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Diagenesis and Its Impact on Reservoir Quality of Volcanic Rocks – Carboniferous of the Western Junggar Basin, China


Volcanic reservoirs have been received more and more attention worldwide, however, reservoir without paleo-crust of weathering is far from detailed understanding. Recent years, high-production oil traps which found in the Junggar basin show this type of reservoir can have a high hydrocarbon potential and suggest the research necessity of this reservoir. This study utilizes core data, thin sections, well-log, fluid inclusion and geochemical data to investigate the lithology and properties of volcanic reservoir according to the research on diagenesis of Carboniferous in western Junggar basin. The study interval is dominated by tuff and andesite, and followed by volcanic breccia and metamorphic rocks. Fractures are the dominant reservoir space, and connecting various types of pores (intergranular, intercrystal, dissolved phenocrystal, and dissolved matrix pores). Diagensis are grouped into constructive and destructive diagenesis. The constructive diageneses consist of devitrification, condensation and contraction, vaporization of volatile matter, tectonic rupture and dissolution. The destructive diagenesis consist of pene-contemporaneous hydrothermal precipitation and crystallization, mechanical impaction and pressure solution, filling and cementation. Three phases of diagenesis and pores evolution are identified, which are condensation phase, post-magmatic hydrothermal phase and burial diagenetic phase. In the condensation phase, primary spaces formed with the influence of devitrification, condensation contraction and vaporization of volatile matter in lava and pyroclastic rocks. In the post-magmatic hydrothermal phase, hydrothermal mineral association precipitation and cement filling decreased the size and number of primary pores. Burial diagenetic phase consists of two sub-phases, which are shallow burial and deep burial period. During the Early Permian to Early Cretaceous, reservoir in shallow-burial diagenetic period, multiphase tectonic fracturing and corrosion contributed to the secondary space and significantly increased the reservoir quality. After the Late Cretaceous, reservoir entered deep-burial diagenetic period, metasomatism cementation reduced the reservoir quality. Condensing constriction induced contraction joints and gas escape induced vesicles which connected by tectonic-induced fractures and dissolved by subsurface fluid are the dominant efficient reservoir spaces for hydrocarbon accumulation.