Thermal maturity assessment of organic matter in the Precambrian–Lower Paleozoic high-over matured marine shale reservoirs: A review

Thermal maturity is one of key parameters for source rock evaluation, which affects not only the effectiveness of hydrocarbon generation and the properties of hydrocarbon but also the formation, evolution and extinction of organic matter-hosted pores in shale reservoirs. Thus, accurate assessment of thermal maturity is of great significance to the evaluation of conventional source rocks and gas-bearing properties of unconventional shale reservoirs. However, thermal maturity assessment of the Precambrian–Lower Paleozoic marine shale deposits has been challenging for several decades. On one hand, the widely used gold standard-vitrinite reflectance (VR_o) cannot be used for maturity assessment due to the absence of vitrinite in the pre-Devonian strata. On the other hand, traditional organic geochemistry approaches (e.g., T_max, biomarkers) cannot be used for maturity assessment of marine shale deposits at the high-over matured stages. In this review, four groups of macerals, including solid bitumen, vitrinite-like maceral, zooclasts and liptinite, can be identified in the Precambrian–Lower Paleozoic sediments, and optical characteristics and origins of these macerals have been summarized. Furthermore, various proxies of thermal maturity assessment are systematically investigated, providing potential solutions for accurate assessment of thermal maturity of the Precambrian–Lower Paleozoic high-over matured marine shale reservoirs. However, accurate identification of different macerals (e.g., solid bitumen, vitrinite-like maceral, graptolite) is still challenged, and moreover, the use of optical reflectance of these macerals is hindered due to optical anisotropies and various conversion relationships that converted their reflectance to equivalent vitrinite reflectance (EqVR_o). In addition, Raman spectrometry is a rapid and non-destructive approach to evaluate thermal maturity of organic matter, and several Raman parameters, including FWHM-G, RBS and I_D/I_G ratio, can be used as the reliable maturity indicators, which overcome the influence of optical anisotropy on the optical indicators. Nevertheless, the application of Raman spectroscopy has been limited due to the lack of uniform test standards (e.g., experimental conditions) and peak fitting methods. In a word, every maturity proxy has its suitable range of application and limitations, so a variety of maturity indicators are essential to comprehensively evaluate thermal maturity of organic matter.