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

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 T3x2 is 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. 摘要: ：烃源岩生气产生的超压是致密砂岩气藏成藏的主要驱动力，提出烃源层生气增压定量计算模型，建立致密砂岩气藏动力平衡方程，制定致密砂岩气成藏模拟流程，并模拟了四川盆地合川—潼南地区须一段生气量及其引起的烃源岩超压史。模拟结果证实：①烃源层生气强度决定生气增压的大小，储集层物性决定气藏的分布，常规气藏与致密砂岩气藏在平面和纵向上的分布均具有互补的特点，前者主要分布于孔隙度大于7%的局部构造高点；后者主要分布于孔隙度较低的构造相对低部位且靠近生气量大的烃源层；②从最大埋深（距今65 Ma）到现今的抬升过程中，由于地层水压力降低，气水压差增大，致密砂岩气运移更远，聚集量增大，但常规气基本不发生运移，聚集量也未增大；③须二段致密砂岩气量是常规气量的5.9倍，未发现资源主要为致密砂岩气，分布在合川的东北部、潼南北部和潼南的东南部等地区。图12表2参37 Key words: tight sandstone gas, accumulation modeling, formation pressure, continuous gas accumulation, resource assessment