Exploring for hydrocarbons in a pyroclastic-affected reservoir is an important research topic. Previous studies have mainly focused on laminated pyroclastic. A large number of dispersed pyroclastic is present in sedimentary rocks, and dispersed volcanic ash strongly influences the diagenetic evolution of sandstone reservoirs. However, these aspects remain understudied. We studied the mechanism of the diagenetic evolution of the Jurassic tuffaceous sandstone reservoir in Qikou Sag of the Bohai Bay Basin by performing inclusion temperature measurements, rock slice identification, and scanning electron microscopy, and using electron microprobes and microzone isotopes. We determined the mechanism of water-rock interaction. Based on microscopic observations, we determined that the main diagenesis included two-stage dolomite cementation, two-stage calcite cementation, quartz cementation, and transformation and dissolution of clay minerals. The hydrolysis and chemical transformation of pyroclastic during burial not only provided an alkaline environment in the early stage of diagenesis but also supplied ions for the formation of microcrystalline quartz and early dolomite and the transformation of clay minerals. Leaching and denudation generated early dissolution caused by a tectonic uplift. Following the epigenetic stage, microbial activity stimulated the formation of early calcite during the shallow burial stage. When the burial temperature of the stratum was 80 °C, the acidic fluid discharged from the thermal evolution of organic matter was neutralized by the soluble components in the pyroclastic, which prevented the formation of a large-scale acidic environment. When the burial temperature exceeded 100 °C, the acidic fluid generated by thermal catalytic decarboxylation of organic matter formed a large quantity of dissolution. The dissolution of plagioclase promoted the overgrowth of quartz and the growth of kaolinite.

Calcite in hydrocarbon reservoirs records abundant information about diagenetic fluids and environments. Understanding the formation mechanisms of calcite is crucial for predicting reservoir characteristics and hydrocarbon migration. This study identifies the types of authigenic calcite present in the Lower Paleozoic carbonate reservoirs of the Bohai Bay Basin through petrographic analysis, cathodoluminescence, and other experimental methods. By integrating electron probe microanalysis, in situ isotopic analysis, and fluid inclusion studies, we further constrain the source of the diagenetic fluids responsible for the authigenic calcite. The results show that there are at least three types of authigenic calcite in the Lower Paleozoic carbonate reservoirs of the Bohai Sea. Calcite cemented in the syn-depositional-to-early-diagenetic stage displays very weak cathodoluminescence, with δ¹³C and δ¹⁸O and paleo-salinity distributions similar to those of micritic calcite. These features suggest that the calcite was formed during burial heating by sedimentary fluids. Calcite filling fractures shows heterogeneous cathodoluminescence intensity, ranging from weak to strong, indicating multiple stages of cementation. The broad elemental variation and multiple cementation events suggest that the diagenetic fluid sources were diverse. Isotopic data show that samples with carbon isotope values greater than −2.9‰ likely formed through water–rock interaction with fluids retained within the strata, whereas samples exhibiting more negative δ¹³C were formed from a mixed-source supply of strata and mantle-derived fluids. Calcite that fills karst collapse pores exhibits alternating bright and dark cathodoluminescence, strong negative δ¹⁸O shifts, and variability in trace elements such as Mn, Fe, and Co. These characteristics indicate a mixed origin of diagenetic fluids derived from both meteoric freshwater and carbonate-dissolving fluids.