Kerogen composition and maturity assessment of a solid bitumen-rich and vitrinite-lean shale: Insights from the Upper Jurassic Vaca Muerta shale, Argentina

Assessment of thermal maturity by solid bitumen reflectance measurements in vitrinite-lean or vitrinite-barren shales is widely used in unconventional shale gas and shale oil plays but the method is commonly challenged by the complexity in the measurements. The Upper Jurassic marine Vaca Muerta shale in the Neuquén Basin in western Argentina is both a prolific conventional source rock and a primary unconventional reservoir for oil and gas. The shale is rich in solid bitumen and lean or barren in vitrinite and offers an excellent opportunity to gain insight into thermal maturity assessment of a shale with a complex organic matter composition. The kerogen composition of a set of immature to highly mature Vaca Muerta shale samples was investigated by organic petrography and reflectance measurements were taken on vitrinite and solid bitumen. The results were integrated with available Rock-Eval data and stable carbon isotope values of shale gas (ethane and propane) collected at the same depth as the rock samples. The maceral composition of immature Vaca Muerta shale is highly sapropelic and oil-prone with Hydrogen Index values of 600 mg HC/g TOC or higher. The liptinite macerals consist primarily of alginite with minor amounts of inertinite and/or oxidized vitrinite. Indigenous vitrinite is generally scarce and in some samples absent. Vitrinite reflectance values range from 0.79%R_o to more than 1.94%R_o corresponding to early peak oil to dry gas maturity. Solid bitumen is abundant in all mature samples. Two general types of solid bitumen morphology were observed: (i) BR I, a dark reflecting porous variant disseminated in the clayey mineral matrix and (ii) BR II, a lighter reflecting more ‘dense’ textured variant. Bands of denser solid bitumen were commonly seen ‘flowing’ in more porous or weak zones in the matrix and around mineral grains. Degassing pores were frequently observed. Solid bitumen reflectance measurements on BR I and BR II provided two populations with different reflectance ranges. The correlation between vitrinite and BR II reflectances is considered to have highest confidence. The correlation can be expressed by the equation VR_eqv = (BR II + 0.3043)/1.026. The established VR_eqv-BR II correlation yields results comparable to other published correlations but also demonstrates significant discrepancy to others. This warrants caution when assessing the maturity of complex unconventional shale gas and oil plays by translating solid bitumen reflectance values into vitrinite reflectance equivalents using correlations not calibrated to that specific shale play. Both Hydrogen Index values and ethane and propane stable carbon isotope values of Vaca Muerta shale gas show strong correlations to VR_eqv derived from BR II. Thus, in cases with absence of vitrinite and questionable solid bitumen measurements reasonable maturities (vitrinite reflectance equivalents) can be inferred from these relationships.