榆林-神木地区上古生界盒8段及山2段气层的成岩作用和成岩相

Diagenesis and diagenetic facies of gas reservoirs in the Upper Paleozoic 8th member of Shihezi Fm and 2nd member of Shanxi Fm in Yulin-Shenmu area,Ordos basin

榆林-神木地区上古生界盒8段和山2段储集岩以石英砂岩、岩屑砂岩和岩屑质石英砂岩为主,成岩作用类型包括压实作用、胶结作用、交代蚀变作用、重结晶作用和溶蚀作用。砂岩经历了早期成岩阶段A,B期和晚期成岩阶段A1,A2和B期的成岩演化过程。压实作用和胶结作用是砂岩孔隙度降低的主要原因,分别造成22%和15%的原生孔隙丧失。砂岩中粘土矿物晶间微孔隙占总孔隙的60%以上,是砂岩的主要储集空间;溶解作用形成的次生孔隙在一定程度上改善了砂岩的储集性能;构造裂隙孔对砂岩储集性能的改善不明显。研究区发育6种不同成因类型的成岩相带,其中石英加大胶结组合孔隙相和自生粘土衬边胶结溶蚀孔相是最有利的天然气储集相带,二者的叠加和改造部位是气藏发育的最有利地带;粘土杂基充填微孔相和自生粘土胶结晶间孔相是较有利的成岩储集相带;杂基蚀变水云母充填压实紧密相和钙质胶结交代致密相不利于孔隙的发育和天然气的储集。

Sandstone reservoirs in the Upper Paleozoic 8th member of Shihezi Fm and 2nd member of Shanxi Fm in Yulin-Shenmu area are dominated by quartz sandstone, lithic sandstone and lithoclastic quartz sandstone, and the diagenetic types includes compaction, cementation, metasomatic alteration, recrystallization and corrosion. The sandstones have experienced diagenetic evolution processes of A and B phases during early diagenesis and A1 , A2 and B phases during late diagenesis. Decrease of sandstone porosity is mainly caused by compaction and cementation, which contribute 22% and 15 % to the loss of primary pores, respectively. Intercrystalline micropores of clay minerals in sandstone are the main reservoir space and account for more than 60 % of the total porosity. Secondary pores resulted from dissolution improved the poroperm of sandstone to some extent. While diaclases contribute less in improving the poroperm of sandstone. There are 6 diagenetic facies zones with various clay matrix and intercrystalline pore facies cemented by authigenic clay are relatively favorable diagenetic reservoir facies zones. The compacted tight facies filled with matrix-altered hydromica and the calcarinate and metasomatic tight facies are unfavorable for development of pores and gas accumulation.