准噶尔盆地腹部永进地区砂岩储层中碳酸盐胶结物特征及其成因意义

Characteristics and genetic mechanism of carbonate cement in sandstone reservoirs of Yongjin area in central Junggar Basin

准噶尔盆地腹部永进地区不整合面附近砂岩储层中次生孔隙和碳酸盐胶结物相对发育。通过薄片观察、阴极发光、扫描电镜/能谱分析,认为碳酸盐胶结物是研究区砂岩最主要的胶结物成分,其主要类型为(铁)方解石和铁白云石,垂向上含量随深度增加而增加,多集中分布在白垩系和侏罗系之间的不整合面之下约100m的范围之内。碳、氧同位素分析以及泥岩中各元素测试结果表明,大气淡水通过永进地区白垩系和侏罗系之间存在的角度不整合对其下伏地层物性作用不明显,晚期含铁碳酸盐胶结物和孔隙发育主要与有机酸关系密切。在有机酸作用下,早期发育的碳酸盐胶结物、长石碎屑颗粒以及粘土矿物为晚期碳酸盐胶结物的发育提供了物质基础,而且溶解的物质在酸性流体作用下运移至不整合面附近重新沉淀,造成高孔隙度和高含量碳酸盐胶结物在深度上具有重叠的特征。由于含铁碳酸盐胶结物形成时代晚而且储层未受到其他建设性成岩作用的明显改造,晚期碳酸盐胶结物含量和面孔率呈互相补偿的关系,表明了晚期碳酸盐胶结物对储层物性的破坏作用。

Secondary pores and carbonate cements are quite well developed in sandstones near the unconformity between Jurassic and Cretaceous strata of Yongjin area in central Junggar Basin. Data obtained by thin-sections analysis, cathodolnminescenee emission analysis and SEM/EDX show that carbonate cements seem to be the uppermost cements composed of ferroan sparry calcite and ankerite and are distributed mostly in the depth interval of 100 m below the unconformity, with their contents increasing with the depth. Carbon and oxygen isotope and elements determination in mudstone values indicate that atmospheric fresh water did not have a distinct effect on reservoir characters below the unconformity, but the development of carbonate cements and pores was closely related to the organic acid. With the action of the organic acid, early carbonates, feldspar and clay minerals provided materials for later carbonates , and the dissolved matters moved to the unconformity by acidic fluids and deposited again, which led to the superimposition of high contents of porosity and carbonate cements at the depth. As the ferroan sparry carbonate cements were formed later and the sandstone reservoir was on the whole not subjected to any other constructive diagenesis, the development of carbonate cements must have played an apparent controlling effect on reservoir characteristics.