Integrated assessment of thermal maturity of the Upper Ordovician–Lower Silurian Wufeng–Longmaxi shale in Sichuan Basin, China

Abstract

Abstract The Wufeng-Longmaxi marine shale of the Upper Ordovician (O3w)-Lower Silurian (S1l) is the main source rock and target for shale gas exploration in Sichuan Basin, China. Maturity assessment is very important for shale gas evaluation, and such assessment is difficult because of the absent of vitrinite, which is the main objective for vitrinite reflectance analysis. Graptolite fragments and solid bitumens are the dominant forms of organic matter in the shales. In this article, optical reflectance analysis of solid bitumen and graptolite and Raman spectroscopy characterization of graptolite were investigated to reduce the levels of uncertainty in thermal maturity assessment. The aims were to (1) build the correlation between graptolite reflectance and solid bitumen reflectance, to (2) establish the relationship between graptolite reflectance and Raman spectroscopy, and (3) to re-build the maturity distribution of such gas shale in Sichuan Basin. 32 core samples and 22 outcrop samples have been collected for the optical reflectance analysis, and the mean graptolite random reflectance in Sichuan Basin ranges from 1.21% to 4.91%. The random reflectance of graptolite (GRo) is higher than that of solid bitumen (BRo), and graptolite random reflectance can effectively be used as a maturity indicator. Compared with graptolite maximum reflectance (GRomax), graptolite random reflectance is more precise owing to smaller standard deviation. A natural evolution of maturity series of graptolites have been chosen for Raman spectroscopy analysis. Based on the previous studies of the relationship between Raman spectroscopy and vitrinite reflectance, the correlations between the graptolite random reflectance and vitrinite reflectance (VRo) for over maturity stage were obtained through characteristic of the Raman spectroscopy. The results indicate relationship between GRo and VRo is not a single linear relationship. Through the calculation, the maturities of this gas shale interval range from 1.16% to 3.63%, which provides the basic important information for shale gas evaluation in Sichuan Basin.