川西地区上三叠统异常流体压力的主要形成机制

川西地区上三叠统异常压力发育。立足川西地区现今储层内实测地层压力的分布特点,综合分析了欠压实作用、生烃作用和构造挤压作用在异常压力形成中的作用。研究认为,川西地区上三叠统的沉积作用和岩性组合有助于形成欠压实作用。来自综合压实曲线的证据表明,上三叠统内欠压实作用仍然存在,并自侏罗纪末开始引起了较大幅度的异常压力。因须家河组渗透性较差,生烃作用特别是天然气的大量生成极有可能导致地层压力的快速增长,但具体幅度取决于欠压实增压的持续时间和流体体系的封闭程度等因素。在构造应力充分释放区,构造挤压作用难以引起流体增压;而在应力缓慢释放区,处于较为封闭体系内的流体承担着大部分上覆负荷,构造挤压引起的流体增压所起的贡献占流体异常压力实测值的80%～100%。构造挤压增压的影响主要发生于喜马拉雅构造运动期间。

Analysis on main formation mechanisms of abnormal fluid pressure in the Upper Triassic, West Sichuan area

The overpressured gas reservoirs are common in the Upper Triassic strata, Western Sichuan basin. On the basis of the distribution of measured reservoir pressures in the Western Sichuan basin, the effects of uncompaction, hydrocarbon generation, and tectonic compression on the formation of abnormal pressure are analyzed. The analysis indicates that the Upper Triassic sedimentation and lithologic association are conductive to the formarion of compaction. The evidence from compaction curves also indicates that the undercompaction since the end of Jurassic has caused a highly abnormal pressure and its effect still exists in the Upper Triassic strata. Owing to the low permeability in the Xujiahe Formation, the rapid buildup of reservoir pressure is most likely interpreted to be caused by hydrocarbon generation, especially the generation of larger amount of gas. The actual pressure increment depends on such factors as the duration of pressure increase caused by undercompaction and the closing degree of fluid system. In the areas with rapid tectonic stress relief, the tectonic compression is unlikely to induce fluid pressure buildup. In the areas with slow tectonic stress relief,the fluid within the closed system bears most of overburden load;the abnormal pressure caused by tectonic compression accounts for 80 - 100 % of measured pressure data. The pressure increase by tectonic compression occurred mainly during the period of Himalaya orogeny.