Organic petrology and geochemistry evaluation of upper Permian source rocks in the Lower Yangtze Area

The Upper Permian strata in the Lower Yangtze area represent a promising unconventional resource for exploration. Employing organic petrology and organic geochemistry analysis, this study investigates the characteristics of organic matter abundance, types, maturity, and sedimentary environments within the Upper Permian Longtan and Dalong Formations of drilling wells and outcrop rock samples in the Lower Yangtze area, in both vertical and planar dimensions. Comparative and comprehensive evaluations between offshore and onshore regions are conducted. A range of analytical methods is utilized, including total organic carbon content analysis, rock pyrolysis, organic matter maturity assessment, gas chromatography-mass spectrometry analysis, and kerogen carbon isotope analysis. The results reveal that the Upper Permian source rocks in the Lower Yangtze area exhibit qualities ranging from moderate to good, with the Longtan Formation outperforming the Dalong Formation and onshore regions surpassing offshore ones. In terms of organic matter abundance, the TOC content and hydrocarbon generation potential of source rocks in the Longtan and Dalong Formations reach good to very good levels. Analysis of carbon isotopic signatures and molecular geochemical characteristics indicates that the Longtan Formation source rocks contain Type II-III kerogen. The Dalong Formation exhibits superior source rock quality compared to the Longtan Formation, with significant contributions from Type I kerogen. The distinct kerogen types between these two source rock units are primarily controlled by differences in sedimentary architecture and depositional environments. These source rocks were deposited in a poorly oxygenated environment characterized by anoxic to suboxic conditions. Notably, the Dalong Formation source rocks show significant variations in biological inputs, including substantial contributions from lower aquatic organisms, while the Longtan Formation was deposited under more dynamic water conditions. Cross-validation using multiple maturity indicators, including equivalent vitrinite reflectance, dibenzothiophene ratios (DBT), Ts/Tm, and C₃₁ 22S/(22S + 22R), demonstrates that the maturity of the Upper Permian source rocks mainly falls within the range of 1.3-2.0%, reflecting a high thermal evolution stage with significant spatial heterogeneity. Comprehensive analysis of the available sample data indicates that the Upper Permian source rocks in the Lower Yangtze area possess the essential material foundation for forming large oil and gas fields. The Dalong Formation, in particular, represents a suite of high-quality potential source rocks, providing significant guidance for future petroleum exploration.