四川盆地西北部上三叠统须三段储层超致密与气藏超压成因

关于四川盆地西北部地区上三叠统须家河组储层致密、气藏超压的相关研究较多,但是对于致密储层的形成机理、异常超高压的展布特征与形成机制、热演化作用对大规模致密气聚集的影响等研究则尚不够系统和深入。为此,利用近期获得的大量钻井资料,研究该区须家河组三段(以下简称须三段)储层特征及主控因素,结合烃源岩热演化和成岩作用演化分析储层致密的原因;在分析气藏流体温度压力特征的基础上,结合构造运动分析气藏超压的形成机制;最后在归纳总结大型超压气藏特征的基础上,分析大规模超压天然气聚集机制。研究结果表明:①须三段极致密储层是强成岩压实与沉积物提供大量碳酸盐岩碎屑导致的强钙质胶结的共同结果 ;②须三段储层经历了深埋下的高热演化,储层最致密的地区既是热演化程度最高的地区,也是致密气的主要发育区,超高压致密气的形成受强成岩作用以及白垩纪末期高热演化的影响明显;③剑阁地区须三段砂/砾岩成岩压实使孔隙度下降了20%,安县构造运动使龙门山隆升为须家河组沉积提供的大量碳酸盐岩物源是碳酸盐胶结导致储层致密化的重要原因,使孔隙度损失了10%～20%;④该区坳陷带气藏异常高压的形成并非构造挤压造成的,而是因断裂不发育、生烃增压与构造反转导致泄压不畅所致,形成极致密储层超高压发育区;⑤龙门山、米仓山断裂带前缘断裂发育导致压力释放,为常压区,储层物性也好于坳陷带。结论认为,该区大规模超高压致密气藏的形成机制复杂、影响因素多样,紧密的源储组合关系、白垩纪末期的高热演化生烃增压、喜马拉雅期构造反转与地层隆升剥蚀、圈闭围岩极好的封闭能力所引起的极致密储层泄压不畅等原因,造就了该区异常超高压天然气的大规模聚集。

The formation mechanisms of ultra-tight and overpressured gas reservoir in the third Member of Upper Triassic Xujiahe Formation in the northwestern Sichuan Basin

More studies have been carried out on the tight and overpressured gas reservoir of the Upper Triassic Xujiahe Formation in the northwestern Sichuan Basin, but the formation mechanisms of tight reservoir, the distribution characteristics and formation mechanisms of abnormal ultra-high pressure and the effects of thermal evolution on large-scale tight gas accumulation have not been researched systematically and thoroughly. To this end, this paper studied the reservoir characteristics and main controlling factors of the third Member of Xujiahe Formation (Xu 3 Member for short) in this area using a large quantity of drilling data obtained recently. And combined with the thermal evolution of the source rocks and the evolution of diagenesis, the reasons for the formation of tight reservoir were analyzed. Then, the formation mechanisms of overpressured gas reservoirs were studied based on tectonic movement after the temperature and pressure characteristics of the fluids in gas reservoirs were analyzed. Finally, the mechanisms of large-scale overpressured natural gas accumulation were analyzed after the characteristics of large-scale overpressured gas reservoirs were summarized. And the following research results were obtained. First, ultra-tight reservoir of Xu 3 Member is the product under the joint effect of strong diagenetic compaction and strong calcareous cementation caused by a large amount of carbonate debris in sediments. Second, the Xu 3 Member reservoir has experienced high thermal evolution in great burial depth. The tightest reservoir is the area with the highest thermal evolution and also the main tight gas development area. The formation of ultra-high pressure tight gas reservoir is obviously affected by the strong diagenesis and the high thermal evolution in the late Cretaceous period. Third, in Jian'ge area, the diagenetic compaction of Xu 3 Member sandstone/glutenite leads to the reduction of porosity by 20%. The important reason for the reservoir densification caused by carbonate cementation is that Anxian tectonic movement leads to the uplifting of Longmenshan Mountain, providing sufficient sources of carbonate rocks for the deposit of Xujiahe Formation. And consequently, the porosity is reduced by 10-20%. Fourth, the formation of abnormal high pressure gas reservoir in the depression zone of this area is not caused by structural compression, but by poor pressure release resulted from fracture underdevelopment, hydrocarbon generation pressurization and tectonic reversal, which leads to the formation of an ultra-high pressure development zone of ultra-tight reservoir. The fracture development in the front margin of Longmenshan and Micangshan fault belts leads to pressure release, so it is a normal pressure zone, whose reservoir physical properties are better than those of the depression zone. It is concluded that the formation mechanisms of large-scale ultra-high pressure tight gas reservoirs in this area are complex with various influencing factors, and the close source-reservoir assemblage relationship, the high thermal evolution and hydrocarbon generation pressurization in the late Cretaceous, the tectonic reversal and stratigraphic uplift and erosion in the Himalayan period, the poor pressure release of ultra-tight reservoir caused by the excellent sealing ability of surrounding rock play a significant role in the large-scale accumulation of abnormal ultra-high pressure natural gas in this area.