陕甘宁盆地庆阳地区长8油层砂岩成岩作用及其对储层性质的影响

Diagenesisof Lacustrine Deltaic Sandstones and Its Impact on Reservoir Quality

庆阳地区长 8油层为湖泊三角洲前缘沉积。水下分流河道和河口砂坝组成的指状砂坝控制了水下三角洲沉积格局。砂岩主要由细砂和少量中砂组成 ,砂岩类型主要为细粒岩屑长石砂岩和长石岩屑砂岩。主要成岩矿物为绿泥石环边、方解石胶结物、石英和长石加大生长、伊利石、伊—蒙混层、钠长石和高岭石等。绿泥石环边的发育对原生粒间孔的保存起到有利的作用 ,对石英和长石加大起到抑制作用。烃类注入时储层埋深已较大 ,不利于孔隙保存。储层发育粒间孔隙、粒内溶孔、铸模孔隙、特大孔隙、裂缝孔隙和微孔隙等多种孔隙类型 ,其中以粒间孔隙最为发育。局部发育的特大孔隙和伸长状孔隙表明该区发生了一定规模的次生溶蚀作用。长 8油层主要受埋藏作用的影响 ,成岩演化处于晚成岩A期。主要成岩相类型为绿泥石环边胶结成岩相、压实充填成岩相、压实压溶成岩相和碳酸盐胶结成岩相 ,不同成岩相类型决定了油气储层性质不同 ,绿泥石环边胶结成岩相构成了本区最好的储层类型。

Sedimentation during stage of Chang-8 deposition was controlled by a lacustrine delta system.The Chang-8 member is an important finger-bar reservoir-sandstone in Qingyang area. Sandstone reservoirs can be classified as compositionally immature feldspathic lithareniteand lithic arkose. The main diagenetic minerals of thesandstones include: authigenetic clayminerals, calcite cement, quartz overgrowth, albiteand later ferroan carbonate. Authigenic quartz overgrowths and micro-crystals contain a variety of organic fluid inclusions that indicate the petroleum filling history. Petroleum filling history showsthat the significant later filling process affectedreservoir quality.Porosity preservation caused by grain-coating , authigenic chlorite. Wide porosity variations in sandstones correlate with an abundance of grain-coating chlorite and consequent inhibition of quartz cementation. Secondary porosity has created mainly by feldspar, rock fragment and clay matrix dissolution. The dissolution mechanisim is closely related to organic acids and transformation of clayminerals in adjacentsource rocks. Based on diagenetic events related to temperature, reservoir diagenetic stages can be dividedinto A-substage of later diagenesis. Reservoir quality is controlled mostly by sedimentary microfacies and diagenesis. Subaquoeus distributary channeland mouth barsandbodies commonly have high porosity and permeability, whereas interdistributary and distal sandsheets may have low petrophysical properties.The study shows that the Chang-8 sandstone locally retained excellent porosities controlled by diagenetic facies despite deep burial and thermal conditions.