different types of gas reservoirs adjacent to each other cannot achieve free gas exchange in the vertical direction.