Thermal maturity calibration of extremely high-mature pre-Devonian strata: A case study from the Lower Cambrian Qiongzhusi Formation in the Sichuan Basin, South China

Abstract

Thermal maturity assessment of pre-Devonian strata is challenging due to the absence of humic vitrinite, and conventional organic geochemical methods (i.e., Rock-Eval Tmax, atomic H/C, biomarker parameters) are usually ineffective for the highly-over mature source rocks. The Lower Cambrian Qiongzhusi Formation shale in the Sichuan Basin, as the main high-quality source rock, is an important exploration target for shale gas. In this study, optical reflectance and Raman spectroscopy measurements were performed on eight Qiongzhusi Formation core samples from well JY1 in conjunction with basin modelling to calibrate to equivalent vitrinite reflectance and constrain thermal maturity. The results reveal solid bitumens/pyrobitumens as the predominant organic constituents in the Qiongzhusi Formation. As burial depth increases from 3,300m to 3,600m, the solid bitumen reflectance acquired from optical measurement and Raman calculation increase from 2.53% to 3.86%, and from 2.88% to 3.49%, respectively, along with increasing optical anisotropy. The Raman-calculated solid bitumen reflectance data are consistent with the optical reflectance data at depths <3,500m, while the former become lower than the latter at depths >3,500m and are consistent with reflectance values constrained by basin modelling. We propose that the surface quality and granularity of solid bitumen, hydrothermal fluid alteration, and varied thermal conductivity of mineral matrix result in the higher optical reflectance of solid bitumen occurrence in the Qiongzhusi carbonates. The multidisciplinary calibration approach integrating optical reflectance measurement, Raman spectroscopy and basin thermal modelling is considered effective and more accurate for thermal maturity assessment.