鄂尔多斯盆地镇泾区块延长组长8^1储层成岩作用特征及其对储集物性的影响

Diagenesis and its influence on reservoir quality of Chang 81 sandstones from Upper Triassic Yanchang Formation in Zhenjing area,Ordos Basin

鄂尔多斯盆地镇泾区块上三叠统延长组长8^1储层具有成分成熟度低，结构成熟度中等到好的特点，岩性以岩屑长石砂岩和长石岩屑砂岩为主。长8^1砂岩属于特低孔特低渗储层，以次生溶蚀粒间孔为主要储集空间。目前储层正处于中成岩阶段A期，主要发育压实、胶结、溶解和交代4种成岩作用类型。机械压实作用是造成长8^1储层埋深小于2000m砂岩原生孔隙大量丧失、渗透率急剧减小的主要原因，随着埋深增加，其对储集物性的影响逐渐减弱。胶结作用对储集物性的破坏主要表现为各种胶结物沉淀析出、充填孔隙、堵塞吼道，其中黏土矿物和晚期碳酸盐胶结是致使储集物性变差的主要原因，但孔隙衬里绿泥石、自生高岭石和早期碳酸盐胶结作用对储集物性的影响具有双重性，且孔隙衬里绿泥石胶结作用的建设性大于破坏性，其在减弱机械压实强度，抑制硅质胶结物发育的同时，为后期酸性有机流体的进入以及溶解产物的带出提供了有效通道，有利于溶蚀孔隙的发育。长8^1储层在埋深1800m、2000m、2100m和2300m附近存在4个次生溶蚀孔隙发育带，以长石颗粒的溶解为主。研究表明，溶解和孔隙衬里绿泥石胶结作用均很发育的层段是长8^1储层油气储集的最有利地带。

Chang 8^1 reservoir sandstones of upper Triassic Yanchang Formation in Zhenjing area of Ordos Basin mainly consist of feldspathic litharenite and lithic arkose, with low mineralogical maturity and median-high textural maturity. They belong to extra-low porosity and permeability reservoirs, with dissolved intergranular pores as the chief accumulation space. The Chang 8^1 reservoir sandstones are now in the A stage of mesodiagenesis,and the main diagenesis of which including compaction, cementation, dissolution and replacement. The mechanical compaction, which principally took place in the early time of burial history, is the predominant reason for reservoir quality deterioration of Chang 8^1 sandstones with burial depth above 2 000 m. Accompanied by the different cementation and dissolution taking place progressively with the burial depth increasing, the destruction degree resulted by mechanical compaction was weakened. The destruction impact of cementation is characterized by precipitation of different cement in primary and secondary pores,in which the cementation of authigenic clay minerals and late stage carbonate have exerted a dominant influence on reservoir quality, making the porosity and permeability decreased quickly. However, the pore-lining chlorite, authigenic kaolinite and early stage carbonate cement have dual effects on reservoir quality, and the constructive influence of pore-lining chlorite cementation over the destructive one. The pore- lining chlorite has preserved the reservoir quality by weakening the degree of mechanical compaction, inhibiting the subsequent generation of quartz cement, and providing the passageways for the entrance of acid fluid of the A stage of mesodiagenesis and flowout of dissolved resultant products, making a large number of secondary pores produced. Dissolution is the key factor for the formation of secondary pores and improvement of pore throat structures of Chang 8^1 reservoir sandstones. There are four dissolution zones in the Chang 8^1 sandstones, located in the vicinity of 1 800 m, 2 000 m, 2 100 m, and 2 300 m perspectively,with the feldspar as the main dissolved grain type. The result shows that the sandstone intervals with dissolution and pore-lining chlorite cementation occurring together are the optimum zones for the oil and gas accumulation.