Characteristics and migration mechanisms of natural gas in tight sandstone reservoirs in the Longfengshan sag, Songliao Basin, China.

Abstract To better understand gas migration and accumulation in tight sandstone reservoirs, we conducted an integrated petrographic and geochemical analysis of hydrocarbon and aqueous fluid inclusions. This analysis was combined with the burial and multistage hydrocarbon charging histories of the reservoir. The variability in the types of hydrocarbon inclusions in the tight sandstone reservoirs can be mapped to identify phase fractionation during burial and migration. The oil-bearing inclusions resulted from the heterogeneous capture of oil generated during the first hydrocarbon charging period, when the sandstones had not yet reached tight reservoir conditions. The gas-bearing inclusions in the tight sandstone reservoirs formed during the phase fractionation of dissolved and free gas due to unstable pressure conditions. The gas-bearing inclusions are characterized by a varying vapor volume fraction (φ vap). A higher proportion of gas-bearing inclusions with φ vap < 25% was observed near the source rock with TOC content >1.2%. In contrast, in structural highs, a higher proportion of gas-water inclusions with φ vap > 25% was observed, demonstrating that phase fractionation occurred during migration. The GOI data suggest that the dissolved gas migration mainly occurred in the reservoir near the source rock with TOC content >1.2%. The subsequent free gas separation in relation to upward migration and strata uplift caused free gas to migrate farther into the tight sandstone reservoir. Additionally, the gases displayed benzene/n-hexane ratio, methane and ethane δ13C values increasing initially, then decreasing along the migration pathway, recording the phase state evolution of the natural gas during the burial history of the reservoirs. Highlights • The paleopressure evolution has been reconstructed. • The geochemical fractionation effects in high and low surplus pressure zone are investigated. • Phase fractionation during the burial history is revealed by fluid inclusion petrography and microthermometry. [ABSTRACT FROM AUTHOR]