Coupled plagioclase dissolution and K‐feldspar overgrowth precipitation preserve anomalously high permeability in deeply buried sandstones

This study investigated the formation mechanism responsible for anomalously high permeability (AHP) observed in the Eocene sandstones at depths from 3824 to 3832 m in the Huizhou Sag, Pearl River Mouth Basin, China, using helium porosity and permeability measurements, thin section, scanning electron microscopy, electron probe X‐ray microanalysis, rare earth element, microthermometry and in situ compositions of individual fluid inclusions, X‐ray diffraction, formation water compositions and numerical geochemical simulations. The AHP is ~10‐fold (average) higher than the baseline permeability at the same depths. Results indicate that authigenic K‐feldspar overgrowth is the predominant factor driving AHP formation. The Al 3+ and SiO 2(aq) required for K‐feldspar overgrowth may have been produced by the dissolution of plagioclase. K‐feldspar precipitation may be driven by two potential pathways. The first is in an inherently K + ‐rich connate formation water and driven by the expulsion of organic acids and K + ‐bearing shale formation water and the intrusion of K + ‐rich hydrothermal fluids (either before or after the expulsion of shale formation water). The second is driven by the early intrusion of K + ‐rich hydrothermal fluids, resulting in initial K‐feldspar precipitation and K + ‐rich formation water; subsequent expulsion of organic acids may further promote K‐feldspar precipitation. Numerical diagenetic simulations validate the feasibility of these hypotheses and further indicate that the precipitation of K‐feldspar overgrowth may inhibit the precipitation of kaolinite and quartz during plagioclase dissolution. This diagenetic mineral assemblage represents a novel coupled mineral dissolution–precipitation process in feldspar‐rich sandstones. Therefore, the coupled plagioclase dissolution and K‐feldspar overgrowth precipitation is suggested as the predominant mechanism responsible for the preservation of AHP mainly through resisting mechanical compaction and inhibiting secondary kaolinite and subsequent kaolinite‐to‐illite transformation. This study proposes a novel mechanism for AHP, identifies a novel diagenetic pathway in feldspar‐rich sandstones and is, therefore, important for understanding sandstone diagenesis and reservoir quality prediction.