Origin and significance of "sweet spots" of analcites in shale oil reservoirs in Permian Lucaogou Formation, Jimsar Sag, Junggar Basin

Abstract

As a hydrocarbon indicating mineral, analcite was always found in shale oil reservoirs and has good correspondence with "sweet spots" in the Lucaogou Formation, Jimsar Sag, Junggar Basin. The petrographic and mineralogical characteristics as well as geologic origins of analcite were studied using a polarized light microscope, whole rock XRD analysis, electron probe BSE image analysis and mineral energy spectrum, in order to discuss its significance for shale oil "sweet spots". The analcite is mainly found in sedimentory tuff and carbonate tuff with some in tuff dolomite, and the analcite rock is very rare. The occurrence of analcite in rocks is lamellar, granular, massive and vein-shaped. Analcite was formed in high salinity, alkaline, low temperature hydrothermal conditions. The diagenetic transformation of non-volcanic material during burial forms authigenic analcite. Secondary analcite was formed by reaction and dissolution of volcanic material and hydrothermal during volcanic jetting. Analcite cements in the early diagenesis protected native porosity, and afforded the formation of secondary solution pores during mid and late diagenesis which has a good correspondence with shale oil "sweet spots".

Keywords

• shale oil
• analcite
• secondary dissolved pore
• lucaogou formation
• jimsar sag
• junggar basin