Maturity assessment of solid bitumen in the Sinian carbonate reservoirs of the eastern and central Sichuan Basin, China: insights from FTIR and Raman spectroscopy analyses

Maturity assessment of solid bitumen is significant for the construction of the thermal evolution history of Sinian carbonate reservoirs in the eastern and central Sichuan Basin because of the absence of vitrinite. Occurrence characteristics of solid bitumen in the Sichuan Basin were investigated by petrography observation. Based on Fourier transform infrared (FTIR) spectroscopy and Raman spectroscopy analyses of solid bitumen, the thermal maturity of solid bitumen in the Sinian reservoir was evaluated, and the thermal history of the Sinian reservoirs in the eastern and central Sichuan Basin was further reconstructed. The results show that solid bitumen occurred within intercrystalline pores, intercrystalline dissolution pores, karst caves, fractures, and stylolites in the eastern and central Sichuan Basin. The FTIR spectrums of solid bitumen are characterized by depleted aliphatic carbon and C=O group, and enrichment of aromatic carbon. The FTIR spectrums of solid bitumen indicate that the thermal maturity of solid bitumen in the Sichuan Basin exceeds at least 1.3%. The calculated Ro of solid bitumen in the eastern Sichuan Basin ranges from 3.8 to 4.09%, that of solid bitumen in the central Sichuan Basin ranges from 3.51 to 3.77%, and that of bitumen inclusions in the central Sichuan Basin varies from 3.54 to 3.64% inferred from Raman spectroscopy analysis. The thermal evolution history of Sinian reservoirs in the eastern and central Sichuan Basin can be divided into two heating–cooling stages. At the end of the second heating stage, that is, the Late Cretaceous, the Sinian reservoir reached the highest temperature of 250 °C in the eastern Sichuan Basin and 225 °C in the central Sichuan Basin.