Investigation of thermal maturity of lower Palaeozoic hydrocarbon source rocks by means of vitrinite-like maceral reflectance — a Tarim Basin case study

Evaluation of the maturity of Chinese lower Palaeozoic sediments has not been satisfactorily accomplished because of the absence of vitrinite and the relatively high level of maturity of the organic matter. In the Tarim Basin, one of the largest petroliferous basins in China, important lower Palaeozoic marine sediments contain a widely distributed type of maceral resembling vitrinite, especially characterised by syndepositional occurrence, elongate shape, homogenous texture and weak anisotropy. The term vitrinite-like maceral is used to describe the maceral in this paper because of insufficient evidence for its origin. Thirty-five lower Palaeozoic source rocks, mainly from five boreholes in the Tarim Basin, have been investigated by organic petrologic methods and a positive correlation between the vitrinite-like maceral reflectance and sample depth has been recognized. This correlation is better than that between bitumen reflectance and depth. Artificial maturation data show that the maturation pathway of vitrinite-like macerals is quite different from that of vitrinite. There is a distinct step in the development of reflectance of vitrinite-like macerals with increasing maturation. By combining the reflectance data from naturally and artificially matured samples, the relationship between vitrinite-like maceral reflectance and vitrinite reflectance can be divided into three stages which are well-represented by three linear regression equations. The equations provide a convenient method to evaluate the maturity of Lower Palaeozoic sediments by allowing an equivalent vitrinite reflectance to be derived from the measured vitrinite-like maceral reflectance.