Abstract
Zeolites are important diagenetic minerals in petroleum reservoirs and have complex impacts on reservoir quality. To highlight this critical and challenging issue, we conduct a case study in the Middle Permian Lower-Wuerhe Formation in the Mahu Sag, Junggar Basin, China. Formation mechanism of zeolites and their impacts on the reservoir quality. Our results show that there are five types of zeolite minerals (i.e. laumontite, heulandite, analcime, stilbite, and clinoptilolite) in the Lower-Wuerhe Formation reservoir, with laumontite and heulandite being the main types. Petrographic and geochemical data suggested that laumontites developed in the Lower-Wuerhe Formation were mainly precipitated from pore water, whereas heulandites were formed associated with alteration of volcanic materials. In addition, the distribution of different type of zeolite minerals is generally controlled by sedimentary facies. The heulandite–laumontite zone developed mainly in the front of fan-delta plain, and the laumontite zone developed mainly in fan-delta front. The distal part of fan-delta front is dominated by albite. The zeolite mineral assemblages are generally controlled by geochemical composition of volcanic lithic fragments. The high content of intermediate-basic volcanic lithic fragments in the eastern Mahu Slope sediments is responsible for authigenic minerals such as heulandite, chlorite, and laumontite. However, the content of intermediate-basic volcanic lithic fragments in the western Mahu Slope sediments is low which results in the cement in this region is dominated by laumontite and mixed-layer illite/smectite. In general, conglomerates deposited in fan-delta front are favorable for the formation of early laumontite and late dissolution of laumontite due to resistance to compaction by coarse fraction and accumulation of acidic fluids in structural highs, which resulted in the formation of a high-quality reservoir. Our results have general implications for hydrocarbon exploration of the zeolite-bearing conglomerate reservoirs in non-marine petroliferous basins.
Highlights
Secondary pore development via the dissolution of zeolite minerals in reservoirs is an important new field for oil and gas exploration research
We focused on conglomerates of the Lower-Wuerhe Formation from 30 wells in the Mahu Sag, and 436 thin sections were prepared for petrography study
Our results suggest that the zeolites in the Lower-Wuerhe Formation could be formed by both alteration of volcanic materials and precipitation from pore water
Summary
Secondary pore development via the dissolution of zeolite minerals in reservoirs is an important new field for oil and gas exploration research. Previous studies have proposed that zeolite cementation leads to a significant reduction in intergranular pores, which degrades reservoir quality (Lian and Yang, 2017). Other studies have suggested that zeolite cementation protects intergranular pores and can be dissolved during later diageneses (Fu et al, 2010; Yang et al, 2005). Zeolites are microporous framework silicate minerals that are sensitive to temperature and pressure, and can be formed in high heat flux, volcanically active, deep sea, soil, nearsurface, and saline lake settings (Do Campo et al, 2007; Hay and Sheppard, 2001; Sun et al, 2014; Zhang et al, 2015). Various mechanisms for zeolite formation have been suggested: (1) heulandite, analcime, and other zeolite minerals transform from each other when the pH of formation water, ion species, and their concentrations in formation water changed; (2) laumontite forms by the alteration of intermediate volcanic rock and tuff fragments; (3) laumontite forms by low temperature metamorphism between 200C and 260C (Li et al, 2012; Zhang et al, 2011); and (4) laumontite formed from plagioclase by nitrification (Fu et al, 2010; Zhu, 1985)
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