Noble gases in Paleozoic shale fluids document tectonic events and fluid migration in the Upper Yangtze Block

Major and noble gases of natural gas extracted from the low-permeability Paleozoic Wufeng-Longmaxi shale were measured to reconstruct the multi-stage, spatially varying tectonic evolution of the Upper Yangtze Block, China, one of the oldest parts of the Earth continents. The high gas dryness ratio [C1/(C2 + C3)] and high carbon isotopic ratios (δ13C-C1, δ13C-C2, δ13C-CO2) suggest a late mature thermogenic origin of shale gas. The highly fractionated atmospheric 20Ne/36Ar and 84Kr/36Ar ratios in our gas samples suggest they result from solubility-based partitioning of noble gases between oil and water followed by gas-water partitioning. Calculated volume ratios of oil, water, and gas phases vary spatially and temporally. In particular, the western Yangtze Block shows a lower reconstructed oil/water ratio, suggesting oil leakage promoted by the Triassic exhumation of Paleozoic shale, while a low gas/water ratio in the central-eastern Yangtze Block suggests gas leakage promoted by basin-wide Jurassic fold-thrust faulting. The lowest C1/36Ar volume ratio around faults at the basin edges indicates extensive gas expulsion. Delineated radiogenic 4He in gas samples are several orders lower than calculated in-situ produced radiogenic 4He, likely suggesting widespread 4He loss. Spatially-varying 4He/nucleogenic 21Ne ratios in the shale indicated that 4He loss in the western Yangtze Block predated that in the central-eastern portion. Such He loss was also coupled with the Triassic exhumation and the Jurassic fold-thrust faulting episodes. In summary, noble gas in pore fluids extracted from low-permeability shale can preserve reliable records of tectonic events produced during upper crust evolution.