Abstract
Thermal maturity is commonly assessed by various geochemical screening methods (e.g., pyrolysis and organic petrology). In this contribution, we attempt to establish an alternative approach to estimating thermal maturity with Raman spectroscopy, using 24 North American oil shale samples with thermal maturity data generated by vitrinite reflectance (VRo%) and pyrolysis (Tmax)-based maturity calculation (VRe%). The representative shale samples are from the Haynesville (East Texas), Woodford (West Texas), Eagle Ford and Pearsall (South Texas) Formations, as well as Gothic, Mancos, and Niobrara Formation shales (all from Colorado). The Raman spectra of disordered carbonaceous matter (D1 and G bands separation) of these samples were directly obtained from the rock chips without prior sample preparation. Using the Gaussian and Lorentzian distribution approach, thermal maturities from VR were correlated with carbon G and D1. We found that the Raman band separation (RBS) displayed a better correlation for equivalent VRe% than vitrinite reflectance VRo%. The RBS (D1–G) distance versus total organic carbon, free hydrocarbons from thermal extraction (S1), and the remaining hydrocarbon generating potential (S2) indicate that the RBS (D1–G) distance is also related to kerogen type. Data presented here from three methods of maturity determination of shale demonstrate that Raman spectroscopy is a quick and valid approach to thermal maturity assessment.
Highlights
Geochemical evaluation is a critical aspect in evaluating source rock reservoir plays, assessing the quantity, quality, and thermal maturity of sedimentary organic matter, and the implication for hydrocarbon generation, retention, and expulsion (Carvajal-Ortiz and Gentzis 2015)
The the maximum rate of hydrocarbon generation (Tmax) versus hydrogen index (HI) cross-plot of pyrolysis data showed that half the samples are II–III and III type and thermally mature to the postmature window which represents an oilto-gas generation condition
The samples used for the present study show a marked difference in thermal maturity between vitrinite reflectance and equivalent reflectance from Tmax
Summary
Geochemical evaluation is a critical aspect in evaluating source rock reservoir plays, assessing the quantity, quality, and thermal maturity of sedimentary organic matter, and the implication for hydrocarbon generation, retention, and expulsion (Carvajal-Ortiz and Gentzis 2015). The distance between G and D peaks is called Raman band separation (RBS), and it has been used as a reliable maturity indicator (Kelemen and Fang 2001; Marshall et al 2012; Wilkins et al 2014; Sauerer et al 2017; Schito et al 2017) Another important issue in recent works is that only the vitrinite reflectance is considered as the primary benchmark for correlation with Raman spectral data. In contrast with other studies, we examine here the correlation of different Raman spectral parameters data with maximum, minimum, and average vitrinite reflectance thermal maturity and equivalent vitrinite reflectance (derived from pyrolysis experiments Tmax value) Another unique aspect of our present study is that it focuses on the various types of kerogen materials—type II–III, type III, and type IV
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