Reflectance Analysis of Organic Matter in Early Paleozoic Organic-Rich Shales

Abstract

Organic maturity is typically defined in terms of vitrinite reflectance (VR). In pre-Devonian strata devoid of vitrinite, the reflectance of the other organic matter is used as a proxy. Graptolites, where they exist, are often used as that proxy with a graptolite reflectance (Grap_RO) value being defined before it is transformed to vitrinite reflectance equivalent (VRe). Often, this Grap_RO value is determined from relatively few reflectance values obtained from a polished rock surface. The frequent inability to confidently identify the organic component — since graptolites, chitinozoans and massive Brownish Organic Matter (BOM) can look very similar in reflected light — means that the values are reported as Grap_RO from “graptolite-like material.” The use of polished grain mounts of isolated organic material rather than polished rock surfaces permits analysis of the morphological features of an organic component along its length no matter what its orientation, thereby enabling correct component identification. Additionally, a large number of individual reflectance values (n=50) can be obtained for each organic component type. This poster illustrates knowledge gained during the application of this process to a proven hydrocarbon sourcing pre-Devonian organic-rich shale. Application of the detailed results has been presented elsewhere (Hayton et al., 2018). In using isolated organic material grain mounts, two things become apparent: a) The different organic matter types have very different average reflectance values, so correct component identification is essential for obtaining a true specified organic component _RO value. b) Evidence for significant reworking and degradation was observed in these samples, meaning that the average reflectance data did not reflect the true in-situ maturity and that a large number of reflectance values are required to differentiate reworked and in-situ populations. In trying to determine the VRe from in-situ Grap_RO it was realized that significant variability exists in the conversion ratios used by various service companies and within the published literature. This is due to a number of factors including:  The use of averaged RO values from the total graptolite-like material population.  The conversion from VR to VRe for graptolites is via bulk geochemistry Tmax values, which in themselves are a weighted average of the characteristics of all the organic component populations within a sample. These may not necessarily provide a true reflection of in-situ maturity where significant reworking has occurred or significant variability in the organic component composition exists.

AAPG Datapages/Search and Discovery Article #90333©2018 AAPG Middle East Region, Shale Gas Evolution Symposium, Manama, Bahrain, December 11-13, 2018