Position-specific carbon isotope compositions of propane as a novel proxy for tracing sealing integrity of shale systems

Shale sealing capacity critically governs hydrocarbon retention and subsurface storage security, yet robust geochemical proxies for its evaluation remain underdeveloped. Our study investigates the position-specific carbon isotope compositions (PSICs) of propane as a novel indicator of sealing efficacy through comparative hydrous pyrolysis experiments (closed versus semi-closed systems) on the Triassic Chang-7 shale of the Ordos Basin, China, and isotopic analysis of Silurian shale gases from the Weiyuan and Changning areas, south China. The experimental results demonstrate distinct PSIC evolution patterns under two contrasting sealing conditions: the closed systems exhibit a ΔC-T (central-to-terminal carbon isotope difference) peak of 19.3‰ at high maturities followed by a decline probably due to hydrogen abstraction, while the semi-closed system shows a monotonic ΔC-T increase to 12.1‰, reflecting suppressed secondary cracking from hydrocarbon expulsion. Linear regressions of PSIC versus bulk δ13C3 reveal sealing-dependent isotopic fractionations with a steeper slope of the δ13C3 versus δ13C of the central carbon (δ13Cc) correlation in the semi-closed systems (1.69 ± 0.20) compared to 1.31 ± 0.11 in the closed system, indicating enhanced isotope heterogeneity. The natural shale gases corroborate these trends: Weiyuan samples (ΔC-T = −2.3‰ to +2.9‰, with a slope of δ13C3 versus δ13Cc correlation = 1.57) align with the semi-closed dynamics, whereas the Changning samples (ΔC-T = −3.2‰ to −2.4‰, slope = 1.34) mirror more confined signatures. These findings establish propane PSIC, particularly δ13C3-PSIC correlations, as a robust geochemical tool to discriminate shale sealing capacities, offering critical insights into hydrocarbon preservation in low-permeability systems.