四川盆地川西坳陷须家河组三段致密砂岩储层成岩作用及孔隙演化

Diagenesis and porosity evolution of the 3^(rd) member of Xujiahe Formation tight sandstone reservoir in the Western Sichuan Depression,Sichuan Basin

综合利用岩心、铸体薄片、阴极发光、X-射线衍射、扫描电镜、包裹体分析和孔渗数据等资料,对四川盆地川西坳陷须三段致密砂岩储层成岩作用和孔隙演化进行了分析。研究发现川西坳陷须三段主要发育岩屑石英砂岩和岩屑砂岩,结构成熟度与成分成熟度差—中等;储集空间类型主要为以岩屑粒内溶孔和长石粒内溶孔为主的次生孔隙,孔隙度多小于6%,渗透率多小于1×10^(-3)μm^(2),属于典型的致密砂岩储层。须三段砂岩主要经历了压实、溶蚀、胶结和交代等成岩作用改造,具体演化过程为压实作用→早期大气淡水溶蚀→早期石英、长石、高岭石等黏土矿物、方解石和白云石胶结→中期有机酸溶蚀→中期石英、高岭石等黏土矿物、铁方解石和铁白云石胶结→晚期粗晶—巨晶方解石充填裂缝。通过定量计算研究区砂岩储层的成岩演化过程可知,砂岩原始孔隙度为37.30%,经压实作用孔隙度减小为9.55%,早期溶蚀使孔隙度增至12.15%,此后因早期胶结作用孔隙度减小为9.49%,随后有机酸溶蚀使得孔隙度增至10.14%,最后经中期胶结作用和晚期粗晶—巨晶方解石充填裂缝,现今孔隙度减小为3.95%。尽管溶蚀作用产生次生溶孔(增孔率8.71%),但压实作用和胶结作用导致的减孔效应(减孔率分别为74.39%和23.72%),为须三段砂岩储层致密化的关键因素。

The diagenesis and porosity evolution of the 3^(rd) member of Xujiahe Formation(Xu3 member)tight sandstone reservoir in the Western Sichuan Depression were analyzed by combining data from cores,thin sections,cathode luminescence(CL),X-ray diffraction(XRD),scanning electron microscope(SEM),fluid inclusion,porosity and permeability.It was found that lithic quartz sandstone and lithic sandstone are mainly developed in the Xu3 member in the Western Sichuan Depression,and the maturity of structure and composition is poor to medium.The reservoir space is mainly composed of feldspars and rock fragment dissolution pores,with porosity < 6% and permeability < 1×10^(-3)μm^(2),which belongs to typical tight sandstone reservoir.The diagenesis of the Xu3 member sandstones included compaction,cementation,dissolution and replacement.The diagenetic evolution process is compaction → early atmospheric fresh water dissolution → early quartz,feldspar,calcite and dolomite,clay minerals such as kaolinite cementation → medium-term organic acid dissolution → medium-term quartz,clay minerals such as kaolinite,ferrocalcite and ankerite cementation → late coarse-giant crystal calcite filling fractures.Through quantitative calculation of the diagenetic evolution process of sandstone reservoir in the study area,it can be found that the original porosity of sandstone is 37.30%,which decreases to 9.55% after compaction,increases to 12.15% after early dissolution,decreases to 9.49% after early cementation,and increases to 10.14% after organic acid dissolution.Finally,the porosity is reduced to 3.95% after medium-term cementation and late coarse-giant crystal calcite filling.The results indicate that although the dissolution produced secondary pores(increasing pore rate by 8.71%),the porosity reduction effect(decreasing pore rate by 74.39% and 23.72%,respectively)caused by compaction and cementation is the key factor for the densification of the Xu3 member sandstone reservoirs.