Evaluating transformation of marine kerogens from Rock-Eval measurements: A. Derivation of a scaled thermal maturation path from laboratory maturation data

Abstract

Identification of kerogen type in source rocks and quantification of its transformation to hydrocarbon products upon thermal maturation are vital for petroleum exploration. These elements can be evaluated using the open pyrolysis of the Rock-Eval. Despite the importance of sulfur-rich marine kerogens (Type II-S) in petroleum systems, the maturation path of only Types I, II and III kerogens were characterized by the Rock-Eval parameters, and a comparison of the maturation path (defined as the HI-Tmax relation) for Type II and II-S is needed.A comparison of the HI-Tmax relation between Type II and II-S kerogens is presented here by compiling literature data of laboratory thermal maturation for six well-known source rocks. The source rocks include the Toarcian Shale, Kimmeridge, Barnett and Woodford for Type II and the Ghareb and Monterey for Type II-S. When comparing the HI-Tmax relation among the different source rocks, the extent of kerogen transformation cannot be readily evaluated because of variations in the immature properties of kerogens. This variation is overcome by normalizing the HI and scaling the Tmax of mature samples to their immature values. These operations create a new thermal maturation scale defined here as HIrelative-ΔTmax. While the suggested scale minimizes the impact of the heterogeneity within each kerogen type, it generates two distinctive maturation paths for each of them. Although Type II-S kerogens have a lower Tmax at the onset of maturation, the maturation path needed for full transformation is longer than that for Type II kerogens. The two maturation paths are statistically analyzed to generate an empirical law for each of them. Thus, the extent of marine kerogen transformation can be estimated using the empirical law and the suggested HIrelative-ΔTmax thermal maturation scale. The longer maturation path of Type II-S is hypothesized to result from enhanced thermal stabilization of the residual kerogen, driven by the rearrangement of sulfur moieties.