Geochemical characteristics of tight sandstone gas and hydrocarbon charging history of Linxing area in Ordos Basin, China

摘要

Genetic type, source, charging history and reasons for secondary biogenic transformation of tight sandstone gas in the Linxing area in China's Ordos Basin have been studied through a combined analysis of gas compositions, isotopes of alkane and carbon dioxide, fluid inclusions and basin simulation. Tight sandstone gas in this area is predominated by methane, and the dryness coefficient is between 0.864 and 0.997 (average = 0.969). The delta C-13(1) values of methane and ethane in this area range from -44.5 parts per thousand to -33.2 parts per thousand (average = -39.4 parts per thousand) and from -27.5 parts per thousand to -24.4 parts per thousand (average = -25.9 parts per thousand), respectively. The non-hydrocarbon gases are nitrogen, followed by carbon dioxide and argon, and delta C-13 of carbon dioxide ranges from -14.6 parts per thousand to 8.6 parts per thousand. The homogenization temperatures of gas and gas-liquid inclusions in P(2)s and C(3)t in Linxing range from 80 degrees C to 170 degrees C, which are concentrated at 120-130 degrees C. The identification of gas origin indicates that tight sandstone gas is coal-type gas, which is mainly due to thermal evolution, followed by the mixing of secondary biogenic gas. Geological conditions and gas geochemical characteristics suggest that tight sandstone gas was produced by coal seam and carbonaceous shale in Upper Paleozoic in the Linxing area. The homogenization temperature of inclusion reflects the continuous accumulation process of tight sandstone gas, and the Cretaceous was the main period for gas charging. The monoclinal structure, maturity of source rocks and activity of underground water in this area are the important factors for secondary biogenic gas formation.