An explanation for the positive organic carbon intercepts on Rock-Eval S2 versus toc plots : an example from Saudi Arabia

Abstract EN

During the course of screening samples for source rock quality by measurement of total organic carbon (TOC) and/or Rock-Eval pyrolysis, samples with poor source rock parameters (TOC of <0.5 to 1.0%, S2 yields of <1.0 to 2.0 mg HC/g rock) are often not analyzed by more sophisticated techniques.

This effectively results in dealing with “selected” samples for more detailed interpretations of a source rock suite within a formation.

Often, kerogen within a formation or a group of apparently related samples (e.g., samples of the same lithology) exhibits heterogeneity.

This heterogeneity may be caused by changes in the depositional environment, such as variation between oxic and anoxic regimes, and may go unrecognized if data are collected only on samples above a threshold TOC or pyrolytic yield.

When samples from an entire source rock unit (both non-source and source) are screened for organic richness and respective pyrolytic potential, and then plotted on an S 2 versus TOC diagram, they usually show a positive intercept on the TOC axis.

Several causes for this intercept have been advanced, including mineral matrix effects and a mathematical origin.

Results from this study advance a third possibility: an underlying inert to gas-prone or a variably mixed component present in many kerogens which is manifested only in the low to moderate TOC samples.

This means that we may now have developed a simple method for the evaluation of reactive (oil-prone and gas-prone components) versus non-reactive (inertinitic) kerogens from a single formation.

Additional support for our conclusions is derived from two independent techniques: visual kerogen assessment and iron-sulfur-carbon relationships.