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. © 2022, Editorial Office of ACTA PETROLEI SINICA. All right reserved.