Characteristics and Genetic Types of Authigenic Quartz in Paleogene ES 3 and ES 4 Shales, Dongying Sag

ABSTRACT Authigenic quartz is a key mineral controlling reservoir quality and rock brittleness in continental shales; however, its silica sources and diagenetic evolutionary pathways in lacustrine fine‐grained sediments remain insufficiently constrained. Taking the Paleogene ES3 and ES4 shales of the Dongying Sag in the Bohai Bay Basin as the study object, this study systematically characterises the morphology, elemental composition and mineral assemblages of quartz using integrated petrographic thin sections, field‐emission scanning electron microscopy with energy‐dispersive spectroscopy and X‐ray diffraction analyses, with the aim of clarifying the distinguishing features and evolutionary sequences of quartz with different genetic origins. The results indicate that quartz in the Dongying Sag shales can be grouped into three major genetic types, including terrigenous detrital quartz, mineral‐transformation–derived nanometre‐scale silica spherules and microcrystalline quartz, and microcrystalline quartz laminae of possible biogenic affinity. On this basis, a quartz genetic discrimination scheme applicable to continental lacustrine basins is established. Within a burial depth range of approximately 3300–4600 m, the diagenetic evolution of authigenic quartz can be divided into three successive vertical stages: initial precipitation of silica spherules, coexisting development of silica spherules and microcrystalline quartz, and stable growth of microcrystalline quartz. Silica released during organic‐matter‐driven smectite to illite transformation and feldspar dissolution represents the principal source of quartz formed through mineral transformation, whereas the crystallisation and development of localised microcrystalline quartz laminae are primarily controlled by silica‐enriched biogenic sediments. From a reservoir perspective, nanometre‐scale silica spherules and microcrystalline quartz may contribute to pore preservation through framework support and the potential generation of newly formed interparticle pores, thereby potentially improving reservoir brittleness and fracability. Microcrystalline quartz laminae are characterised by both high porosity and good connectivity, serving as important mineralogical indicators of high‐quality reservoirs and efficient hydrocarbon migration pathways. These findings demonstrate that the genetic types and vertical evolutionary patterns of authigenic quartz provide critical constraints on favourable exploration depth windows and hydraulic fracturing optimisation in the Dongying Sag and analogous lacustrine shale systems, offering new diagenetic–mineralogical insights for the prediction of high‐quality continental shale reservoirs.