Modeling of the tight sandstone gas accumulation for the Xujiahe Formation, Hechuan-Tongnan Area, Sichuan Basin

摘要

The tight sandstone gas accumulation is mainly driven by overpressure arising from the gas generation in source rocks. This paper proposes a quantitative computation model for the pressurization from gas generation in source rocks, establishes a dynamic equilibrium equation and a modeling process for tight sandstone gas accumulation, and simulates the gas generation quantity and source rock overpressure history in the Hechuan-Tongnan area of the Sichuan Basin. The modeling reveals that: (1) The gas generation intensity determines the degree of pressurization from gas generation, whereas the reservoir property determines the distribution of gas reservoirs; the conventional gas reservoir is complementary with the tight sandstone gas reservoir in terms of horizontal and vertical distribution, the former is mainly distributed at the top of reservoir with porosity over 7% on local structural highs while the latter is mainly distributed at the bottom of reservoir on local structural lows near source rocks of large gas generation quantity. (2) During the uplifting process from the maximum burial depth (65 Ma) to the current burial depth, because of the pressure drop of formation water and the increase of gas-water pressure difference, the tight sandstone gas migrates farther and increases in accumulation quantity; however, the conventional gas does not migrate basically and has no increase in accumulation quantity; (3) The quantity of the tight sandstone gas in the T3x2is 5.9 times that of the conventional gas, and the undiscovered resource is mainly tight sandstone gas distributed in northeast of Hechuan, and north and southeast of Tongnan.