Indication of natural gas migration and accumulation by three-dimensional fluorescence characteristics of gas-field waters: A case study of the Permian-Triassic in the central-western Sichuan Basin

Permian-Triassic natural gas migration paths and accumulation patterns in the central-west Sichuan region have been studied by way of examining the three-dimensional shape, light intensity excitation and fingerprint characteristics of aromatic hydrocarbons fluorescence spectra of gas-field waters. The Permian-Triassic gas-field waters in the central-western Sichuan Basin have (condensate) gas reservoir characteristics, where three subclasses (base peak, double peak and odd peak, but mainly the stable type of base peak subclass) and seven types of three-dimensional fluorescence characteristics have been identified. Three-dimensional fluorescence characteristics of the waters of the Permian-Middle and Lower Triassic differ greatly from that of the Upper Triassic Xujiahe Formation. The former is mainly influenced by hydrocarbons from high to over mature Lower Paleozoic marine shales and the Upper Permian Longtan Formation coals, but the latter is mainly influenced by its own source rock maturity differences and organic matter type difference. Research on the three-dimensional fluorescence characteristics indicates that the Upper Permian and Middle to Lower Triassic reservoirs in the Moxi structure are predominately dominated by vertical migration and accumulation. Gas in the reservoirs below the Leikoupo Formation originates mainly from the Longtan Formation coals, while the Leikoupo Formation gas reservoir includes gas from a gypsum-salt rock section. Gas in the Xujiahe gas reservoir in the Pingluoba structure is mainly migrated from central-southern faults and the reservoirs have strong lateral heterogeneity.