Linking Volcanism to Quartz Types and Origins in Carboniferous–Permian Shales of North China: Insights Into Reservoir Properties

ABSTRACT Elucidating the potential relationships between authigenic quartz and its silica origins and volcanic activity greatly enhances our understanding of quartz formation mechanisms and offers valuable guidance for shale gas exploration. The Late Carboniferous to Early Permian period represents a pivotal interval in Earth's history marked by intense volcanic activity and extensive accumulation of organic‐rich shales worldwide. This study focuses on shale from the upper Carboniferous‐lower Permian strata in the Ningwu Basin, North China (NC). Scanning electron microscopy (SEM), cathodoluminescence (CL), elemental geochemistry, XRD, and nitrogen (N 2 ) adsorption analysis were used to aid in revealing the quartz types and silica sources in shale deposited pre‐, during, and end‐stage of volcanic eruptions, as well as the effects of different silica sources on porosity evolution. Our results reveal the coexistence of extrabasinal detrital quartz and several forms of authigenic quartz, including silica skeletal fragments, nano‐micrometre‐sized quartz aggregates, and matrix‐dispersed microquartz grains in these deposits. Extrabasinal terrigenous quartz appears to be the dominant source of silica, whereas volcanic ash alteration and dissolution of biogenic silica constitute the primary silica sources for authigenic quartz in shales deposited during volcanic episodes (Kasimovian‐Gzhelian and Gzhelian‐Asselian intervals, abbreviated as KGB‐VA and GAB‐VA). This authigenic quartz forms a rigid framework that effectively inhibits compaction and preserves shale porosity. In contrast, authigenic silica formed during pre‐eruption (KGB‐PVA) and end‐eruptions (GAB‐EVA) stages is primarily originated from clay mineral transformation. Shale gas exploration in the Carboniferous‐Permian strata in NC should prioritise organic‐rich intervals, which were influenced by multiple volcanic events and restricted water column during deposition and are deeply buried with high thermal maturity. The results of this study offer additional insights into the quartz formation mechanism in Carboniferous‐Permian shales and deliver conceptual support for guiding shale gas exploration efforts.