Geochemical and stable carbon isotope composition variations of natural gases in tight sandstones from the West Sichuan Basin, China

Abstract

Recent exploration has revealed the great potential of gas resources, complicated gas distributions and geochemical characteristics of natural gases in tight sandstones of the West Sichuan Basin, China (WSBC). The chemical, stable carbon and light hydrocarbon compositions of natural gases from WSBC were investigated to assess the genesis, sources, and migration pattern leading to variations in the geochemical and stable carbon isotope compositions. The analysis shows that the natural gas in the Jurassic tight sandstones occurs as dry gas or wet gas in some places; however, most of the gas in the deep Triassic tight sandstones is apparently dry gas. Carbon isotope distributions increased in the order δ13C1 < δ13C2 < δ13C3 < δ13C4. The light hydrocarbon content increased in the order of chain (i.e. non‐cyclic) alkanes, cycloalkanes, and aromatic hydrocarbons. Relatively high ethane carbon isotope values and methylcyclohexane contents indicated the presence of lacustrine coal‐formed gases. The Jurassic natural gas is in a lower thermally mature stage and possibly originated from a mixture of T3X3 and T3X5 black mudstones. The deep Triassic natural gas was possibly originated in the deep source rocks because the gas has a relatively higher thermal maturity. Positive correlations between burial depth and methane content, dryness coefficient, δ13C1, and δ13C1–δ13C2 for the deep Triassic gas suggest that the migration distance was short and the source was nearby or the gas accumulated in situ. The chemical compositions and geological studies revealed that the fractionation of the Jurassic natural gases in a tight reservoir was predominantly caused by vertical migration rather than the maturity of the local source rocks. Copyright © 2016 John Wiley & Sons, Ltd.