--> Assessing Thermal Maturity of Different Oil Shale Samples Using Laser Raman Spectroscopy

AAPG ACE 2018

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Assessing Thermal Maturity of Different Oil Shale Samples Using Laser Raman Spectroscopy

Abstract

Maturity assessment of oil shale formations is one of the critical issues in the exploration of shale hydrocarbons. Conventionally vitrinite reflectance VRo is used as a reliable method for maturity analyses, but it is a time-consuming and expensive process. In this contribution, we used Laser Raman spectra of kerogens, G and D bands of disordered carbonaceous matter, to relate the thermal maturity of representative shale oil samples of Haynesville (East Texas), Woodford (West Texas), Eagle Ford and Pearsall (South Texas) and Gothic shale (Colorado) from various depths. Using Gaussian distribution approach, we correlate Raman signals (G and D band distance, height and peak ratio) of samples of known maturity from VRo and estimate unknown maturity of oil shale samples. The (D/G) area ratio vs. known VRo % shows a positive linear correlation. The Raman Shift distance (G-D) vs. known VRo % suggests that there are two oil shale groups of different maturity: one of high maturity cluster (G-D = 240-250 cm-1), and another with low maturity cluster (G-D = 260-270 cm-1). In addition, most of the samples show a higher (G-D) distance with increasing depth (and probably an increase in thermal maturity). The Raman Shift distance (G-D) vs. known total organic carbon (TOC), free hydrocarbons (S1) and the remaining hydrocarbon generating potential (S2) indicates that the (G-D) distance is directly related with TOC, S1, and S2. However, some matured samples show a high (G-D) distance with low values of TOC, S1, and S2, which indicate expulsed hydrocarbons. Based on these correlations, it is possible to establish an empirical equation to determine thermal maturity directly using Laser Raman spectra with minimal sample preparation for potential ‘on rig’ use including the down-hole environment.