Calcite cements are volumetrically dominant among the most diagenetic constituents in turbidite sandstones of the Paleogene Shahejie Formation in the Dongying Depression. The results show carbonate cements mainly occur in three phases: Calcite I (pre‐compaction), Calcite II (post‐compaction) and ankerite in the turbidite sandstone. The isotopic composition of Calcite I and Calcite II range from –4.3‰ to 4.4‰, –3.5‰ to 4.3‰ PDB, respectively, suggesting that the calcite cements are mainly derived from the interbedded shales. The early calcite cement precipitated from the mixing of lacustrine and meteoric waters at 38–69°C. The late‐phase calcite precipitated from pore waters modified by water‐rock interactions at 64–126°C. Two distribution patterns of calcite cements can be identified, depending on their position within the sandstone. The peripheral cementation of the sandstones is near the sand‐shale contact. The development of cement is controlled by sandstone thickness and the properties of the interbedded shale. For scattered cementation, the distribution of the calcite cement is not controlled by the sand‐shale contact. Calcite cement was preferentially distributed in coarsegrained sandstones, indicating that sandstones with high original permeability are the preferred pathway for the migration of early diagenetic fluids.

Authigenic carbonate cement is one of the most abundant diagenetic minerals in sandstone reservoirs. Determining its origin and distribution may provide useful information for understanding the sandstone reservoir quality. In this study, we report results from a suite of analytical techniques to investigate the origin and evolution of carbonate cements in the third member of the Shahejie Formation in the Bohai Bay Basin. Our data indicate that the carbonate cements mainly occur in three phases: the early-phase calcite, late-phase ferroan calcite, and late-phase ferroan dolomite and/or ankerite. The early-phase calcites show depleted δ18O of the early-phase calcite (−11.8% to −7.8%), suggesting an 18O-depleted fluid origin from the mixing between lacustrine and meteoric waters. They were precipitated earlier than the quartz overgrowth at 40°C–63°C based on the oxygen isotope. The late-phase calcites were precipitated at 70°C–115°C, and they originated from water-rock interaction modified pore water at the same time or later than feldspar leaching. They show a lower average δ13C value (1.27%) than the early-phase calcite (1.65%), indicating that the interbedded shales within the sandstones most likely provided the required components for the precipitation of the late-phase calcite. Also, some Fe2+ was released during the organic acid release and then precipitated the late-phase ferroan calcites. The late-phase ferroan dolomite and/or ankerite were precipitated at 90°C–137°C in a deep diagenetic condition. They show depleted δ13C values (mean −0.42%), and the carbons were mainly sourced from the thermal decarboxylation of organic matter and lacustrine carbonate. The early-phase calcite inhibits compaction while filling the pores, and the dissolution of ferroan calcite cements was the main reason for the development of secondary pores in the sandstone reservoirs. The late-phase ankerite reduces the reservoir porosity.

The storm deposition types within the Dabeiwang area of Xuzhou City, Jiangsu Province, China, showcase a wide range of diversity and distinctly apparent characteristics. Based on measured sections and thin-section observations, we have studied the typical characteristics of storm deposition in this area, divided the sedimentary sequence, and summarized a comprehensive model of storm deposition. Various rock types such as thick-bedded dolomitic limestone, oolitic limestone, and calcirudite are developed in the study area. The calcirudite exhibits bamboo leaf-like morphology and various distribution forms. Typical storm sedimentary structures such as scour surfaces and mound cross-bedding were identified in the study area. Based on the distance of the gravel layer from the storm center and the depositional environment, the storm sedimentary sequence in the study area can be divided into allopatric-proximal-lagoon facies storm deposit, allopatric-distal-oolitic shoal facies storm deposit, autochthonous-supratidal facies storm deposit, allopatric-distal-interbeach marine storm deposit, sub-allopatric-tide zone facies storm deposit, and autochthonous-tide zone facies storm deposit. We summarize the comprehensive model of Cambrian storm deposition in the Dabeiwang area of Xuzhou, which has enriched the research results of Cambrian storm deposition in Xuzhou and provides support for the construction of a lithofacies paleogeographic pattern.

The dissolution of feldspar is the main mechanism for the formation of secondary pores in deep sandstone reservoirs, which is controlled by pore fluid properties and fluid dynamics. The simulation experiment analyzed the effect of different fluid/rock ratios (hydrodynamics) on the orthoclase dissolution kinetics under reservoir diagenesis conditions. The experimental temperature was set at 100°C; the solutions were divided into acetic acid solutions and hydrochloric acid solutions, with pH values at 2 and 4; the reaction time was between 30 days and 80 days. The experimental results show that the fluid/rock ratio is an important factor controlling the feldspar dissolution. The higher the fluid/rock ratio, the stronger the fluid’s ability to dissolve at orthoclase, and the more the release of Al and Si. With the increase of fluid/rock ratio, it is possible to observe more noticeable dissolution characteristics from the feldspar morphology. The lower the fluid/rock ratio, the higher the concentration of Al and Si ions released by orthoclase dissolution, and the easier it is to cause the precipitation of clay minerals, which is not conducive to the improvement of reservoir physical properties. The fluid/rock ratio is an important controlling factor on the formation and distribution of authigenic minerals such as kaolinite. The research results are of great implications in the feldspar dissolution to form secondary pores in the burial diagenesis process. In the sandstone-shale contacts or sandstone reservoirs with good initial physical properties, the strong fluid flow is conducive to the secondary pores formation. The experimental findings are helpful for us to understand better the process and mechanism of feldspar alteration and secondary pores formation.