四川盆地西北部地区中三叠统——古生界压力封存箱与油气成藏

Relationship between pressure compartment and hydrocarbon accumulation in the Middle Triassic–Paleozoic in NW Sichuan Basin

四川盆地西北部地区(以下简称川西北)龙门山断裂带北段存在着常压、高压和超高压等多个压力带,前人在研究该区"压力封存箱"时,通常以目前该盆地超压区的分布为依据划分出"压力封存箱"的范围,但是对于压力封存箱的演化及其与油气成藏关系的研究则鲜见。为此,利用井震联合解释、Easy%R_o热动力学模型、平衡剖面技术等手段,刻画川西北中三叠统—古生界压力封存箱的构成,恢复其形成过程,并研究封存箱内中二叠统栖霞组和中泥盆统观雾山组气藏的形成演化过程。研究结果表明:①川西北中三叠统—古生界存在自源型异常高压封存箱,封存箱范围西至①号隐伏断裂,东至九龙山构造,北至米仓山古隆起南缘山前断褶带构造,南至双鱼石构造南段,该压力封存箱以九龙山构造至双鱼石构造带中线为界分为东、西2个次箱;②该压力封存箱距离龙门山断裂带越远超压程度越高,压力封存箱与外界存在着明显的压力差,箱内压力系数介于1.4～2.0,箱外为正常压力系数;③古生界压力封存箱形成于印支运动早期,经燕山运动和喜马拉雅运动调整形成现今格局;④中三叠统—寒武系油气为喜马拉雅期所形成的箱内成藏,压力封存箱内存在着多套烃源灶,气源充足,顶板和隔板具有较强的封盖性,源储匹配良好,油气优先富集于距离箱顶板较近的优质储层中。结论认为,该区古生界深层海相地层具有较大的油气勘探潜力,其中双鱼石—中坝地区栖霞组为最有利的勘探层系,观雾山组是潜在的有利勘探层系。

There are multiple belts of normal pressure, high pressure and overpressure in the north section of Longmenshan fault zone in the northwestern(NW) Sichuan Basin. The previous studies on the "pressure compartment" in this area usually took the distribution of overpressure belts in the Sichuan Basin as the basis for defining the range of "pressure compartment", but rarely paid attention to the evolution of pressure compartment and its relationship with hydrocarbon accumulation. In this paper, the compositions of the Middle Triassic–Paleozoic pressure compartment in NW Sichuan Basin were described and its formation process was reproduced by means of well–seismic joint interpretation, Easy%R_o thermodynamic model and balanced profile technology. Based on this, the formation and evolution process of the gas reservoirs of Middle Permian Qixia Fm and Middle Devonian Guanwushan Fm in the pressure compartment was studied. And the following research results were obtained. First, there is an authigenic abnormal pressure compartment in the Middle Triassic–Paleozoic in NW Sichuan Basin, which is bounded by No.1 buried fault on the west, Jiulongshan structure on the east, the piedmont fault fold belt along the southern margin of Micangshan paleo-uplift on the north and the southern section of Shuangyushi structure belt on the south. This pressure compartment is divided into two sub-compartments(i.e., east and west compartment) with the central line of Jiulongshan structure through Shuangyushi structure belt as the boundary. Second, the farther this pressure compartment is from Longmenshan fault zone, the higher the overpressure degree is. There is a distinct pressure difference between the pressure compartment and the environment. And the pressure coefficient inside the compartment is between 1.4 and 2.0 and that outside the compartment is normal. Third,the Paleozoic pressure compartment was formed in the early stage of Indosinian movement and its current pattern emerged after being adjusted by Yanshan movement and Himalayan movement. Fourth, oil and gas reservoirs of Middle Triassic–Cambrian are the reservoirs which were formed inside the pressure compartment in the Himalayan stage. There are multiple sets of source kitchens inside the pressure compartment, which provide sufficient gas sources, with the roof and the barriers having strong sealing capacity, and the source–reservoir matching relationship is good. And hydrocarbon preferentially accumulates in the high-quality reservoir which is closer to the roof of the pressure compartment. In conclusion, the Paleozoic deep-seated marine reservoirs in this area have extensive oil and gas exploration potential. And among them, Qixia Fm in Shuangyushi–Zhongba area is the most favorable exploration series of strata, and Guanwushan Fm is the potential favorable exploration series of strata.