鄂尔多斯盆地姬塬西部三叠系延长组长8油层组成岩作用与致密化过程

摘要

The mechanics of diagenesis and densification of the reservoir rocks from Chang 8 Interval,Triassic Yanchang Formation,western Jiyuan area,Ordos Basin were investigated through analyses of cores,cast thin sections,scanning electron microscope images,carbon and oxygen isotopes,and fluid inclusion.The reservoir rocks are mainly composed of lithic arkose and feldspathic litharenite.The reservoir rocks went through diagenesis processes include compaction,pressure solution,cementation,alteration,dissolution and structural fracturing so that the reservoirs are considered to be at late middle-diagenetic stage A.The typical diagenesis sequence includes mechanical compaction,cementation of cholorite rim,cementation of early micritic calcite,cementation of quartz overgrowth,dissolution of feldspar and rock fragments,cementation of authigenic kaolinite,cementation of authigenic quartz and cementation of medium term (ferrous) calcite in choronological order.The results show that:(1) The close proximity towards provenance and rapid fluctuation of lake level contributed to sediment supply.Facies distribution is the main control of reservoir properties where distributary channel facies and interdistributary bay facies show favorable and low reservoir qualities,respectively.(2) The original porosity of Chang 8 Interval during early burial stage is 41.35％ and decreased rapidly due to compaction,resulting in a 50.67 percent drop of original porosity.Cementation resulted in an additional 37.48 percent decrease of original porosity.Dissolution increase the porosity by 3.26％,howeverthe reservoir quality is still limited with this slight increase of porosity.(3) The sedimentary and diagenetic factors above lead to the densification of the Chang 8 Interval reservoirs in western Jiyuan area,and the densification process of Chang 8 Interval can be divided into four periods:Rapid porosity reduction through compaction,porosity reduction through early cementation,porosity increase through dissolution and densification through cementation.(4) The porosity of the reservoir is far below 10％ during hydrocarbon accumulation,therefore,the reservoirs of Chang 8 Interval are interpreted to be densified through compaction prior to hydrocarbon accumulation.%利用岩心、铸体薄片、扫描电镜、碳氧同位素及流体包裹体等资料,对鄂尔多斯盆地姬塬西部三叠系延长组长8油层组致密储集层特征进行了研究,重点分析了储集层成岩作用特征和储集层致密化成因机制及过程.研究区长8油层组砂岩主要为岩屑长石砂岩和长石岩屑砂岩,经历了压实—压溶、胶结、交代、溶蚀及构造破裂等成岩作用,整体处于中成岩A期晚期.典型成岩序列依次为机械压实、绿泥石黏土膜、早期(泥晶)方解石胶结、石英次生加大、长石、岩屑溶蚀、自生高岭石胶结、自生石英胶结、中期(含)铁方解石胶结.综合研究认为:(1)近物源伴随湖平面快速上升的沉积环境提供了储集层致密化的物质基础,不同沉积微相储集层物性差别较大,分流河道最好,分流间湾最差;(2)长8油层组原始孔隙度为41.35％,压实作用损孔率为50.67％,造成储集层孔隙度急剧降低,胶结作用损孔率为37.48％,导致孔隙度进一步减小,溶蚀作用仅增加了3.26％的孔隙度,难以有效改善储集层质量;(3)上述沉积与成岩因素共同导致储集层致密,致密化过程可分为压实孔隙骤减阶段、早期胶结减孔阶段、溶蚀作用增孔阶段和晚期胶结致密阶段共4个阶段;(4)油气大量充注期储集层孔隙度远低于10％,长8油层组先致密后成藏.