东海盆地西湖凹陷中西部低渗近致密—致密砂岩气成因、来源及运聚成藏

Genesis,Origin and Migration-Accumulation of Low-Permeable and Nearly Tight-Tight Sandstone Gas in the Central Western Part of Xihu Sag,East China Sea Basin

以东海盆地西湖凹陷中西部西斜坡平湖组中下段及宝石组和西次凹花港组下段两个靶区为例,基于钻测资料结合激光共聚焦扫描显微镜和扫描电镜等技术对目前低渗近致密—致密砂岩气层的宏观分布和储层微观特征进行了描述。利用气样稳定碳同位素组成、组分及轻烃分析成果,使用盆地模拟和流体包裹体技术,结合构造-沉积地质背景,对西斜坡和西次凹低渗近致密—致密砂岩气成因、气源和运聚成藏机制进行了综合研究。研究结果表明:两个区带已钻遇的近致密—致密砂岩气主要为煤型气。西次凹致密气主要来源于本地平湖组煤系源岩,因暗色泥岩有少量的腐泥母质,可能有少量油型气存在;此外部分天然气还有本地花港组下段煤系源岩微量影响;西次凹生成的成熟后期—高成熟天然气于3.9~0Ma借助断裂大规模垂向运聚成藏,具有近源、晚期、一次成藏机制;西斜坡近致密气充注时期为2.8~0Ma,近致密气可分为两类,一类来自本地平湖组煤系烃源岩的成熟中期天然气,于晚期充注成藏,另一类来源于异地西次凹平湖组煤系烃源岩的成熟后期—高成熟气,也于相近时期,通过断裂与三角洲前缘砂坝构成的阶梯状运移通道大规模充注西斜坡储层,具有"远近双源"、晚期、一次成藏机制。

With middle-lower Pinghu and Baoshi formations in western slope zone and lower Huagang formation in western sub-sag in Xihu Sag, East China Sea Basin as the objectives, this study describes the macro and micro distribution characteristics of low-permeable and nearly tight-tight sandstone gas reservoirs using drilling data, confocal laser scanning microscope and scanning electron microscope. Based on tectonic- sedimentary settings, stable carbon isotopic compositions of natural gas and analysis result of light hydrocarbon, this study carried out comprehensive analysis of genesis, origin and migration-accumulation of tight sandstone gas using source rock simulation and fluid inclusion technology. The results indicate that nearly tight-tight sandstone gas in two study regions is mainly coal type gas. Tight sandstone gas in western sub-sag mainly comes from coal-measure rocks in the Pinghu Formation, and dark sapropelic matter may indicate existence of minor amount of oil-type gas. In addition, part of natural gas may be slightly affected by coal-measure rock of the lower Huagang Formation. Nearly tight sandstone gas in western slope zone can be divided into two groups, moderately mature gas from local source rocks and mature late stage-highly mature gas from source rocks in western sub-sag. The mature late stage-highly mature gas in western sub-sag vertically migrated and accumulated via faults at 3.9-0 Ma. Moderately mature gas was injected into reservoirs in western slope zone at 2.8-0 Ma. Meanwhile, the mature late stage-highly mature gas from western sub-sag migrated into western slope zone via ladder-like channel which consists of faults and deltaic front channel sand bar.