砂岩储层致密化及其与油气充注成藏的耦合关系——以鄂尔多斯盆地延安气田上古生界为例

The densification of sandstone reservoir and its coupling relationship with hydrocarbon charging and accumulation:a case study of the Upper Paleozoic in Yan′an Gas Field,Ordos Basin

针对鄂尔多斯盆地延安气田上古生界致密砂岩储层,文中采用薄片鉴定、扫描电镜分析及流体包裹体测试等方法,开展储层特征、天然气成藏期次等研究,恢复孔隙度演化过程,讨论了各类砂岩储层致密化与油气充注成藏的耦合关系。研究表明:在早成岩阶段,凝灰质石英砂岩、高塑性岩屑砂岩及钙质胶结砂岩已演变为特低孔岩石,平均孔隙度为8%,不发生烃类充注,之后持续保持致密状态。石英砂岩、低脆性石英砂岩经历了早成岩机械压实、溶蚀及高岭石胶结作用,在第1期烃类充注时,平均孔隙度为23%,随后在石英压溶和胶结作用下孔隙度继续减小;在第2期大量烃类充注时,平均孔隙度为18%,随着烃类不断充注,储层越来越致密。因此,石英砂岩和低脆性石英砂岩储层在早期成藏阶段孔渗条件较好,且至成藏晚期储层致密化过程中仍发生广泛的油气充注,其致密化过程与油气充注成藏的耦合关系为“边成藏边致密化”。

For the tight sandstone reservoir of Upper Paleozoic in Yan′an Gas Field of Ordos Basin,by using many methods such as thin section identification,scanning electron microscope analysis and fluid inclusion testing,the reservoir characteristics and natural gas accumulation period are studied,and the porosity evolution process is restored.The coupling relationship between densification of each type sandstone reservoir and hydrocarbon charging and accumulation are discussed.The research shows that in the early diagenetic stage,the tuffaceous quartz sandstone,highly plastic lithic sandstone and calcareous cemented sandstone evolve into ultra-low porosity rocks,with average porosity of 8%,without hydrocarbon charging,and then continue to maintain a tight state.The quartz sandstone and low brittleness quartz sandstone undergo early diagenetic mechanical compaction,dissolution and kaolinite cementation,and the average porosity is 23%during the first stage of hydrocarbon charging.Then the quartz dissolution and cementation continue to reduce the porosity.During the second stage of large-scale hydrocarbon charging,the average porosity is 18%.With the continuous hydrocarbon charging,the reservoir is gradually densified.Therefore,the quartz sandstone and low brittleness quartz sandstone have good porosity and permeability conditions in early accumulation stage,and extensive oil and gas charging can still occur in the process of reservoir densification until the late accumulation stage,and the coupling relationship between the densification process and hydrocarbon charging is"densification while hydrocarbon accumulation".