Correction criteria for the suppression of vitrinite reflectance in hydrogen-rich kerogens: preliminary guidelines

Abstract

Thermal maturity is an important factor when assessing the petroleum potential of a basin or particular source rocks. Although the vitrinite reflectance (VR) technique provides excellent results from coal samples, it has limitations when source rocks are used. One problem is the anomalously low VRs (the suppression of VR, VR suppression) in hydrogen-rich kerogens. The anomalously low (suppressed) VRs discussed in this article are mainly caused by the problems that organic petrographers encounter in finding and measuring appropriate vitrinite particles in hydrogen-rich kerogens. VR suppression is recognized when lower-than-normal VRs are measured on exinite-rich (or amorphous-rich) intervals in a VR-well profile (VR vs depth). It can also be recognized when anomalously low VRs, relative to the regional coalification (maturation) pattern, or to regional oil/gas occurrences, are encountered in an interval. Although VR suppression in organic-rich rocks, coals and oil shales is well documented, guidelines for its correction are lacking. A preliminary model (Fig. 1) is presented to help correct the VR suppression in hydrogen-rich kerogens. Measured (suppressed) VR (in % R o) along with original (when immature) hydrogen index (HIo, estimated via Rock-Eval pyrolysis and/or organic petrography data) of a sample can be used to estimate the maximum true (unsuppressed) VR. Although the VR and HIo data for an individual sample is sufficient to estimate the maximum true VR we recommend the use of data from several closely-spaced samples. The model was tested with examples from published data and found to reasonably correct VR suppression in hydrogen-rich kerogens. We plan to refine the model using data from rock samples (suppressed-unsuppressed pairs) and pyrolysis data.