Application and Assessment of Open-system vs. Closed-system Kerogen Isolation Methods for Characterization of Gas Shale Kerogens
Pernia, Denet 1988-
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Increasing interest in unconventional natural gas resources has spurred much research into the origin as well as the mode of entrapment and expulsion of natural gas. A pre-requisite of all such research is the isolation and separation of the kerogen, the insoluble organic matter from which hydrocarbons are formed, in its purest and most intact form from the rock matrix. Previous work showed that the isolation methods, closed-method versus open-method, used can impact kerogen recovery, purity, and elemental composition of the kerogen. This study takes it further and also addresses the effects of the isolation processes on the elemental, isotropic, spectroscopic, and structural (physical) properties of the kerogen recovered. Four major gas shales, including the Barnett, the Marcellus, the Haynesville, and a Polish gas shale, were chosen to test the impact of the closed-conservative versus open-conventional isolation methods on the properties of kerogen. The Monterey shale, though not strictly a gas shale, was included to address the effects on sulfur-rich, Type II-S kerogens. Standard screening procedures including Total Organic Carbon analysis, RockEval Pyrolysis, and X-ray Diffraction (XRD)-mineralogy were carried out on the native rocks, as well as sulfur forms. The carbon and sulfur isotopic compositions, organic elemental composition, solid-state 13C Nuclear Magnetic Resonance (NMR) spectroscopic properties, and Pyrolysis-GC pyrolytic behavior of the isolated kerogens were investigated. Higher ash content of kerogens isolated via the open-method and corresponding XRD results show more impurities than the closed-method kerogens. The closed-method resulted in higher kerogen recovery efficiencies. Pyrolysis-GC n-alkene/n-alkane ratios and relative quantities of alkanes, alkenes, and aromatics show much variability across samples isolated by the three methods, as though the same sample’s kerogen-cracking behavior has been altered. The 13C NMR yielded no significant change in the kerogen structure. The closed-method kerogens generally had higher carbon weight % than the open-method. Stable carbon isotopes showed no significant differences in the kerogen δ13C, whereas the stable sulfur isotopes showed significant change in the kerogen δ36S. Still to be investigated are the impacts of the isolation methods on the kerogen’s gas-retention and storage capacity as well as its density.