Natural Gas Geochemistry and Paleoenvironmental Controls on Hydrocarbon Enrichment in the Western Slope of the Songliao Basin

ABSTRACT

The Western Slope of the Songliao Basin is a key frontier for hydrocarbon exploration, but the origin of natural gas accumulations and the depositional controls on deep Permian source rocks remain poorly constrained. This study combines gas compositional and stable carbon isotope data from 66 wells with elemental geochemical data from 35 core samples recovered from well MKD1 to characterise both the genetic types of natural gases and the paleoenvironmental conditions governing source rock development. Gas geochemical analyses show the recovered gases are dominated by CH ₄ , with concentrations of 74.89% to 97.41%, and generally low CO ₂ contents. Carbon isotope signatures indicate CH ₄ occurs mainly as oil‐associated thermogenic gas and secondary cracking product, with primary microbial gas restricted to the Western overlap zone. CO ₂ is predominantly of inorganic origin, though carbon isotope data alone cannot uniquely discriminate carbonate decomposition from other potential inorganic sources. Paleoenvironmental reconstructions from the MKD1 core section record a systematic transition through the Middle–Upper Permian succession, shifting from saline, reducing marine settings to transitional and lacustrine fresh to brackish water conditions. Organic matter enrichment is governed primarily by strongly reducing bottom‐water conditions, humid paleoclimate and elevated paleoproductivity, rather than direct salinity forcing. These depositional conditions favoured the development of Type II kerogen, and subsequent thermal cracking of this kerogen accounts for the oil‐associated thermogenic gas signatures observed in overlying Cretaceous reservoirs. These findings provide a basis for evaluating deep hydrocarbon potential across the western Songliao Basin.