The Sinian Dengying Formation in western Hubei Province represents a potential target for marine natural gas exploration in the Middle Yangtze region, where natural gases with moderate helium contents have been discovered. Marine carbonate reservoirs of the Dengying Formation occur in both western Hubei and the Weiyuan Gas Field of the Sichuan Basin. However, marked differences in helium abundance and gas-reservoir occurrence between the two regions suggest that the formation of helium-rich natural gas was closely associated with regional geological processes. The Dengying Formation in western Hubei underwent a complex tectonic evolution and contains abundant high-angle calcite veins that crosscut sedimentary bedding. These veins record fracture opening, reservoir reconfiguration, and mineral filling, thereby providing evidence for reconstructing the evolutionary history of the natural gas reservoirs. Calcite U-Pb dating results indicate that fracture opening and mineral filling occurred in the Dengying Formation during the early Yanshanian period. The calcite veins are generally characterized by negative Ce and positive Eu anomalies in their rare earth element distribution patterns, whereas their C-O-Sr isotopic compositions are close to those of seawater contemporaneous with deposition of the Dengying Formation. These characteristics suggest that the vein-forming fluids were derived predominantly from the Dengying Formation. Raman spectroscopic analyses reveal hydrocarbon-bearing inclusions in the veins, composed mainly of methane and solid bitumen. This observation indicates that products generated by the cracking of paleo-oil accumulations were trapped during fracture opening and calcite precipitation. Early Yanshanian tectonic activity may have reopened reservoir fractures and promoted the redistribution of natural gas within the Dengying Formation. Subsequent calcite filling reduced fracture connectivity and may have partially impeded the later dissipation of natural gas. Compared with the Weiyuan Gas Field in the Sichuan Basin, the lower helium abundance in the Dengying Formation of western Hubei may be attributed to regional differences in helium generation conditions, reservoir scale, hydrocarbon-gas charging intensity, tectonic modification, and preservation conditions.

Keywords: Dengying Formation; helium-bearing natural gas; petroleum accumulation conditions; preservation history; western Hubei Province.

Author Profile:

Shaojie Li, Male, Associate Professor, PhD, mainly engaged in petroleum geochemistry and geochronology research. E-mail: shaojie.li@yangtzeu.edu.cn.