Late-fracture calcite veins decode natural gas preservation

Abstract

The preservation of natural gas can be easily dismissed by late crustal deformations, which are often concealed by fracture-filling calcite vein networks. This study introduces a novel approach to confine the timeframe of natural gas-leakage processes by employing coupled in situ U–Pb dating and clumped isotope (Δ47) analysis of late-fracture calcite veins associated with brittle deformation in caprock/unconventional reservoirs during basin uplift. Specifically, Δ47 and U–Pb data were acquired for fracture calcite veins that formed in high-angle or vertical fractures within the early Silurian shale reservoirs of the Southeastern Sichuan Basin in China. Absolute U–Pb ages revealed two distinct calcite precipitation events at 6.89 ± 0.18 and 3.55 ± 0.11 Ma, respectively. The primary mechanism for shale gas escape from early Silurian shale reservoirs was the leakage of natural gas through activated high-angle faults that had remained unsealed since at least 6.89 Ma. At approximately 3.55 Ma, the overlying weathered layer was lifted to the surface of the Earth, facilitating the connection between meteoric water and shale reservoirs through activated fractures. Natural gas leakage occurred concurrently with meteoric water infiltration, and a non-linear mixing effect between methanogenesis and meteoric water, as well as intense ventilation along the water-bearing fracture surface, is identified as the cause of delayed late-fracture calcite precipitation, leading to an abnormal δ13C of calcite vein (up to +22.1‰). This approach holds significant implications for assessing natural-gas preservation in uplifted basins worldwide.