四川盆地震旦系灯影组天然气的差异聚集分布及其主控因素

为了揭示四川盆地震旦系灯影组油气藏之间的成生联系和古今气藏的转换机理,采用地质、地球物理与地球化学研究方法相互支撑,宏观、中观与微观研究结果相互印证的思路,利用多年来的钻探资料对该盆地灯影组天然气的差异聚集分布及其主控因素进行了研究。结果表明:①距今100 Ma以前灯影组古油藏的原油原位裂解形成早期超压古气藏,而在距今100~20 Ma期间,四川盆地持续隆升,特别是构造高点的迁移和威远构造的初步形成,使得早期超压古气藏调整形成晚期超压古气藏;②随着距今20 Ma以来隆升幅度的加大,威远构造上覆地层被快速剥蚀,盖层封盖能力大大降低,保存条件变差,致使灯影组天然气从威远构造顶部地表下三叠统嘉陵江组天窗开始泄漏和逸散,启动了四川盆地华蓥山以西地区灯影组天然气的差异聚集和逸散过程,位于资阳、金石和龙女寺—磨溪—安平店—高石梯等气藏构造圈闭溢出点以外的天然气沿灯影组顶部不整合面向威远构造运移聚集,并通过其顶部的嘉陵江组天窗向地表逸散,气藏压力系数由异常高压向正常压力过渡。四川盆地震旦系灯影组天然气的差异聚集分布模式与经典油气差异聚集理论既有相同点又有差异性,对该区天然气的勘探有着重要的启示和指导作用。

Differential accumulation and distribution of natural gas and their main controlling factors in the Upper Sinian Dengying Fm,Sichuan Basin

This paper aims to disclose the genesis-pooling relationship between the hydrocarbon reservoirs and the mechanism of transformation between ancient and modern gas reservoirs in the Sinian Dengying Fm in the Sichuan Basin. Therefore, based on the drilling data and the integrated study of geology, geophysics and geochemistry, we discussed the differential accumulation and distribution of natural gas and their main controlling factors in this study area. The results demonstrated that the crude oil in the paleo-oil pools of the Dengying Fm cracked into gas to form the early paleo-gas pools with overpressure 100 Ma ago. During the period of 100 Ma - 20 Ma, the Sichuan Basin uplifted and the Weiyuan anticline formed primarily, which made the structural high migrate and form the late paleo-gas pools with overpressure. Those processes were followed by a significant uplift, stronger deformation and cooling episode during the last 20 Ma. It resulted in a substantial erosion of the strata in the surface and a decrease of the sealing ability in the caprocks and overburden rocks of the the Dengying Fm. Therefore, the natural gas in the Dengying Fm leaked off from the eroded window of the Lower Triassic Jialingjiang Fm located on the top of the Weiyuan anticline. It started a process of differential accumulation and dissipation of the natural gas in the Dengying Fm across the Sichuan Basin. The gas below the spillpoint of the structural gas pools migrated to the Weiyuan anticline along the unconformity of the Dengying Fm from the Ziyang, Jinshi and Longnüsi - Moxi - Anpingdian - Gaoshiti gas pools. The gas escaped through the eroded window of the Jialingjiang Fm on the top of the Weiyuan anticline, resulting in a decrease of pressure to a normal pressure in the Dengying Fm. In conclusion, such a differential accumulation mode in the Dengying Fm in the Sichuan Basin shares some similarities and differences with the classic hydrocarbon differential entrapment theory, and will play a great inspiration and guiding role in the further gas exploration in this study area.