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

Abstract

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.