Xihu Basin is one of the Cenozoic sedimentary basins within the continental shelf of the East China Sea, within which eight oil and gas fields and four hydrocarbon-bearing structures have been found. Our systematic analysis of potential petroleum systems in the basin has identified the Eocene Pinghu Formation as the most important source rock in the basin. The Eocene Pinghu Formation consists of mudstone and coal deposited in an embayment and tidal-flat environment and is characterized as containing type III kerogen. The Pinghu Formation is mature and, at the present time, is within the oil and wet-gas windows with determined vitrinite reflectance values in the range of 0.55–2.2% measured vitrinite reflectance (Rom). Modeling results suggest that the main stage of hydrocarbon expulsion occurred during the Miocene. The main reservoir consists of fine-grained sandstones of the Huagang and Pinghu formations deposited within shallow lacustrine and estuarine environments. The measured porosity from core samples of reservoir rock ranges from 10 to 35%, whereas permeability values range from 1 to 4000 md; both porosity and permeability decrease with depth. The mudstones of the upper Huagang and Longjing formations also occur as regional seals, which developed overpressure zones as determined by two-way sonic transit times. The overburden includes all the Oligocene, Miocene, Pliocene, and Quaternary strata. The hydrocarbon traps in the basin are mainly structural traps, including faulted blocks, faulted structural noses, and anticlines. Most traps were developed at the ends of the Eocene or Oligocene. Hydrocarbons produced from the Xihu Basin are predominantly natural gases with minor amounts of crude oil. The crude oil in the basin is characterized by a low density, low sulfur, low wax, low vanadium/nickel ratio, a low resin and asphaltene content, and a high proportion of saturated hydrocarbons. The natural gas in the basin is composed predominantly of methane, with an average C1/∑C1–5 ratio of 0.89 and a C1/∑C2–5 ratio of 8.6. Two petroleum systems have been identified in the basin: the known Pinghu-Huagang and the hypothetical Huagang-Huagang systems. The Pinghu-Huagang is the major petroleum system and most important for oil and natural gas exploration in the basin.

The Qixia Formation and Maokou Formation of Middle Permian in the southwestern Sichuan Basin were pervasively dolomitized during the diagenetic history. Petrographically, four types of dolomites, namely three replacive dolomites (Rd1, Rd2, and Rd3) and one dolomite cement (Cd), were distinguished. Rd1 dolomite occurs as very fine (<50 µm), planar-s to nonplanar crystals; Rd2 dolomite shows planar-e to planar-s crystal shapes with fine crystal sizes (50–250 µm) and is characterized by center-frog and margin-clear; Rd3 dolomite occurs as medium to coarse (250 µm–2 mm), nonplanar crystals; and Cd dolomite is characterized by saddle crystals filling dissolution pores and/or fractures, translucent white color in the hand samples, and strong sweeping extinction under cross-polarized light. In areas close to reactivated basement faults (Zhangcun outcrop and well Hanshen1), Rd3 (~65% by abundance) was the dominant type of replacement dolomite and minor amounts of Rd1 and Rd2 (~10%) were found in this area. Cd (~25%) was extensively developed in fractures and dissolution pores, whereas, in areas far away from the fault zones (Xinjigu outcrop), Rd1 (~20%) and Rd2 (~55%) were dominant replacement dolomites, and only a small portion of them were recrystallized to form Rd3 (~20%), with minor Cd (~5%) dolomite occurring in some dissolution pores. The δ¹³C_V-PDB (−0.37‰ to 4.32‰) and δ¹⁸O_V-PDB values (−7.41‰ to −5.19‰), ⁸⁷Sr/⁸⁶Sr ratios (0.707085 to 0.707795), and rare earth elements (REE) patterns (flat REE patterns with slight light rare earth element (LREE) enrichment and slight negative Ce anomalies) suggest that Rd1 dolomite was formed penecontemporaneously in an evaporitic tidal flat evaporation environment with salinities higher than seawater. The Rd2 dolomite, characterized by δ¹³C_V-PDB (−0.18‰ to 4.89‰) and δ¹⁸O_V-PDB values from −6.6‰ to −5.5‰, ⁸⁷Sr/⁸⁶Sr ratios from 0.707292 to 0.707951, and LREE enrichment and slight negative Ce anomalies, was interpreted as forming from the recrystallization of Rd1 at shallow burial. The δ¹⁸O_V-PDB values (−12.01‰ to −8.23‰), the prominent positive anomaly of Eu, high ⁸⁷Sr/⁸⁶Sr ratios (0.7081–0.7198) and high fluid inclusion homogenization temperatures (149–255 °C) suggest that Rd3 and Cd dolomite were formed from hot fluids. Based on regional stratigraphic data, the Rd3 and Cd were likely formed at depths less than 1500 m; thus, the ambient burial temperature would be lower than 85 °C. The high fluid temperatures recorded by fluid inclusions, thus, indicate that the dolomitization was of hydrothermal nature. The δ¹⁸O_V-SMOW values, homogenization temperatures, and salinities of the fluid inclusions of Rd3 and Cd in proximal areas were systematically higher than those in distal areas, suggesting that the hydrothermal fluid ascended along faults in proximal areas and then migrated laterally along the strata to distal areas. The dolomites of the Middle Permian carbonates in the southwestern Sichuan Basin, thus, resulted from different dolomitization phases, and the latter hydrothermal dolomitization was controlled by a combination of strata and structures.

The identification of structural diagenesis and the reconstruction of diagenetic paragenesis in fault damage zones is important for understanding fault mechanisms and fluid flow in the subsurface. Based on the examination of core and sample thin section data, we deciphered the diagenetic parasequence and their fault controls for Ordovician carbonates in the northern Tarim intracratonic basin in NW China (Halahatang area). In contrast to the uniform nature of diagenesis observed in country rocks, there is a relatively complicated style of compaction and pressure solution, multiple fracturing, and cementation and dissolution history along the carbonate fault damage zones. The relative paragenetic sequence of the structure related diagenesis suggests three cycles of fracture activities, following varied fracture enlargement and dissolution, and progressively weaker calcite cementation. These processes of structure related diagenesis are constrained to the fault damage zones, and their variation is affected by the fault activities. The results of this study suggest that the carbonate reservoir and productivity could be impacted by the structure related diagenesis locally along the fault damage zones.