Maturity Assessment of the Lower Cambrian and Sinian Shales Using Multiple Technical Approaches

Abstract

The Lower Cambrian Niutitang and Sinian Doushantuo shales are the most important and widespread source rocks and target layers in South China. Reliable data of the thermal maturity of organic matter (OM) is widely used to assess hydrocarbon generation and is a key property used in determining the viability and hydrocarbon potential of these new shales. Nevertheless, traditional thermal maturity indicators are no longer suited to the vitrinite-lack marine shales. This study aims to combine high throughput Raman and infrared spectroscopy analysis to confirm and validate the thermal maturity in comparison with the bitumen reflectance (Rb). Raman parameters such as the differences between the positions of the two bands (VG-VD) are strong parameters for calculating the thermal maturity in a large vitrinite reflectance (Ro) ranging from 1.60% to 3.80%. The infrared spectroscopy analysis indicates that the aromatic C=C bands and CH2/CH3 aliphatic groups both are closely correlated with thermal maturity. The calculated Ro results from Raman and infrared spectroscopy are in strong coincidence with the Rb. The relationships between Rb and pore volumes/surface areas (calculated from N2 adsorption) indicate that the sample with Rb of 3.40% has the lowest pore volumes and surface areas. Focused ion beam scanning electron microscopy (FIB-SEM) observations of OM pores indicate that Ro of approximately 3.60% may be an upper limit for OM porosity development. Obviously, kerogen Raman and infrared spectroscopy can indicate methods for reducing the risk in assessing maturity with practical, low-cost accurate results. Exploration of shale gas in the high maturity (>3.40%–3.60%) region carries huge risks.