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Distinguishing solid bitumens formed by thermochemical sulfate reduction and thermal chemical alteration

Kelemen, Simon R. and Walters, Clifford C. and Kwiatek, Peter J. and Afeworki, Mobae and Sansone, Michael and Freund, Howard and Pottorf, Robert J. and Machel, Hans G. and Zhang, Tongwei and Ellis, Gregory S. and Tang, Yongchun and Peters, Kenneth E. (2008) Distinguishing solid bitumens formed by thermochemical sulfate reduction and thermal chemical alteration. Organic Geochemistry, 39 (8). pp. 1137-1143. ISSN 0146-6380. doi:10.1016/j.orggeochem.2008.04.007.

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Insoluble solid bitumens are organic residues that can form by the thermal chemical alteration (TCA) or thermochemical sulfate reduction (TSR) of migrated petroleum. TCA may actually encompass several low temperature processes, such as biodegradation and asphaltene precipitation, followed by thermal alteration. TSR is an abiotic redox reaction where petroleum is oxidized by sulfate. It is difficult to distinguish solid bitumens associated with TCA of petroleum from those associated with TSR when both processes occur at relatively high temperature. The focus of the present work was to characterize solid bitumen samples associated with TCA or TSR using X-ray photoelectron spectroscopy (XPS). XPS is a surface analysis conducted on either isolated or in situ (>25 μm diameter) solid bitumen that can provide the relative abundance and chemical speciation of carbon, organic and inorganic heteroatoms (NSO). In this study, naturally occurring solid bitumens from three locations, Nisku Fm. Brazeau River area (TSR-related), LaBarge Field Madison Fm. (TSR-related), and the Alaskan Brooks range (TCA-related), are compared to organic solids generated during laboratory simulation of the TSR and TCA processes. The abundance and chemical nature of organic nitrogen and sulfur in solid bitumens can be understood in terms of the nature of (1) petroleum precursor molecules, (2) the concentration of nitrogen by way of thermal stress and (3) the mode of sulfur incorporation. TCA solid bitumens originate from polar materials that are initially rich in sulfur and nitrogen. Aromaticity and nitrogen increase as thermal stress cleaves aliphatic moieties and condensation reactions take place. Organic sulfur in TCA organic solids remains fairly constant with increasing maturation (<3.4 sulfurs per 100 carbons) due to offsetting preservation and H2S elimination reactions. In contrast, TSR solid bitumens are sulfur rich and nitrogen poor solids. These heteroatom distributions are attributed to the ability of TSR to incorporate copious amounts of inorganic sulfur (>3.5 to not, vert, similar17 sulfur per 100 carbons) into aromatic structures and to the low levels of nitrogen in their hydrocarbon precursors. Hence, XPS results provide organic chemical composition information that helps to distinguish whether solid bitumen, either in situ or removed and concentrated from the rock matrix, was formed via the TCA or TRS process.

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Additional Information:Copyright © 2008 Elsevier Ltd. Received 24 September 2007; revised 31 March 2008; accepted 3 April 2008. Available online 12 April 2008. We thank R. Worden and B. Horsfield, for constructive comments, and our respective affiliations for permission to present and publish this work.
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ID Code:11755
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Deposited On:23 Sep 2008 03:27
Last Modified:08 Nov 2021 22:02

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