Paleo-Temperature and -Pressure Characteristics of Fluid Inclusions in Composite Veins of the Doushantuo Shale (Yichang Area, South China): Implications for the Preservation and Enrichment of Shale Gas

Abstract

The Sinian Doushantuo Formation has been identified as a promising shale gas reservoir in South China and potentially worldwide. This study aims to restore the paleo-temperature and paleo-pressure characteristics of the Doushantuo Formation, found in the Yichang area, western Hubei, and to study the significance of overpressure on shale gas preservation and enrichment using petrographic, cathodoluminescence (CL), microthermometry, and Raman spectroscopy techniques on the composite veins. Single-phase methane inclusions and two-phase aqueous inclusions were consistently observed in the same fluid inclusion assemblage (FIA), indicating that the fluid inclusions trapped during fracture cementation were saturated with methane-rich hydrocarbon fluid. The CL image analysis demonstrated that the formation of veins in the shale occurred in two different stages, consistent with the results of the homogenization temperature (Th) analysis. The trapping pressures of the methane inclusions ranged from 81.95 to 95.00 MPa and from 57.13 to 63.62 Ma in the calcite and quartz veins, respectively, as predicted by the combination of the methane inclusion density and Th of the coexisting aqueous inclusions and the equations of state for the supercritical methane. The calculated pressure coefficient (Pc) of 1.90–2.31 indicates that fluid inclusions were trapped in the medium-to-high overpressure state. The Ths of the fluid inclusions are lower than that of the maximum paleo-temperature (∼230 °C), indicating that these fluid inclusions were trapped after the maximum burial and first rapid uplifting process (∼165 Ma). These trapped diachronous fluid inclusions can provide crucial evidence for the formation and evolution of the identified overpressure. Our results suggest that the paleo-overpressure in the Doushantuo shale was continually released through high-angle fractures during the multistage uplifts, deposition, and reuplifting in the Yanshanian and Himalayan movements, resulting in normal pressure and low gas content.

Shale Gas Exploration and Development in China: Current Status, Geological Challenges, and Future Directions

“…The Cambrian Niutitang Formation and its equivalent layers in and around the Sichuan Basin have been explored early in China. The shales are characterized by high organic matter content and large thickness, but the old age and high maturity result in complex preservation conditions for shale gas, ,,, and that is the reason why this formation has not yet been commercially developed except for low gas output from the Cengong area of Southeast Guizhou, the Yichang area, and the central Sichuan paleo-uplift. In the high maturity stage, evaluation of gas generation potential, gas content, and preservation conditions is essential in the exploration research of the Cambrian shale gas in the Sichuan Basin and its surrounding areas.…”