Origin of authigenic albite in a lacustrine mixed-deposition sequence (Lucaogou Formation, Junggar Basin) and its diagenesis implications

Authigenic albites occur widely in clastic reservoirs with important implications for diagenesis and reservoir formation. The middle Permian Lucaogou Formation in the Jimusaer Sag (Junggar Basin, NW China), where major exploration breakthroughs in shale oil have been achieved, reveals a new phenomenon that authigenic albites are abundant in unique mixed carbonate-volcanic-clastic sequences. This has not been reported in the literatures. To fill the knowledge gap, the origin of these authigenic albites and their relationship with dissolution pores (i.e. diagenesis implications) were investigated. Results show that two types (I and II) of authigenic albite were identified within the shale oil reservoirs. Euhedral Type I authigenic albites with 3-10 mu m only occur in dolarenite intraclasts and are symbiotic with amorphous dolomite minerals with a pure chemical composition of >99% albite-end-member content. Larger Type II authigenic albites with 10-50 mu m are widely distributed in reservoirs, primarily in dissolution pores, and coexist with authigenic dolomite minerals or dolomite overgrowths. Their chemical composition is less pure with anorthite-end-member contents that range from undetectable to 9.77%, with an average of 1.34%. A symbiotic relationship, pure chemical composition, size, and euhedral morphology indicate that Type I authigenic albites precipitated during syngenetic hydrothermal action. However, the morphology of dissolution pores, residual symbiotic orthoclase, impure chemical composition and carbon-oxygen isotope indicate that Type II were the products of the dissolution and reprecipitation of perthite crystal pyroclasts influenced by acid organic fluids in latter diagenesis. The differential dissolution of orthoclase and albite components in perthite crystal pyroclasts formed enormous intergranular secondary pores in the presence of dolomite minerals in the shale oil reservoirs.

Automated summary

Authigenic albites are widely present in unique mixed sequences, with two identified types formed either within dolarenite intraclasts or in dissolution pores. The study reveals that Type I authigenic albites precipitated during syngenetic hydrothermal action, while Type II were formed through dissolution and reprecipitation of perthite crystal pyroclasts influenced by acid organic fluids in latter diagenesis. These findings provide significant insights into diagenesis and reservoir formation.