Assessing extreme maturities – Challenging examples from immature Jordanian to overmature Far Eastern unconventional formations

Abstract

Thermal maturity is often readily available from optical vitrinite reflectance measurements, but in marine sediments with absence of terrigenous organic material (vitrinite macerals) or formations older than the Devonian, this technique has limitations. Raman spectroscopy has been applied as an alternative measurement to infer thermal maturity in challenging formations. Organic-rich outcrop samples from the Jordanian Lajjoun and Sultani deposits and from a Far East shale were characterized for their key geochemical properties (bulk chemical and mineral composition, total organic carbon content) and thermal maturity. Bulk mineral and chemical abundances were quantified using standard X-ray diffraction and X-ray fluorescence techniques. Total organic carbon (TOC) content was calculated from programmed temperature pyrolysis. Thermal maturity was determined and compared using conventional vitrinite reflectance and a novel, rapid laser Raman spectroscopy method. The Jordanian samples are organic-rich (16.3–17.4 wt% TOC), while the Far Eastern sample is relatively organic-lean (2 wt% TOC). Programmed pyrolysis shows that the Jordanian samples seem immature with high remaining hydrocarbon generative potential (S2), whereas the sample from the Far East appears to be overmature with extremely low S2 value. These findings were confirmed by the results from organic petrography as well as through Raman spectroscopic measurements. The Far Eastern samples are suggested to be highly mature (4.6–5.7% VRE), whereas the Jordanian samples were found to be immature (0.4–0.5% VRE). Robust Raman maturity determination allows to define and refine petrophysical properties of kerogen and thus enhance the accuracy in predicting hydrocarbon in place and the potential for its recovery.