Abstract 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 Neuquen 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%Ro to more than 1.94%Ro 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 VReqv = (BR II + 0.3043)/1.026. The established VReqv-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 VReqv 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.

中文翻译：

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固体富沥青和贫镜质页岩的干酪根成分和成熟度评估：来自阿根廷上侏罗统 Vaca Muerta 页岩的见解

摘要 通过固体沥青反射率测量来评估贫镜质或贫镜质页岩的热成熟度在非常规页岩气和页岩油区中得到广泛应用，但该方法通常受到测量复杂性的挑战。阿根廷西部内乌肯盆地的上侏罗统海相 Vaca Muerta 页岩既是多产的常规烃源岩，也是主要的非常规油气储层。页岩富含固体沥青和贫或贫镜质，为深入了解具有复杂有机质成分的页岩的热成熟度评估提供了绝佳机会。通过有机岩相学研究了一组未成熟到高度成熟的 Vaca Muerta 页岩样品的干酪根成分，并对镜质体和固体沥青进行了反射测量。结果与可用的 Rock-Eval 数据和在与岩石样品相同深度收集的页岩气（乙烷和丙烷）的稳定碳同位素值相结合。未成熟 Vaca Muerta 页岩的基质成分具有高度腐泥性和油性，氢指数值为 600 mg HC/g TOC 或更高。liptinite macerals 主要由藻酸盐和少量惰性和/或氧化的镜质组成。本土镜质体通常稀缺，并且在一些样品中不存在。镜质体反射率值范围从 0.79%Ro 到超过 1.94%Ro，对应于早期的石油到干气成熟度高峰。所有成熟样品中都含有大量固体沥青。观察到两种一般类型的固体沥青形态：(i) BR I，一种散布在粘土矿物基质中的暗反射多孔变体和 (ii) BR II，更轻的反射更“密集”的纹理变体。密度较大的固体沥青带通常在基质中和矿物颗粒周围的多孔或薄弱区域中“流动”。经常观察到脱气孔。BR I 和 BR II 上的固体沥青反射率测量提供了具有不同反射率范围的两个群体。镜质体和 BR II 反射率之间的相关性被认为具有最高的置信度。相关性可以用公式 VReqv = (BR II + 0.3043)/1.026 表示。已建立的 VReqv-BR II 相关性产生的结果与其他公开的相关性相当，但也表明与其他相关性存在显着差异。在评估复杂的非常规页岩气和油田的成熟度时，使用未针对特定页岩油藏进行校准的相关性将固体沥青反射率值转换为镜质体反射率当量，这需要谨慎。Vaca Muerta 页岩气的氢指数值以及乙烷和丙烷稳定碳同位素值都显示出与来自 BR II 的 VReqv 的强相关性。因此，在没有镜质体和有问题的固体沥青测量值的情况下，可以从这些关系中推断出合理的成熟度（镜质体反射当量）。