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

Haiguang Wu,Junjun Zhou,Wenjun He,Funing Sun,Wenxuan Hu,Chengcheng Feng,Xun Kang,Yunfeng Zhang

doi:10.1177/01445987211042702

Abstract

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 μm 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 μm 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.

Highlights

Authigenic albites are common diagenetic minerals in sedimentary rocks
This study aimed to clarify the origin of authigenic albites and to determine the diagenesis implications, that is, the genetic mechanism of the secondary dissolution pores in the unique carbonate– volcanic–clastic shale oil reservoirs in the Jimusaer Sag
Two types (I and II) of authigenic albites were identified in the mixed dolomitic–volcanic–clastic shale reservoirs in the Lucaogou Formation

Summary

Introduction

Authigenic albites are common diagenetic minerals in sedimentary rocks. In clastic and volcaniclastic rocks, they are generally formed through albitization (e.g. Boles and Coombs, 1977; Boles and Ramseyer, 1988; Kang et al, 2016; Zhu et al, 2016), which is an important alumino-silicate reaction in the shallow crust of the Earth (Perez and Boles, 2005). Albitization is considered to a coupled process of dissolution and reprecipitation (González-Acebrón et al, 2012; Morad et al, 1990; Perez and Boles, 2005). Extensive albitization means consuming a mass of Na+ in formation water This process changes chemical composition of pore water (Perez and Boles, 2005) and reflects mass transfer (Aagaard et al, 1990; Bjørlykke and Jahren, 2012; Boles, 1982; Morad et al, 1990).

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