NEW APPROACHES TO DETECTION AND CORRECTION OF SUPPRESSED VITRINITE REFLECTANCE

Abstract

Vitrinite reflectance is commonly used as an indicator of thermal history and the likely maturity of source rocks. However, comparison of reflectance with other thermal indicators, and assessment of downhole gradients, indicate that vitrinites sometimes have anomalously low reflectance. This phenomenon is referred to as suppression, and can affect vitrinite in both marine and terrestrial sedimentary rocks, and in coal seams. For coals it is possible to diagnose and correct reflectance suppression using bulk chemistry; however, dispersed organic matter in sedimentary rocks requires a petro- logical approach. Therefore, a new technique has been developed combining reflectance with quantitative vitrinite fluorescence (VRF). A VRF plot allows objective differentiation of sedimentary Type III and IV organic matter into suppressed vitrinite, normal vitrinite, recycled vitrinite, and inertinite, based on graphical relationships. This takes the guesswork out of vitrinite identification and overcomes the problem of suppression. Measurements are made in a nitrogen atmosphere to avoid fluorescence alteration. Examples of VRF plots show that fresh samples are not essential for vitrinite fluorescence, as previously assumed.