A new approach to interpreting Rock-Eval S2 and TOC data for kerogen quality assessment

A simple method for application in source potential mapping is used to assess the original oil and gas potentials in source rock horizons based upon Rock-Eval potential (S) and total organic carbon (TOC) values. The method assumes that kerogens consist of mixtures of end-members with assigned hydrogen index values. Based on suggested algorithms, the average amounts of oil-prone, gas-prone and inert organic material over source rock intervals are determined in TOC units. The method uses regression lines from plots of remaining hydrocarbon potentials (S-2) versus total organic carbon (TOC), and "quick-look" transparent overlays are used to read the appropriate kerogen mixture. Mineral matrix effects during pyrolysis, when strong, can cause erroneous results. This effect which occurs for oil-prone kerogens and adsorptive minerals can cause problems particularly for lean samples (S-2 = 0-3 mg HC/g rock) whilst the errors for richer samples are less. The method is applied on three sections of Upper Jurassic organic-rich rocks from the Danish North Sea sector, which are at different maturity stages. One of these sections is dominated by gas-prone material, one is dominated by oil-prone material and the third section contains a mixture of oil- and gas-prone material. The method has been compared with other methods that split kerogens in oil and gas generating potential and has given reasonable results. Experience using the method and a presented example suggest that sedimentological, system tract information may be derived from S-2 to TOC cross-plots. A constructed modelling example suggests that the end-member concept used in this approach may be used in forward type source rock prediction models when combined with sedimentological models. The resulting S-2-TOC plots can be used in order to check the forward modelling results against observed values. (C) 2004 Elsevier Ltd. All rights reserved.