Evaluating aromatization of solid bitumen generated in the presence and absence of water: Implications for solid bitumen reflectance as a thermal proxy

https://doi.org/10.1016/j.coal.2022.104016Get rights and content

Highlights

  • Organic-rich mudrock was pyrolyzed under hydrous and anhydrous conditions.

  • Solid bitumen generated during pyrolysis was evaluated via petrography.

  • Hydrous pyrolysis enhanced expulsion and formed low viscosity, aromatic solid bitumen.

  • Anhydrous pyrolysis formed cross-linked, less aromatic solid bitumen.

  • Radical disproportionation promotes aromatization in the presence of water.

Abstract

Geological models for petroleum generation suggest thermal conversion of oil-prone sedimentary organic matter in the presence of water promotes increased liquid saturate yield, whereas absence of water causes formation of an aromatic, cross-linked solid bitumen residue. To test the influence of hydrogen from water, organic-rich (22 wt% total organic carbon, TOC) mudrock samples from the Eocene lacustrine Green River Formation Mahogany zone oil shale were pyrolyzed under hydrous and anhydrous conditions in closed system batch reactors at temperatures between 300 and 370 °C for 72 h. Pre- and post-pyrolysis samples were characterized using petrographic approaches including optical microscopy, reflectance, Raman spectroscopy, and scanning electron and transmission electron microscopy to quantify differences in relative appearance, abundance, and composition of solid bitumen newly generated during the pyrolysis experiments. Petrographic analyses were supplemented by geochemical screening measurements (TOC content and programmed temperature pyrolysis). Results show post-hydrous pyrolysis residues contain lower TOC, are comprised of solid bitumen with greater aromaticity, and have textures indicative of lower viscosities, relative to anhydrous residues from the same temperature pyrolysis conditions. These observations suggest solid bitumen forming from thermal conversion of oil-prone sedimentary organic matter under anhydrous conditions may be less aromatic, although more cross-linked, than solid bitumen forming under hydrous conditions at the same time-temperature combination. To explain these results, we suggest that a radical disproportionation mechanism is favored in the presence of hydrogen donated from water, and that this disproportionation promotes aromatization in the solid residue with concomitant expulsion of saturated hydrocarbons.

Introduction

Consideration of the role of water in petroleum generation is crucial to understanding hydrocarbon generation rates, expulsion and primary migration mechanisms, thermal stability regimes for petroleum, and proxy indicators for thermal maturity conditions in petroliferous sedimentary basins (Lewan, 1997). In particular, water is thought to enhance or facilitate chemical reaction mechanisms during petroleum formation by providing a source of hydrogen which reacts with sedimentary organic matter during catagenesis, ultimately resulting in the generation and expulsion of saturated hydrocarbons, primarily n-alkanes and cycloalkanes, via a free radical mechanism (Hoering, 1984; Kissin, 1987; Lewan, 1991, Lewan, 1994). Long established work by petroleum geochemists has shown that hydrous pyrolysis is the most analogous laboratory technique available for generating and expelling a saturate-rich liquid similar in composition to naturally generated crude oil (Comet et al., 1986; Lewan et al., 1979; Spigolon et al., 2015), although anhydrous methods can produce a similar effluent when generated oil vapors are separated from the rock during reactor cooling (Jin et al., 2012). Newer laboratory approaches for hydrous simulation of petroleum generation have also introduced lithostatic pressure simulation combined with open-system expulsion and effluent collection (Stockhausen et al., 2020), although similarities of expelled oil generated in this approach are still being compared to that expelled in classic hydrous pyrolysis.

As described by Lewan (1994), the presence of liquid water in hydrous pyrolysis experiments causes a water-saturated bitumen network to develop in thermally maturing organic-rich source rocks, where bitumen is operationally defined in a geochemical context as a tarry substance composed of high molecular weight organic components noncovalently bound in the sedimentary organic matter and extractable by typical organic solvents. Thermal rupture of covalent bonds in the oil-prone sedimentary organic matter during high temperature hydrous pyrolysis begins a chain of radical reactions, until termination of a free radical site is initiated by hydrogen radicals from dissolved water. Termination prevents β-scission of covalent carbon‑carbon bonds in the β-position from the termination site, whereas non-terminated radical sites initiate β-scission to form a separated aliphatic fragment. Hydrophobicity of the separated hydrocarbon in the water saturated bitumen results in an immiscible oil phase, which is then expelled due to buoyancy and the overall net volume increase during petroleum generation.

In contrast to hydrous pyrolysis, pyrolysis in the absence of water as a liquid medium in the experiment (anhydrous pyrolysis, as described in the methods section below) is thought to result in formation of a cross-linked ‘pyrobitumen’ through polymerization (e.g., Koopmans et al., 1998), where pyrobitumen is operationally defined as an insoluble carbon-rich organic residue. In the model of Lewan, 1991, Lewan, 1994, Lewan, 1997, free radical sites are not terminated by hydrogen donors under anhydrous conditions because of the absence of dissolved water in the bitumen. Therefore, more free radical sites are present, causing more β-scission, which is only terminated by cross-linking recombination with other free radicals in the same macromolecular structure, forming a cross-linked, aromatic pyrobitumen (Lewan, 1997). This model for the role of water is widely accepted, as reviewed and described in many studies of natural and simulated petroleum generation (Cavelan et al., 2019; Schimmelmann et al., 1999; Schimmelmann et al., 2006; Seewald, 2003; Whitelaw et al., 2019).

Several previous studies have reported geochemical and petrographic evidence to elucidate the role of water in petroleum generation, although results are mixed and sometimes contradictory (Behar et al., 2003; Huang, 1996; Lewan, 1997; Lewan and Roy, 2011; Moyer and Prasad, 2017). These works have generally compared the residues and effluents of anhydrous and hydrous pyrolysis experiments which were conducted with the same temperature and time conditions. Higher total pyrolyzate yields from hydrous experiments and lower total organic carbon content and S2 (programmed temperature pyrolysis pyrolyzate) from spent-rock residues created under hydrous conditions are interpreted as due to enhanced petroleum expulsion in the presence of water. Another line of evidence supporting enhanced expulsion under hydrous conditions is the presence of expansion fractures in residues of hydrous experiments and their absence in anhydrous residues (Lewan and Roy, 2011). Evidence for formation of a more aromatic carbon residue under anhydrous conditions is less forthcoming. For example, vitrinite reflectance (VRo, a petrographic proxy for aromaticity, e.g., Carr and Williamson, 1990) has been reported to be lower in anhydrous residues than in hydrous residues from the same experimental temperature (Behar et al., 2003; Huang, 1996). Several works have reported generally greater aromatic abundances in liquid products from hydrous experiments, compared to anhydrous experiments at the same temperature (Behar et al., 2003; Wu et al., 2018). However, Lewan (1997) reported lower H/C, and higher VRo and NMR-derived aromaticity (fa) values in anhydrous residues. Petrographic measurements of VRo are determined on the terrigenous maceral vitrinite, which is present throughout the pyrolysis experiment. That is, the vitrinite is not a newly formed carbon material created during petroleum generation, i.e., a solid bitumen, or coke, from which measurements could provide direct evidence to further support the role of water in petroleum generation as described above.

Herein, we created solid bitumen (a petrographically observed solid petroleum which is an immobile solid at room temperature) from oil-prone amorphous organic matter via hydrous and anhydrous pyrolysis at temperatures of 300 to 370 °C. The solid bitumen was evaluated via optical and electron microscopies, Raman spectroscopy, and geochemical screening methods, to test the hypothesis that petroleum generation in the presence of water would promote formation of a saturate-rich exudate (oil) associated with an aliphatic higher H/C solid bitumen residue, whereas anhydrous pyrolysis would create a more aromatic, cross-linked solid bitumen residue. The results are evaluated in context of potential implications to hydrocarbon generation in sedimentary basins, where variable degrees of water saturation in oil-prone source rocks may result in the formation of relatively higher or lower volumes of pyrobitumen versus oil during thermal maturation. The results are also evaluated in context of the now common use of solid bitumen reflectance (BRo) as a thermal proxy in source-rock reservoir characterization (e.g., Hackley et al., 2020a).

Section snippets

Summary of methodology

Aliquots from crushed (8 mesh), homogenized Green River oil shale were isothermally pyrolyzed under hydrous and anhydrous conditions at temperatures ranging from 300 to 370 °C for 72 h. Residues were analyzed by optical and electron microscopies, Raman spectroscopy, programmed temperature pyrolysis, and carbon analyzer. Due to funding, time, and sample availability limitations, not all residues were analyzed by each analytical technique, e.g., Raman spectroscopy was conducted only for the 340,

Geochemical screening analyses

TOC content decreased progressively with increasing pyrolysis temperature, to a low value of 4.09 wt% in the hydrous pyrolysis residue from 360 °C (Table 1; Fig. 1A). An exception to the progressive decrease in TOC is present in the anhydrous residue from 370 °C, which showed a slight increase of 0.66 wt% relative to the anhydrous residue from 360 °C. TOC content is systematically higher in residues from anhydrous pyrolysis experiments relative to hydrous experiments at the same temperature.

Summary

Solid bitumen was created from oil-prone amorphous organic matter in the lacustrine Green River Formation Mahogany zone oil shale via hydrous and anhydrous pyrolysis at temperatures ranging from 300 to 370 °C and evaluated via optical and electron microscopies, Raman spectroscopy, and geochemical screening methods. This work tested the hypothesis that petroleum generation in the presence of water would favor the generation and expulsion of a saturate-rich exudate associated with an aliphatic

Author contributions

PCH conceived the idea to test aromaticity in solid bitumen created during pyrolysis experiments in the presence and absence of water. PCH performed pyrolysis experiments and organic petrography analyses. AMJ, PCH and PJB collected and interpreted Raman spectra. RJM and PLS performed SEM-EDS and TEM analysis. BJV and JJH performed pyrolysis experiments and sample preparation. JEB contributed the APM sample and associated characterization data. PCH wrote the manuscript with input from all

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This manuscript benefited from the critical review of Michael Lewan (USGS-Emeritus) and three anonymous journal reviewers. This work was supported by the Energy Resources Program of the USGS. Augusta Warden (USGS) supervised geochemical screening analyses. Joshua Schumacher (National Institute for Standards and Technology) performed transmission electron microscopy (TEM) lamellae preparation. Lynette Ballast, Berdie Valdez, and Jim Connor (Cerium Laboratories) assisted with TEM analyses. All

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