Diagenetic thermal evolution of organic matter by Raman spectroscopy

Vitrinite reflectance is considered one of the most reliable indicators of thermal maturity in sedimentary basins and is widely used to calibrate numerical models that assess hydrocarbon generation/expulsion from source rocks. Nevertheless, ambiguities in vitrinite reflectance can occur from variations in composition and preservation state of organic facies, temperature/pressure conditions and the abundance of organic matter resulting in incorrect or uncertain assessments of petroleum systems. This study presents an original application of Raman spectroscopy to assess the diagenesis - catagenesis of kerogen between the immature and mid-mature stages of hydrocarbon generation. Kerogens were isolated from 33 intervals of drill cuttings from a 5 km thick Oligocene-Miocene siliciclastic section of the Malembo Formation (Lower Congo Basin, Angola). Their Raman spectra were obtained and derived parameters were compared to depth and to previously reported equivalent vitrinite reflectance values. The best correlations between thermal maturity and Raman parameters were found for D-G band distance; FWHM of the G band; D/G area ratio; RA2 ratio (calculated as S + Dl + D/Dr + Gl + G band ratio) and the D/G width ratio. The Raman parameters were not influenced by varying kerogen composition (mixed type II/III). The technique offers the potential to reduce the risk of source rock thermal maturity assessment even when organic facies rich in amorphous organic matter are present.