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The geochemical characteristics of saturated and aromatic hydrocarbons from different formations and lithologies provide ob-vious evidence for transgressions that occurred during Upper Triassic Xujiahe stage in Sichuan Basin with a great impact on the source input and depositional environment.A clear dual peak distribution for normal alkanes and obvious abundant com-pounds sourced from bacteria and algae in whole oil gas chromatogram indicates the abundance of lower organisms input.The ratio of Pr/Ph is low,ranging from 0.33 to 0.86 with an average of 0.60,quite different from Pr/Ph >2.0 for coal measures in swamp environment,representing source rocks from saline lake or marine facies.In the gas source rocks extracts,abundant β-carotane,-carotane,and their degradated series were detected in the whole oil chromatogram,indicating a reducing envi-ronment.The concentrations of methyl steranes and dinosteranes are high.The content of polycyclic aromatic sulfur heterocy-cles(PASH) is relatively higher in aromatic fraction and the assemblage of fluorene,dibenzofuran,and dibenzothiophene is different from the typical saline lake and the regular swamp facies source rocks,manifesting the transgression effects on gas source rocks.
The geochemical characteristics of saturated and aromatic hydrocarbons from different formations and lithologies provide ob-vious evidence for transgressions that occurred during the Upper Triassic Xujiahe stage in Sichuan Basin with a great impact on the source input and depositional environment. A clear dual peak distribution for normal alkanes and significantly abundant com-pounds sourced from bacteria and algae in whole oil gas chromatogram indicates the abundance of lower organisms input. The ratio of Pr / Ph is low, ranging from 0.33 to 0.86 with an average of 0.60, quite different from Pr / Ph > 2.0 for coal measures in swamp environment, representing source rocks from saline lake or marine facies. The gas source rock rocks extracts, abundant β-carotane, -carotane, and their degraded series were detected in the whole oil chromatogram, indicating a reducing envi -ronment. The concentrations of methyl steranes and dinosteranes are high. The content of polycyclic aromatic sulfur heterocycles (PASH) is relati vely higher in aromatic fraction and the assemblage of fluorene, dibenzofuran, and dibenzothiophene is different from the typical saline lake and the regular swamp facies source rocks, manifesting the transgression effects on gas source rocks.