Diagenetic alterations in Eocene, deeply buried volcaniclastic sandstones with implications for reservoir quality in the Huizhou depression, Pearl River Mouth Basin, China

This study investigates the origin of various diagenetic alteration products in Eocene, deeply buried volcaniclastic sandstones in the Huizhou depression, Pearl River Mouth Basin, China, to evaluate the influence of diagenesis on reservoir quality. Evi-dence from petrography and geochemistry of diagenetic minerals illustrates a relatively complicated diagenetic history in the volca-niclastic sandstones. Eogenetic hydration of volcanic glass materi-als led to precipitation of grain-coating chlorite and heulandite cements. First-generation laumontite cement (60 degrees C-70 degrees C) formed via heulandite transformation during eogenesis. Variable mesogenetic alteration products developed in an organic-CO2- based system and mainly included (1) precipitation of pore -filling chlorite, quartz, kaolinite, second-generation laumontite (90 degrees C- 100 degrees C), and calcite cements (d13CVPDB,-20.96%o to-7.8%o; d18OVPDB,-15.78%o to-10.88%o); (2) dissolution of volcanic glass, orthoclase and laumontite; and (3) chloritization and albiti-zation. Kinetic modeling results, coupled with petrographic observations, reveal that although total reservoir quality was not significantly enhanced by mesogenetic dissolution, diffusion of dissolved ions (e.g., Fe2+, Mg2+, and Ca2+) among different depositional microfacies produced a heterogeneous distribution of diagenetic alteration products and different reservoir evolu-tionary pathways. This diffusive transportation process led to local enhancement of reservoir quality in distributary-channel microfacies and local destruction of reservoir quality in mouth-bar and sheet-sand microfacies.