Comparative Analysis of Maturity between Solid Bitumen and Bitumen Inclusions in Marine Shale of the Sichuan Basin and Its Geological Significance

Organic matter maturity serves as a critical indicator for evaluating hydrocarbon generation potential of source rocks and shale gas preservation conditions. In recent years, laser Raman spectroscopy has been widely applied to study organic matter maturity in ancient marine shales due to its non-destructive nature and high resolution. Although scholars have established quantitative models for thermal evolution degree based on Raman spectral characteristics of solid bitumen in shales, significant research gaps remain in characterizing the maturity of bitumen inclusions within fracture-filling minerals. This study focuses on the Lower Cambrian Qiongzhusi Formation shale and bitumen inclusions in fracture-filling minerals from the Sichuan Basin. Through micro-laser Raman spectroscopy combined with peak deconvolution, maturity differences between bitumen inclusions and solid bitumen in shales are systematically compared, providing new perspectives for marine shale organic matter maturity evaluation. Results demonstrate that the calculated maturity of shale solid bitumen based on peak height ratio (ID1/IG) ranges from 3.32% to 3.64%, whereas bitumen inclusions range from 3.51% to 3.71% with lower data dispersion. At identical depths, bitumen inclusions generally exhibit higher maturity than solid bitumen, with absolute differences of 0.02%–0.22%, indicating that closed systems better preserve thermal evolution information of organic matter. Bitumen inclusions are more suitable for reconstructing organic matter thermal history, whereas solid bitumen maturity in shale should primarily serve as an evaluation index for source rocks.