Assessment of thermal maturity in Lower Cambrian organic-rich shale in south China using integrated optical reflectance and Raman spectroscopy of pyrobitumen

Abstract

It is challenging to accurately determine the thermal maturity of Lower Cambrian overmature shale, which is widely distributed in South China. In this study, we report an integrated method of measuring optical reflectance and Raman spectroscopy of pyrobitumen to determine the thermal maturity of Lower Cambrian organic-rich shales that contained a highly aromatized organic matter chemical structure. The optical reflectance measurement was conducted on large-particle-size angular pyrobitumen to eliminate the interference of clay mineral matrix, and the Raman spectrum analysis is a statistical response of a highly thermally evolved pyrobitumen chemical structure. Raman analysis compensates for the limitation of optical anisotropy interference in optical reflectance measurements. There is a good agreement between optically measured pyrobitumen reflectance (PBRo) and pyrobitumen reflectance calculated by Raman parameters (RmcPBRo), reflecting only minimal effects of optical anisotropy on the average optical reflectance (PBRo) values of pyrobitumen with fine or irregular mosaic texture, further indicating reliability of this integrated method for analyzing overmature shale. Thermal maturity varies significantly in the Lower Cambrian organic-rich shales, ranging from 2.60 to 2.92% EqVRo in the western Hubei province (WHBP), 2.98–3.49% EqVRo in the southern Shaanxi province (SSXP), 3.20–3.38% EqVRo in the southern Sichuan province (SSCP) and 4.06–4.31% EqVRo in the southeastern Chongqing (SECQ). The degree of aromatization and the structural order of organic matter (OM) are abruptly intensified when thermal maturity reaches about 3.5–3.7% EqVRo, as indicated by the deconvolution of the D and G bands in the Raman spectrum of pyrobitumen. The abrupt change in chemical structure of OM provides evidence for further dividing the early and late stages of overmaturation at a boundary of 3.5–3.7% EqVRo. The Lower Cambrian organic-rich shales distributed in the Cambrian shale gas breakthrough areas are normally at the early stage of overmaturation (2.0% < EqVRo < 3.5–3.7%). Development of paleo-uplifts in the Cambrian shale gas breakthrough areas led to relatively shallow burial depth of Lower Cambrian shale distributed on the peripheral areas of paleo-uplifts and created favorable shale gas preservation condition with only rare and small-scale faults.