准噶尔盆地环玛湖凹陷二叠系砂砾岩储层沸石类胶结物的形成及石油地质意义

准噶尔盆地环玛湖凹陷二叠系砂砾岩储层中广泛发育沸石类胶结物。X射线衍射、铸体薄片、扫描电镜、包裹体测温和能谱分析等分析结果表明,研究区沸石类胶结物中浊沸石最为发育,其次为片沸石,方沸石含量较低,其形成条件为富含Na⁺、Ca²⁺、Mg²⁺的弱碱性还原环境。研究区沸石矿物的晶型较为完整,罕见方沸石与片沸石、浊沸石共生,也没有发生方沸石向片沸石和浊沸石转化的现象,表明沸石矿物是在成岩阶段从孔隙水中自形结晶形成;方沸石形成于早成岩阶段A期,其形成温度较低,为60~70℃,片沸石和浊沸石形成于早成岩阶段B期,片沸石形成温度为90~100℃,浊沸石形成温度为100~110℃。岩石学及矿物学特征表明,研究区沸石类胶结物更易发育于杂基含量较低、分选和磨圆度较好、原生孔隙发育、孔隙水易于流动的扇三角洲前缘水下分流河道砂砾岩中。早期形成的方沸石易发生溶蚀但其含量较低,片沸石和浊沸石形成时间较晚,更接近油气充注时期,且稳定性较高,一般很难发生强烈溶蚀,因此研究区沸石的溶蚀作用对储层的储集性能改善不大。研究区压实作用强烈,适度含量的沸石类胶结物可有效抵御砂砾岩储层的压实作用,保留良好的剩余粒间孔及连通的孔隙喉道,为后期酸性流体的进入提供必要的空间基础,有利于溶蚀作用;大量沸石胶结会破坏砂砾岩原有的储集空间,堵塞孔隙喉道,阻碍后期酸性流体的进入,使储层孔隙度和渗透率大幅度降低,因而不利于储层的改善。

Formation of zeolite cements in Permian sandy conglomerate reservoir in the circum-Mahu sag,Junggar Basin and its petroleum geological significance

Zeolite cements are developed widely in the permian sandy conglomerate reservoir in the circum-Mahu sag, Junggar Basin. The results of X-ray diffraction, casting thin section, scanning electron microscope, inclusion temperature measurement and energy spectrum analysis show that laumontite is most developed in the zeolite cements in the research area, followed by heulandite. In contrast, the content of cubicite is low, and it is formed in the weakly alkaline reduction environment rich in Na⁺, Ca²⁺ and Mg²⁺. In the study area, the zeolite crystal form is relatively complete. The co-occurrence between cubicite, heulandite and laumontite occurs rarely, nor is there conversion of cubicite to heulandite or laumontite, suggesting that zeolite minerals are formed by automorphic crystallization from pore water in the diagenetic stage; cubicite is formed at Stage A in the early diagenetic period, at a low temperature ranging from 60℃ to 70℃; heulandite and laumontite are formed at Stage B in the early diagenetic period, at a temperature ranging from 90℃ to 100℃ and 100℃ to 110℃, respectively. The petrological and mineralogical characteristics show zeolite cements are more likely to develop in the sandy conglomerate with low matrix content, good gradation, fine psephicity, developed primary pores and easy-to-flow pore water in the front underwater distributary channels in fan delta. Although the early-formed cubicite is easily subject to corrosion, its content is low. Heulandite and laumontite are formed at a later stage approaching to the period of hydrocarbon charging; generally, strong corrosion rarely occurs to them owing to their high stability. Therefore, the corrosion of zeolite in the study area has little effect on improving the reservoir performance. The study area is dominated by strong compaction, so a proper amount of zeolite cements can effectively resist the compaction of sandy conglomerate reservoirs and preserve good residual intergranular pores and connected pore throats, which has provided a necessary space for acidic fluids in the later stage and is helpful for corrosion. A large amount of zeolite cementation will destroy the original reservoir space of glutenite, clog pore throats, impede the entry of acidic fluids in the later stage, and largely reduce the reservoir porosity and permeability, which is unfavorable to the reservoir improvement.