Comparing natural and artificial thermal maturation of a Type II-S source rock, Late Cretaceous, Israel

Abstract

Marine source rocks that are sulfur-rich (i.e., Type II-S kerogen) start to mature earlier than their sulfur-lean equivalents (Type II). This affects the amount of petroleum generated with the increase in thermal maturation. However, the evolution of HI with Tmax upon thermal maturation is not described quantitatively (or semi-quantitatively) for Type II-S kerogen. Here we explore the natural thermal maturation of the Ein Zeitim/Ghareb Formation (Late Cretaceous) type II-S source rock in the Golan Basin, north Israel. This is done by comparing geochemical data on cuttings from the section in the Golan Basin with an artificial maturation experiment conducted on an immature analog core taken from the Shfela Basin, central Israel. Using bulk Rock-Eval analyses of unextracted and extracted samples, the natural and artificial maturation paths are found to be comparable and show sub-parallel trends for the relative decrease in hydrogen index (HI) and the increase in the normalized bitumen content (mgBitumen/gTOC) with increasing thermal maturation (measured as Tmax). This comparison suggests that maturation in the Golan Basin increases with depth across the ~350 m thick section from an early maturation state to the peak of bitumen generation. While the original HI (HIo) is estimated from the field data, the original S2 prior to maturation (S2o) is estimated based on the empirical relationship derived from the artificial maturation data trend in order to estimate hydrocarbon generation. Second-order differences between the artificial and natural maturation trends may be due to small differences in the sulfur content of the immature kerogens from the two basins (0.5–1 wt% from the kerogen), leading to an offset in the Tmax maturation scale. Thus, a lower-case scenario is considered to estimate lower values for S2o and the hydrocarbons generated across the source rock section. As sulfur in the immature kerogen has a substantial effect on the onset of maturation, we postulate that the maturation path presented here is more representative for sulfur-rich kerogens then that of the sulfur-lean Type II kerogens. However, variability in the sulfur content within sulfur-rich kerogens, will probably change the specific maturation path of a given kerogen.