Chemical, Molecular, and Microstructural Evolution of Kerogen during

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Chemical, Molecular, and Microstructural Evolution of Kerogen during Thermal Maturation: Case study from the Woodford Shale of Oklahoma Paul R. Craddock, Kyle D Bake, and Andrew E Pomerantz Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b00189 • Publication Date (Web): 21 Mar 2018 Downloaded from http://pubs.acs.org on March 21, 2018

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Energy & Fuels

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Chemical, Molecular, and Microstructural Evolution of Kerogen during

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Thermal Maturation: Case study from the Woodford Shale of Oklahoma

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Paul R. Craddock*, Kyle D. Bake, Andrew E. Pomerantz

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Schlumberger-Doll Research Center, Cambridge, MA, USA

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* Corresponding author: [email protected], +1-617-768-2042

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Manuscript submitted to: Energy & Fuels

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Original submission: January 15, 2018

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Revised submission: March 20, 2018

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Abstract

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Integrated elemental, spectroscopic (infrared spectroscopy, X-ray absorption near edge

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structure), and gas intrusion (helium pycnometry, nitrogen adsorption) analyses are used to

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characterize the bulk chemical, molecular, and physical microstructures of kerogen spanning a

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thermal maturity transect (vitrinite reflectance, Ro, from 0.5% to 2.6%) across the Woodford

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Shale of the Anadarko Basin, Oklahoma. The integration takes advantage of novel procedures to

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prepare kerogen isolates that preserve both the chemical and physical properties of the organic

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matter in the bulk shale. The Woodford kerogens follow the expected trends in H/C and O/C

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coordinates during thermal maturation for type II kerogen. Infrared spectra show that loss of

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hydrogen from kerogen is related to cracking of hydrogen-rich aliphatic (alkyl) carbon structures

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from aromatic carbons. Within the range of Ro values < 1.5%, peripheral aromatic carbons

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remain highly substituted with alkyl (methyl, methylene) and probably heteroatom functional

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groups. At Ro values > 1.5%, these substitutions are substantially removed and replaced by

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hydrogen. The evolution of carbon structures inferred from the IR spectra is supported by known 1 ACS Paragon Plus Environment

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carbon bond dissociation energies for carbonaceous materials. Total organic sulfur and sulfur-

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XANES data show that sulfur in Woodford kerogens is dominated by aromatic sulfur

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(thiophene) and that reactive, aliphatic sulfur (sulfide) is eliminated at low degrees of thermal

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stress (Ro ≤ 0.9%). At higher thermal stress, sulfur speciation is stable and dominated by

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thermally stable thiophene (sulfur in aromatic rings). The physical properties of Woodford

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kerogens evolve during thermal maturation in a manner consistent with their molecular

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characteristics. Skeletal density of kerogen increases during maturation in a manner that is

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linearly correlated to its atomic H/C ratio and inferred aromatic carbon content. Specific surface

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area of kerogen also increases during maturation, reflecting development of internal pores within

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the kerogen skeletal framework as aliphatic carbon structures are preferentially cracked and

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expelled from solid kerogen. The quantitative chemical and structural changes expressed by the

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Woodford kerogens during thermal maturation, including their hydrogen and carbon content,

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carbon speciation, and skeletal density, are shown by comparison to not be measurably different

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for other type II kerogens from numerous oil- and pas-producing shale plays, indicating that

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thermal stress acts to drive maturation of type II kerogen in a similar way globally.

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2 ACS Paragon Plus Environment

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Energy & Fuels

1. Introduction

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Kerogen is the most abundant form of sedimentary organic matter (OM)1 and a defining

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component of unconventional petroleum source rocks, commonly termed shale. From a

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geochemical perspective, kerogen is the solid, insoluble, and nonvolatile sedimentary OM. Its

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molecular structure (comprised essentially of C, H, N, S, and O) controls the type, amount, and

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quality of hydrocarbons generated during thermal maturation.2 In a geological context, kerogen

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is commonly studied using petrographic inspection (e.g., vitrinite reflectance) and thermal

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decomposition methods (e.g., elemental analysis, programmed pyrolysis). Vitrinite reflectance3-6

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and programmed pyrolysis7-9 (e.g., Rock-Eval) provide a determination of the thermal maturity

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of kerogen from which structural characteristics may generally be inferred, but they give no

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direct insights on structural properties. From a chemical perspective, elemental analysis10 offers

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bulk chemical concentrations, but also provides no direct molecular structural information.

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Several solid-state spectroscopic methods are available for the analysis of specific

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molecular structures in carbonaceous materials such as kerogen, including infrared (IR)

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spectroscopy, Raman spectroscopy,

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near-edge structure (XANES), and X-ray photoelectron spectroscopy (XPS). Raman

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spectroscopy11-18 and

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environment of carbon comprising aliphatic and aromatic moieties, as well as their absolute

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abundance. Advanced

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discrimination and quantification of specific C, CH, CH2, and CH3 functional groups including

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their connectivity or proximity, from which parameters such as the average carbon number of

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aromatic clusters and of aliphatic chains, can be estimated.22,

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routinely identify functions associated with C, H, and O, including aromatic C=C and CH,

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C nuclear magnetic resonance (NMR), X-ray absorption

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C NMR19-27 are two techniques capable of probing the bonding

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C NMR spectral acquisition and editing techniques now offer


28, 29

Infrared spectroscopy can

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aliphatic CH, CH2, and CH3, and oxygenated C–O and C=O groups.26,

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analysis of organic sulfur and nitrogen species is now established using sulfur- and nitrogen-

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XANES.25, 48-67 Solid-state XPS is capable of quantifying several functionalities in carbonaceous

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materials associated with carbon, oxygen, sulfur, and nitrogen.51,

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several of these techniques and their applications in the analysis of kerogen and other complex

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carbonaceous materials is given by Pomerantz.73 Separately, methods such as scanning electron

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microscopy,74-79 gas adsorption,80-84 and small angle neutron and X-ray scattering,84-88 exist for

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analysis of aspects of the physical microstructure of kerogen, such as its pore size distribution

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and surface area, performed on either bulk shales or kerogen isolated from the rock.

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Measurements run on bulk shales can be challenged by presence of volumetrically-dominant

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inorganic matrix that obscures the signal from the organic matter. Kerogen isolates have

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generally been prepared using acid demineralization and drying procedures that preserve its

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chemical structure but alter its physical microstructure, such that microstructural properties of

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kerogen isolates are not representative of the organic matter in situ.87 More often than not,

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methods for the analysis of kerogen are used in isolation, providing a far-from-complete picture

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as to the molecular makeup of these complex materials.

55, 67-72

27, 30-47

Quantitative

A recent review of

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Kelemen et al.25 carried out one of the most comprehensive surveys of kerogen

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compositions of different types and from several organic-rich sedimentary facies. These kerogen

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types [see

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algal OM in freshwater environments), type II (initial intermediate H/C and O/C ratio, derived

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predominantly from planktonic OM in marine environments) and type III (initial low H/C and

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high O/C ratio, derived from terrestrial higher-order plants). Their results from 13C NMR, XPS,

89, 90, 91

] included type I (initial high H/C and low O/C ratios, typical of aliphatic-rich


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and XANES provided a detailed comparison of kerogen chemical structure of different types and

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organo-facies, and to a limited extent as a function of thermal maturity.

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Here, we present an integrated analysis of kerogen, with a focus on a single organic-rich

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sedimentary formation (predominantly type II kerogen of the Woodford Shale, Anadarko Basin,

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Oklahoma) that presents a natural thermal maturity transect from immature to post-mature

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(vitrinite reflectance, Ro, range from at least 0.5 to 4%). Samples from a single formation were

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chosen to minimize differences in original kerogen composition as is encountered when

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comparing kerogens of different types (i.e., types I, II, and III) and/or from different organo-

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facies. In this way, measured differences in kerogen characteristics among the samples are

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predominantly, if not entirely, a result of their level of thermal maturation. The study focuses on

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type II kerogen because it represents by far the major fraction of sedimentary OM in all paleo-

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marine petroleum source rocks of North America and elsewhere that are producing substantial

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and economic quantities of unconventional oil and gas. As such, the results here may be used

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more broadly as an analog to infer or contrast kerogen characteristics in other shale plays.

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Using novel procedures for the preparation and analysis of isolated kerogen, here we

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assess both its chemical and physical microstructural characteristics. The results reveal detailed

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and complementary trends in chemical composition and physical microstructure of type II

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kerogen as a function of thermal maturity. This type of information is critical, for example, to

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optimize the properties of kerogen needed for petrophysical interpretations of shale, as well as to

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constrain molecular models of kerogen structure92-97 that can provide insights to hydrocarbon

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storage and transport.

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2. Materials and Methods


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2.1. Nomenclature

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Kerogen is defined here as all solid, insoluble, and non-volatile fractions of sedimentary

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organic matter (OM), following established geochemical conventions.1 This convention arises

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from the methods by which bulk kerogen is chemically isolated from forms of sedimentary OM

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(e.g., source-rock bitumen) that are soluble in usual organic solvents and from the inorganic

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mineral matrix of the bulk rock, which makes kerogen amenable to study by numerous analytical

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techniques. This definition is not precisely the same as the petrographic definition of kerogen as

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it is studied by optical inspection, in which this term is restricted to “primary” [e.g.,

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“depositional” OM [e.g.,

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“secondary”—being formed from thermal maturation of previously soluble source-rock bitumen

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or liquid oil98—but also insoluble. This secondary insoluble OM is commonly termed

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“pyrobitumen” or “post-oil solid bitumen” [e.g., 6, 78, 79, 98], and it is distinguished optically from

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primary insoluble OM on the basis of texture, such pore-filling characteristics.6 These two

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insoluble fractions are inseparable using chemical techniques for the isolation of bulk insoluble

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OM, and are together termed kerogen herein. New techniques are being developed to study the

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chemical and mechanical properties of insoluble OM in situ without the need for its isolation,99

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such that certain characteristics of primary and secondary insoluble OM may be investigated

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separately.

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2.2. Samples

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78, 79

] or

]. There may exist a fraction of sedimentary OM that is

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The Woodford Shale is a Late Devonian-Early Mississippian paleo-marine, organic-rich

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sedimentary facies, containing predominantly kerogen derived from marine planktonic

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organisms (type II kerogen) at concentrations up to 25 wt% total organic carbon (TOC) and is an

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important hydrocarbon source rock in Oklahoma.100 The Woodford Shale is widely distributed 6 ACS Paragon Plus Environment

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across a geologic province called the Southern Oklahoma Aulacogen,101, 102 with recent drilling

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and exploration concentrated in four plays: the Anadarko Basin in western Oklahoma, the

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Arkoma Basin in eastern Oklahoma, the Ardmore Basin in southern Oklahoma, and the

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Cherokee Platform in northwest Oklahoma.103 The tectonic history of the Southern Oklahoma

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Aulacogen and general characteristics of the Woodford Shale have been detailed in earlier

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publications [e.g., 102, 104, 105].

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The samples studied in this paper are core and cuttings from wells drilled into the

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Woodford Shale in the Anadarko Basin, which were curated and studied previously for their

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vitrinite reflectance by the Organic Petrography Laboratory (OPL) of the Oklahoma Geological

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Survey.106 The samples (Table 1) range from immature (vitrinite reflectance mean Ro = 0.55%)

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to post-mature (Ro = 4.05%), which can be used to assess trends among kerogen properties as a

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function of level of thermal maturation within a coherent sedimentary facies. The method used

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for previous determination of the mean, random vitrinite reflectance values of these samples is

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described elsewhere.104 For this study, between 15 and 90 g of material was received for each

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sample, of which ~ 15 g was crushed to fine powder (~ 10 µm) using an auto-mortar. The

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powders were cleaned of extractable organics (e.g., residual drilling fluid, source-rock bitumen,

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oil) with dichloromethane in a Soxhlet extractor. After air-drying, the powders were split into

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homogeneous fractions for subsequent whole-rock screening comprising TOC and programmed

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pyrolysis (Rock-Eval). This screening was used to select a subset of samples for kerogen

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isolation that both spanned a range of thermal maturity and contained sufficient mass of kerogen

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required for a complete chemical and microstructural analysis. Kerogen isolates were obtained

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from the bulk powders using open-system chemical treatment procedures.107 Briefly, a series of

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concentrated HF and HCl acid additions were made to remove silicate and carbonate minerals,


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followed by addition of acidic CrCl2 to remove metal sulfides (predominantly pyrite). Removal

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of inorganic minerals was confirmed by high-temperature (~ 1000 °C) ash analysis, with ash

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concentrations of 0.1–2 wt% relative.

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The isolated kerogen fractions were suspended in ethanol and subsequently dried using

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critical point drying (CPD) techniques to preserve their microstructure.108 The procedure

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employs liquid-gas phase transition without crossing the phase boundary. Kerogen isolates were

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flushed with liquid CO2 at 5.5 × 106 Pa (800 psi) and 0 °C for 2400 s (40 min) to remove

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miscible ethanol, followed by vaporization and purging of supercritical CO2 above its critical

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point (31.1°C, 1071 psi). The cycle was repeated using a 1200 s (20 min) flush. The dry kerogen

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isolates were then ready for study. This procedure is demonstrated to preserve aspects of kerogen

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microstructure, such as surface area, which collapse under traditional oven-drying procedures.108

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2.3. Total organic carbon and Rock-Eval pyrolysis

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Total organic carbon (TOC) measurements were run on the cleaned, bulk powders using

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established combustion-coulometry methods.109 Because the samples were solvent-cleaned, the

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reported TOC values represent only the organic carbon content from the insoluble organic

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fraction, kerogen.

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Programmed pyrolysis (Rock-Eval) is a common alternative to vitrinite reflectance for

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determination of thermal maturity, and was run in this study to evaluate performance against the

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vitrinite reflectance benchmark. The technique uses temperature-programmed pyrolysis of a

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powdered sample in an inert atmosphere from which the amounts of free hydrocarbons (S1 peak,

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units of mg of hydrocarbon per g of sample) and potentially generative hydrocarbons (S2 peak,

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units of mg of hydrocarbon per g of sample) are quantified from the measured pyrograms. For


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this set of samples, the S1 peak is negligible (< 1 mg/g) because they were solvent-cleaned of

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existing hydrocarbons. Rock-Eval hydrogen index (H.I.) and Tmax are two metrics commonly

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calculated from the measured pyrograms. H.I. is defined as equal to 100 × S2/TOC (units of mg

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hydrocarbon per g of total organic carbon). Tmax is the temperature (°C) at which the maximum

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amount of hydrocarbon is generated from kerogen decomposition (i.e., temperature of the S2

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peak maximum), and can be calibrated specifically to equivalent vitrinite reflectance, for

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example using the correlation: %VRe = 0.018 × Tmax − 7.16.110

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2.4. Elemental Analysis

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Kerogen bulk compositions were measured using routine combustion elemental

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analysis.111 Carbon, hydrogen, nitrogen, and sulfur were quantified by flash combustion in a

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stream of oxygen at ~ 1000 °C. Oxygen was quantified by flash pyrolysis in an inert atmosphere

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at ~ 1000 °C.

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2.5. Infrared spectroscopy

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Infrared (IR) spectroscopy quantifies the distribution of vibrational frequencies of a

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sample and has been used extensively to study the functional group abundances associated with

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carbon, hydrogen, and oxygen in various carbonaceous materials including coals30-37, kerogens26,

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27, 38-45, 112

, bitumens43, and asphaltenes45-47, 64.

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Isolated kerogens were prepared for IR analysis using the potassium bromide (KBr) pellet

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method, which enables the collected IR spectra to be interpreted quantitatively according to the

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Beer-Lambert Law and thus for the IR spectra of different analytes to be compared directly.

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Splits of kerogen isolates were gently hand-ground using a clean mortar and pestle, and a

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precisely known mass of disaggregated kerogen powder (2.000 ± 0.010 mg) was added to a 59 ACS Paragon Plus Environment

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mL tungsten carbide mixing vial containing two 7-mm diameter agate spheres. Precisely 900.00

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± 0.50 mg of KBr powder (XL Spectrograde powder form, International Crystal Laboratoroes,

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Garfield, NJ) was weighed out, and the KBr was added incrementally to the mixing vial, with the

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mass of KBr added being approximately equal to the total mass of material in the mixing vial.

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The mixture after each addition was mixed using a Retsch MM400 mixer mill (Verder Scientific

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Inc., Newtown, PA) run at 23 Hz for 5 min. In total, six additions of KBr were made for a

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mixing time of 30 min. Next, 200.00 ± 0.50 mg of the homogeneous powdered mixture was

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placed into a prepared die and pressed into a 13-mm diameter pellet at 10 ton/cm2 for 10 min

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under vacuum. The prepared kerogen pellets were scanned in transmission mode using a Bruker

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Vertex70 dual-range Fourier transform infrared (FTIR) spectrometer. The measurement

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comprised 25 individual scans of 2 s each across the mid-IR range (~ 5200–450 cm-1) with a

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resolution of 2 cm-1. The collected spectrum was calculated as the mean of the 25 scans in

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reported in absorbance units. A background (blank) absorption spectrum was scanned at the

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beginning of each measurement and subtracted automatically from the kerogen absorption

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spectrum.

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Several IR structural indices were computed from the acquired spectra, based on band

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assignments and conventions established by earlier studied.35, 36, 39, 42 The IR intensity of each

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band of interest was computed as its integrated area following the curve deconvolution

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procedures described in a previous publication.43 Table 2 defines the IR structural indices. The

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CH3/CH2 ratio is inversely related to the average length of aliphatic chains and is positively

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related to the degree of chain branching. Aromaticity is positively correlated to the relative

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abundance of aromatic versus aliphatic carbons that are bound to one or more hydrogens. The A-

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factor and C-factor represent, respectively, the amount of aliphatic carbon and oxygenated


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carbon species relative to that of aromatic carbon. These indices were used to quantify changes

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in the functional group abundances of kerogen during thermal maturation.

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2.6. Sulfur K-edge X-ray absorption near-edge structure spectroscopy

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Sulfur K-edge X-ray absorption near-edge structure spectroscopy (XANES) can quantify

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the abundance of sulfur-containing functional groups in carbonaceous materials such as coal48-52,

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kerogen25, 53-56, bitumen56-58, asphaltenes59-64, and asphalts.113 The techniques measures electronic

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transitions from 1s orbitals to vacant molecular orbitals with 3p character. The energy of the

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transition is a function of sulfur oxidation state (increasing energy with increasing oxidation

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state), which relates to the bonding environment of the sulfur atom. Sulfur chemistry of kerogen

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is shown to impact the kinetics of petroleum (oil and gas) generation in source rocks.114. Sulfur

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content and chemistry can influence the surface polar functionality of carbonaceous materials,

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including coal, kerogen, and bitumen, that may impact the storage (e.g., adsorption) and

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transport of petroleum in the subsurface.56, 115, 116

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Kerogen isolates were prepared for XANES by dilution of ~ 50 mg of analyte in a sulfur-

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free matrix (boron nitride powder; > 99.5 % from Alfa Aesar, Ward Hill, MA) to ~ 1 wt% total

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sulfur to prevent self-absorption effects.56 The powder mixture was then manually pressed into a

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6-mm diameter pellet for analysis. XANES spectra were collected in fluorescence mode at the

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Advanced Photon Source beamline 9-BM using a Stern-Heald-Lytle detector mounted in a

239

helium-purged sample chamber. The range of photon energies for spectral acquisition was 2450

240

to 2600 eV, and energy was calibrated against a Na-thiosulfate pre-edge feature at 2469.20 eV.

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Measured XANES spectra were recorded as the ratio of the intensity of the total fluorescence

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signal, I, to the intensity of the excitation radiation, I0, as a function of the excitation photon

243

energy. Spectra were baseline-corrected by setting I/I0 to zero in the pre-edge region (< 2465 eV) 11 ACS Paragon Plus Environment

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and normalized by setting I/I0 to unity in the post-edge region (> 2490 eV). The relative

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concentrations of organic sulfur species in the kerogens were quantified by fitting the resulting

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spectra to a linear combination of model sulfur compounds (Figure 1). Inorganic sulfur forms

247

such as pyrite and sulfate (anhydrite, barite), if present in the isolate phase, can overlap the

248

XANES spectra of kerogen and so were included in the spectrum fitting, but their abundance was

249

not reported when presenting the molar fractions of organic sulfur species in kerogen. The

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uncertainty on the analysis is approximately 10 mol%.

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2.7. Helium pycnometry

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Kerogen density is known to increase with level of thermal maturation112, 117-119 and is a

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critical parameter for petrophysical determination of formation volumes and porosity. Helium

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pycnometry was run to determine the absolute (“skeletal”) density of bulk kerogen isolates.

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Measurements were made using a Micromeritics AccuPyc II 1340 gas pycnometer that

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determines the volume of a precisely known mass of analyte based on Boyle’s Law.

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Approximately 300–400 mg of sample was analyzed inside of a 1 cm3 volumetric chamber. Ten

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volumetric determinations were made for each sample, and the volume computed as the mean of

259

the replicates. Density was calculated according to mass/volume relationship and is reproducible

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for a given sample within ± 0.02 g/cm3. Densities were corrected for the presence of residual

261

inorganic minerals in the kerogen isolate assuming the ash was from sulfate (as indicated by

262

XANES sulfur species, here represented by anhydrite, ρ = 3 g/cm3), with the correction being

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between 0.02 and 0.06 g/cm3.

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2.8. Nitrogen adsorption surface area analysis


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The preparation of kerogen isolates incorporating novel critical point drying (CPD)

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methods enabled microstructural analysis of these samples. Specific surface area (SSA)

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determinations were made on a Micromeritics ASAP 2420 surface area analyzer using multi-

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stage nitrogen adsorption. The SSA of each sample was calculated from the Brunauer-Emmett-

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Teller (BET) adsorption isotherm. Several hundred milligrams of sample was precisely weighed

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into a surface area tube followed by a fill-rod to minimize head space. The pre-measurement

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mass of the sample plus tube was recorded. The samples were then degassed for 6 h at

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programmed temperature and pressure (60 °C, < 5 µm Hg) to eliminate surface-adsorbed

273

atmospheric gases. The mass of sample plus tube was recorded again to confirm adsorbed gas

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loss. The surface area tube was wrapped in a thermal jacket, loaded onto the SSA analyzer inside

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of a dewar, and immersed in liquid nitrogen. The jacket ensured thermal equilibrium inside of

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the surface area tube during the measurement. The measurement consisted of sequential

277

quantification of adsorbed nitrogen as a function of relative pressure, P/P0, between 0 and 1

278

where P and P0 are the equilibrium and saturation pressure of the adsorbate (nitrogen). The

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specific surface area was then computed from the linear BET adsorption isotherm plot between

280

P/P0 equal to 0.012 and 0.286.

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2.9. Assessing preservation of kerogen properties during acid demineralization

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Tests were performed to confirm that the acid demineralization procedure used to isolate

283

kerogen from the bulk sample did not oxidize or otherwise measurably alter its original

284

composition or structure. For this purpose, it was necessary to study as a proxy for kerogen, a

285

solid carbonaceous material (asphaltene) that could be obtained in a pure form without acid

286

demineralization, allowing the carbonaceous material to be directly analyzed and compared

287

before and after exposure to concentrated acids. Asphaltenes are macromolecular compounds 13 ACS Paragon Plus Environment

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288

representing the heaviest fraction of petroleum and whose composition shares commonalities

289

with that of kerogen.120 Approximately 1 g of solid asphaltene was precipitated from a Middle

290

East black oil (UG8) by diluting in excess volume of n-heptane (40:1 ratio by mass of n-heptane

291

to oil) and stirring for ≥ 12 h. The asphaltene precipitate was filtered under vacuum over a 0.45

292

µm Teflon filter, and then transferred from the filter to a Soxhlet extractor and washed with n-

293

heptane for 24-48 h to eliminate any remaining soluble impurities. The precipitate was split into

294

two equal fractions. One fraction was analyzed by transmission FTIR spectroscopy and XANES

295

in its original form. The second fraction was exposed to the same concentrated acid procedure as

296

used to isolate kerogen from shale and then also analyzed by transmission FTIR spectroscopy

297

and XANES.

298

Figure 2 compares the FTIR and XANES spectra of the original and acid-treated

299

asphaltene splits. The IR spectra are practically identical with respect to the characteristic IR

300

absorption bands (e.g., C—C, C—H, and C—O) for carbonaceous materials. The IR spectra

301

show a 0.1 absorbance unit difference between 1150–1250 cm-1, but this region of the IR

302

spectrum is commonly associated with strongly absorbing inorganic bonds and does not impact

303

the computed IR structural indices. These bands are inferred to represent mineral impurities and

304

are not always observed in the kerogen IR spectra, described below. The XANES spectra and

305

sulfur species abundances are also the same within the reproducibility of the data. The apparent

306

small decrease, albeit within analytical error, in the abundance of oxidized sulfur species

307

(sulfoxide, sulfonate) in the acid-treated asphaltene is inconsistent with oxidation of

308

carbonaceous materials during treatment or storage. These data support the resistance to

309

measurable oxidation or other chemical alteration of carbonaceous materials prepared using the


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310

same acid treatment as the kerogens analyzed here, in line with the similar observations made by

311

earlier studies of kerogen structure.43, 50, 56, 61

312 313

3. Results

314

3.1. Thermal maturity estimates

315

Figure 3 compares Rock-Eval parameters versus benchmark vitrinite reflectance for the

316

set of Woodford Shale samples. There exists the expected positive correlation between Rock-

317

Eval Tmax and vitrinite reflectance to values up to ~ 1.4% Ro (approximately late-oil window),

318

and within this range Ro-equivalence computed from Tmax [VRe = 0.018 × Tmax – 7.16]110 agrees

319

with that from vitrinite reflectance. Beyond this range (condensate and dry-gas windows), Tmax

320

estimates, and thermal maturity estimates from Tmax, are lower than expected. Consistent with

321

previous studies,8,

322

source rock samples with S2 signals less than ~ 1 mg HC/g TOC.

323

3.2. Elemental compositions

44

Tmax is found here to be an unreliable indicator of thermal maturity for

324

Table 3 reports the bulk elemental (C, H, N, S, O) concentrations of the kerogen isolates

325

from the Woodford Shale. It is known from the Van Krevelen diagram (H/C vs. O/C) that the

326

hydrogen and oxygen content of kerogen decreases during increasing thermal maturation,10, 90

327

Figures 4 and 5 plot, respectively, for the Woodford kerogens, their atomic H/C versus atomic

328

O/C and atomic H/C versus vitrinite reflectance, Ro. The data identify the kerogens as type II,

329

confirm their expected decrease in hydrogen content with increasing level of thermal maturation

330

(H/C ratio decreasing from 1.09 to 0.45), and indicate that they have low oxygen content, even at

331

or prior to the onset of oil generation (O/C ratio ranging from 0.04 to 0.08). The increase in the 15 ACS Paragon Plus Environment

Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

332

oxygen content of the most thermally mature samples is unexpected, and likely reflects

333

contamination from inorganic oxygen from trace amounts of refractory oxides in the kerogen

334

isolates or from water in the pyrolysis measurement. For the Woodford kerogens, total sulfur

335

content sharply decreases from 3.1–3.5 wt% to 1.3–2.1 wt% at a vitrinite reflectance, Ro, value

336

of ~ 0.9%. Total nitrogen content is low (2.0–2.6 wt%) and shows no clear evolution during

337

thermal maturation.

338

3.3. Infrared structural indices

339

Figure 6 plots the FTIR spectra of the Woodford kerogens in order of increasing thermal

340

maturity, with their key structural groups indicated. The IR intensities of structural groups reveal

341

clear trends with increasing thermal maturity, comprising a decrease in the intensities of aliphatic

342

C—H bands and, to a lesser extent, in oxygenated (C—O, C=O) bands, and an increase in the

343

intensities of aromatic C=C and C—H bands. Table 3 reports group relative abundances

344

quantified by IR structural indices: CH3/CH2, Aromaticity, A-factor, and C-factor. Figure 7 plots

345

the computed IR structural indices of the kerogens as a function of their thermal maturity

346

(vitrinite reflectance, Ro) and atomic H/C ratio. The increase in ratio of methyl (—CH3) to

347

methylene (—CH2) groups indicates a shortening of the mean length of aliphatic chains in

348

kerogen during maturation. The A-factor, representing the relative abundance of aliphatic C—H

349

versus aromatic C=C groups, decreases during maturation. Aromaticity as defined from IR

350

spectroscopy reflects the relative amount of hydrogen bonded to aromatic carbons versus

351

aliphatic carbons, and increases during thermal maturation. The C-factor, representing the

352

relative abundance of C—O and C=O versus aromatic C=C groups, also decreases during

353

maturation. This FTIR data supports that the unexpected increase in bulk oxygen content of the


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354

most mature kerogens reflects contamination in the bulk chemical analysis from inorganic

355

oxygen in mineral or water impurities.

356

3.4. Sulfur XANES

357

The sulfur functional group abundances of kerogen derived from the sulfur XANES

358

analysis are reported in Table 3. The values are expressed as mole percent of total sulfur. Small

359

amounts (2–9 mol%) of inorganic sulfate were identified in the XANES spectra, and excluded

360

when comparing the organic-sulfur distribution between samples. Figure 8 plots the molar

361

abundances of organic-sulfur species in kerogen as a function of their thermal maturity. Organic

362

sulfur in all kerogens is dominated by thiophene (sulfur bonded to aromatic carbon). The two

363

least mature kerogens studied (0.55–0.66% Ro, 3.1–3.5 wt% Stotal) contain 5–6 mol% of organic

364

sulfide (sulfur bonded to aliphatic carbon) that is not identified in the more mature samples (0.9–

365

2.6% Ro, 1.3–2.1 wt% Stotal).

366

3.5. Skeletal density

367

Table 3 reports the skeletal density of the Woodford kerogens derived from helium

368

pycnometry measurements. Figure 9a shows that the skeletal density of kerogen increases

369

systematically with increasing level of thermal maturation from ~ 1.2 g/cm3 (0.55% Ro) to 1.55

370

g/cm3 (2.6% Ro), reflecting an increase of 30% relative. Fig 9b shows that skeletal density plots

371

along an array that is nearly inverse linear to their atomic H/C ratio.

372

3.6. Specific surface area

373

The Woodford kerogen isolates were prepared using novel acid demineralization plus

374

critical point drying procedures that retain the microstructural properties of kerogen.108 Table 3

375

reports the resulting Brunauer-Emmett-Teller (BET) calculated specific surface area (SSA) of 17 ACS Paragon Plus Environment

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376

the Woodford kerogens, which range over an order of magnitude from ~ 50 to ~ 300 m2/g.

377

Figure 10 plots SSA as a function of thermal maturity, showing the robust, positive correlation

378

between the two.

379

4. Discussion

380

The compositional evolution of kerogen from the paleo-marine Woodford Shale of the

381

Anadarko Basin, Oklahoma, is assessed here as a function of thermal maturation using a

382

combination of solid-state analytical techniques. The focus on sedimentary organic matter (OM)

383

from a single sedimentary formation aids to minimize compositional heterogeneity among

384

kerogens resulting from processes other than thermal maturation (e.g., differences in kerogen

385

type and depositional environment). Our data show that kerogen in the Woodford Shale evolves

386

dramatically and in a systematic manner during thermal maturation with respect to both its

387

chemical (e.g., carbon, hydrogen, and sulfur abundance and structure) and microstructural

388

properties (e.g., absolute density, specific surface area). The compositional characteristics of

389

kerogen in the Woodford Shale are compared, where available, with those measured for

390

kerogens from other paleo-marine sedimentary formations (type II kerogen), which provides

391

insights to whether thermal maturation of kerogen in other major oil- and gas-producing shales

392

follows compositional trends like those demonstrated here for the Woodford Shale.

393

The bulk elemental and FTIR data demonstrate, as expected, a substantial decrease in the

394

hydrogen content of Woodford kerogens during thermal maturation (Figures 5 and 7). The IR

395

structural indices show a decrease in the abundance of hydrogen-rich aliphatic carbons relative to

396

that of hydrogen-poor aromatic carbons (A-factor; Figure 7), as well as a shortening of the mean

397

length of remaining aliphatic chains by an increasing ratio of methyl to methylene groups

398

(CH3/CH2 ratio; Figure 7). The loss of hydrogen-rich aliphatic structural groups is not linear with 18 ACS Paragon Plus Environment

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Energy & Fuels

399

respect to increasing vitrinite reflectance. Rather, the magnitude is greatest at Ro values less than

400

or equal to 1.2% (immature to oil window [e.g.,

401

from kerogen at low thermal maturity is likely responsible for the generation of bitumen as a

402

precursor to generation of crude oils, as is indicated by laboratory artificial maturation

403

experiments.122, 123 Shown for comparison in Figure 5 are published H/C and vitrinite reflectance

404

data for type II kerogens represented by North American, European, and Middle East paleo-

405

marine organic-rich shales with ages from ~ 480 Ma to 90 Ma. Type II kerogens from these

406

different shales follow a H/C compositional trend with vitrinite reflectance that is

407

indistinguishable from that exhibited by the Woodford kerogens.40, 112, 124 The effect of thermal

408

stress on the cracking of carbon-carbon bonds appears to be largely similar for type II kerogen

409

globally.

121

]). This cracking of alkyl (C—C) structures

410

Insights to sulfur evolution during thermal maturation is provided by the bulk elemental

411

and sulfur XANES data. Sulfur has, despite its generally low abundance in type II kerogen,

412

particular significance for the rates of oil generation because of its proposed role in initiating free

413

radicals that promote cracking of alkyl C—C bonds at faster rate and lower thermal stress.114 The

414

sulfur data for the Woodford kerogens show an early and sharp change in both sulfur content and

415

organic-sulfur speciation as a function of thermal maturity. Total sulfur concentrations decrease

416

by ~ 50% relative from 3.1–3.5 wt% (atomic S/C ~ 0.015) to 1.3–2.1 wt% (atomic S/C ~ 0.01) at

417

vitrinite reflectance, Ro, values between 0.7% and 0.9%, but remain relatively uniform thereafter

418

(Figure 11). These data support the reactive nature of one or more sulfur species in kerogen, and

419

are at least consistent with the hypothesis that cracking of sulfur bonds occurs before and

420

initiates the cracking of alkyl C—C bonds, discussed above. The complementary sulfur XANES

421

data indicates that reactive sulfur is mostly aliphatic sulfide (~ 6 mol%), which is observed in


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

422

kerogens with Ro values of less than 0.7%, but not in kerogens of higher thermal maturity

423

(Figure 8). Sulfur species abundances, expressed as the ratio of reactive sulfur (aliphatic sulfide

424

± elemental) to thermally stable sulfur (aromatic thiophene), exhibits the same sharp and early

425

decrease as does the total sulfur content (Figure 11). Considering sulfur mass balance, the factor

426

of two reduction in total sulfur concentration cannot be accommodated only by the removal of

427

reactive sulfide. Thiophenic sulfur dominates the sulfur speciation of all Woodford kerogens

428

(83–89 mol% of organic sulfur), and some aromatic sulfur must be lost of kerogen by direct

429

cracking of aromatic S—C bonds or as collateral loss from cracking of lower-molecular-weight

430

fragments (i.e., bitumen, oil) in which thiophene is present. Indeed, the latter can explain the

431

predominance of thiophene as the major sulfur species in the heaviest molecular fractions of

432

petroleum (i.e., asphaltene).63 The narrow range of thiophene-dominated sulfur distributions of

433

the Woodford kerogens is similar to that documented for kerogen from the Silurian Qusaiba

434

Shale in Saudi Arabia spanning a similar range of thermal maturity (Ro ~ 0.9–2.0%).112 But, the

435

Woodford kerogens do not express as high sulfur concentrations (maximum atomic S/C ~ 0.03),

436

nor the wide range of thiophene abundances (30–90 mol%) measured by Kelemen et al. for other

437

type II kerogens from several shales of mostly lower thermal maturity (mean Rock-Eval Tmax
1.5%.20, 22, 25, 27 These studies have also shown a concomitant decreasing fraction

481

of aromatic carbon bonded to alkyl and heteroatom group attachments. Kelemen et al.25

482

documented for a diverse set of kerogens a robust, negative correlation between their fraction of

483

total carbon in aromatic structures from 13C NMR and XPS measurements and their bulk atomic

484

H/C ratio. The correlation is expected because aromatic carbon structures in kerogen have atomic

485

H/C ratios ≤ 1 whereas aliphatic carbon structures have atomic H/C ratios ≥ 2. Our regression

486

through this correlation [%Caromatic = 110 – 59 × H/C] allows us to estimate an equivalent

487

aromatic carbon fraction in the Woodford kerogens from their measured atomic H/C ratios. The

488

estimated %Caromatic values in the Woodford kerogens increases from ~ 45% at Ro = 0.55% to ~

489

70 % at Ro = 1.55% (Table 3), which is indistinguishable from the values calculated from NMR

490

spectroscopy for other type II kerogens of equal thermal maturity.20,


22, 25, 27

Extending this

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491

correlation to the most mature Woodford kerogen studied (atomic H/C = 0.45) predicts a

492

%Caromatic value of ~ 83% at Ro = 2.6% (Table 3). The relative trends in the abundance of

493

aromatic carbon and its attachments derived from the FTIR analysis are, for the set of Woodford

494

kerogens, consistent with the interpretations of changes in carbon structures for other type II

495

kerogens derived from NMR spectroscopy.

496

Substantial changes in the abundance of molecular functional groups in kerogen during

497

maturation, described above, are shown here to be complemented by evolution of its physical

498

properties. The skeletal density of the Woodford kerogen increases substantially with increasing

499

vitrinite reflectance, from ~ 1.2 g/cm3 to at least 1.55 g/cm3 (Figure 9). This trend is interpreted

500

to reflect the gross aliphatic versus aromatic character of kerogen, which evolves from aliphatic-

501

rich (analogous to alkyl structures such as waxes that have lower density of < 1 g/cm3) to

502

aromatic-rich (analogous to graphite that has higher density of ~ 2 g/cm3). The density versus

503

maturity trend shown by the Woodford kerogens is indistinguishable from that shown by other

504

type II kerogens from numerous organic-rich shales (Figure 9a).112,

505

chemical and physical measurements of the Woodford kerogens here shows that the absolute

506

density of kerogen is nearly linearly correlated to its atomic H/C content, and by extension its

507

aromatic carbon content (Figure 9b). Assuming this linear correlation holds for any given H/C

508

ratio, the inferred density of kerogen is less than that of pure graphite, reflecting presence of

509

heteroatoms and imperfect ordering of aromatic clusters in catagenic kerogen in contrast to

510

graphite that is composed of essentially infinite and perfectly ordered sheets of aromatic carbon.

117-119

Integrating the

511

Not only does the density of Woodford kerogens measurably change during maturation,

512

so does their internal specific surface area (SSA). Novel and quantitative insights to kerogen

513

pore structure is enabled here using combined acid demineralization and critical-point drying 23 ACS Paragon Plus Environment

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Page 24 of 49

514

methods that are shown to preserve kerogen microstructure.108 For these samples, SSA increases

515

by nearly a factor of ten between immature kerogen (~ 50 m2/g) and over-mature kerogen in the

516

dry-gas window (~ 300 m2/g; Figure 10). The calculated SSA values for the Woodford kerogens

517

overlap those of other type II kerogens prepared using the same critical-point drying methods,112

518

as well as with those calculated from surface area measurements of bulk shales after correcting

519

for inorganic porosity.130 The development of internal pore surfaces in Woodford kerogens is

520

consistent, for example, with X-ray and electron imaging studies75,

521

qualitatively show abundant nanometer- to micrometer-sized pores formed within kerogen

522

during maturation. This evolution, when interpreted in context of complementary chemical

523

evolution, is explained by the creation of porous volumes within a skeletal framework of

524

imperfectly ordered aromatic carbons, resulting from the cracking and expulsion of aliphatic-

525

dominated structures during maturation.

526

4. Summary and Conclusions

77-79, 95, 131, 132

that

527

A set of kerogens (defined here as the solid, insoluble, and non-volatile fraction of

528

sedimentary organic matter) spanning a maturity transect (vitrinite reflectance, Ro, between 0.5%

529

to 2.6%) across the Woodford Shale of the Anadarko Basin in Oklahoma (paleo-marine, type II

530

kerogen) was studied using a combination of spectroscopic and gas intrusion methods, providing

531

detailed insights to their bulk chemical, molecular, and physical evolution during geological

532

maturation. The following conclusions are made: Woodford kerogens follow the expected trend of substantially decreasing bulk

533

•

534

H/C content with increasing thermal maturity. IR spectra indicate that the loss of

535

hydrogen from kerogen is related to cracking of hydrogen-rich aliphatic (alkyl) chains

536

from aromatic carbon clusters. Within the oil-generation window (Ro ≤ ~ 1.4%), aromatic 24 ACS Paragon Plus Environment

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Energy & Fuels

537

carbons remain highly substituted with alkyl (methyl, methylene) and probably

538

heteroatom (O, S, N) functional groups. At higher thermal stress (Ro > ~ 1.5%) these

539

aromatic substitutions are substantially removed and eventually replaced by hydrogen.

540

The structural evolution inferred from spectroscopic methods is consistent with known

541

carbon bond dissociation energies for carbonaceous materials.

542

•

543

have low sulfur concentrations (< 4 wt%), dominated by aromatic moieties (thiophene; ~

544

80–90 mol%). Reactive sulfur (aliphatic sulfide) is eliminated at low thermal maturity

545

(Ro ≤ 0.9%). At higher thermal stress, sulfur content and speciation remains stable.

546

•

547

manner consistent with their molecular characteristics. Kerogen skeletal density increases

548

by at least 30% relative (1.2 g/cm3 to 1.55 g/cm3) for the range of thermal maturity

549

studied. Density is linearly and negatively correlated with H/C ratio, and by inference is

550

linearly and positively correlated with aromatic carbon content, evolving from aromatic-

551

lean (i.e., alkyl-rich) to aromatic-rich (graphitic).

552

•

553

procedures to isolate and dry kerogen, and show a nearly order of magnitude increase (~

554

50 m2/g to 300 m2/g) for the range of thermal maturity studied. Increasing surface area is

555

consistent with development of organic-hosted pores within an imperfectly ordered

556

framework of aromatic carbons, resulting from preferential cracking and expulsion of

557

aliphatic carbons on the periphery of aromatic clusters during maturation and petroleum

558

generation.

Organic sulfur contents and sulfur XANES results show that Woodford kerogens

Physical properties of Woodford kerogens evolve during thermal maturation in a

Internal pore surface area measurements of kerogen are enabled by novel


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Globally, chemical and physical properties of type II kerogens from numerous

559

•

560

unconventional oil and gas shale plays follow maturity trends indistinguishable from that

561

exhibited by Woodford kerogen, with respect to carbon and hydrogen content, carbon

562

speciation, and density. Subtle differences may occur among type II kerogens with

563

respect to heteroatom (e.g., S, O) content and speciation, although their evolution with

564

maturity is similar.

565

Acknowledgements

566

We gratefully acknowledge Brian Cardott of the Oklahoma Geological Survey for

567

providing the Woodford Shale samples and their vitrinite reflectance data, for numerous

568

beneficial discussions and a review of an earlier draft of this manuscript, and for support to

569

publish these results. We appreciate the constructive reviews from three reviewers on our

570

original submission. We thank Schlumberger-Doll Research PetroLabs for assistance with

571

sample preparation and analysis.


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572

Figure Captions

573

Fig. 1. Sulfur K-edge XANES spectra for sulfur standards, arranged top-to-bottom in order of

574

increasing sulfur oxidation state.

575

Fig. 2. Transmission FTIR and sulfur K-edge XANES spectra of asphaltene extracts from a

576

Middle East black oil (UG-8) before and after treatment with concentrated acids. The spectra and

577

the resulting IR and XANES structural indices of the two fractions are the same within

578

uncertainty, indicating that the acid treatment does not measurably impact the chemical

579

composition of solid carbonaceous materials.

580

Fig. 3. Rock-Eval Tmax and VRe maturity parameters plotted against vitrinite reflectance, Ro.

581

Fig. 4. Van Krevelen diagram showing that the Woodford kerogens follow the expected

582

evolution of atomic H/C and O/C ratios during thermal maturation for type II organic matter.

583

Fig. 5. Atomic H/C ratio of Woodford kerogen plotted versus their vitrinite reflectance, Ro.

584

Previously published data for other type II organic matter are plotted for comparison.40, 112, 124

585

Fig. 6. Transmission FTIR spectra of Woodford kerogens, arranged top-to-bottom in order of

586

increasing thermal maturity.

587

Fig. 7. IR structural indices (CH3/CH2, A-factor, Aromaticity, and C-factor) plotted against

588

measured vitrinite reflectance, Ro, and atomic H/C.

589

Fig. 8. Sulfur K-edge XANES spectra and calculated distribution of organic sulfur species for

590

Woodford kerogens, arranged top-to-bottom in order of increasing thermal maturity. Kerogen

591

sulfur species are dominated by thermally stable thiophene (sulfur in aromatic rings) and show

592

the early loss of reactive sulfide (sulfur in aliphatic chains).

593

Fig. 9. Skeletal density of Woodford kerogen plotted versus (a) vitrinite reflectance, Ro, and (b)

594

atomic H/C ratio. Published data for other type II kerogen are shown for comparison.112, 118, 119,

595

133

596

Fig. 10. Specific surface area of Woodford kerogen plotted versus vitrinite reflectance, Ro.

597

Published data for other type II kerogens from bulk shale and kerogen isolate analyses are shown

598

for comparison.112, 130 27 ACS Paragon Plus Environment

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599

Fig. 11. Sulfur content (weight percent and atomic S/C) and sulfur speciation (molar ratio of

600

aliphatic sulfur [sulfide + elemental] to aromatic sulfur [thiophene]) of Woodford kerogen

601

plotted versus vitrinite reflectance, Ro, and atomic H/C.

602 603

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105. Comer, J. B., Organic geochemistry and paleogeography of Upper Devonian formations in Oklahoma and western Arkansas. In Source Rocks in the Southern Midcontinent, 1990 Symposium: Oklahoma Geological Survey Circular 93, Johnson, K. S.; Cardott, B. J., Eds. 1992; pp 70-93. 106. Cardott, B. J., Thermal maturation of the Woodford Shale in the Anadarko basin. In Anadarko Basin Symposium 1988: Oklahoma Geological Survey Circular 90, Johnson, K. S., Ed. 1989; pp 32-46. 107. Ibrahimov, R. A.; Bissada, K. K., Comparative analysis and geological significance of kerogen isolated using open-system (palynological) versus chemically and volumetrically conservative closed-system methods. Organic Geochemistry 2010, 41, 800-811. 108. Suleimenova, A.; Bake, K. D.; Ozkan, A.; Valenza, J. J.; Kleinberg, R. L.; Burnham, A. K.; Ferralis, N.; Pomerantz, A. E., Acid demineralization with critical point drying: A method for kerogen isolation that preserves microstructure. Fuel 2014, 135, 492-497. 109. Jackson, L., L.; Roof, S. R., Determination of the forms of carbon in geological materials. Geostandards Newsletter 1992, 16, 317-323. 110. Jarvie, D. M.; Claxton, B. L.; Henk, F.; Breyer, J. T., Oil and shale gas from the Barnett Shale, Ft. Worth Basin, Texas (abs). AAPG Annual Meeting, Program with Abstracts, p. A100 2001. 111. Durand, B.; Monin, J. C., Elemental analysis of kerogens (C, H, O, N, S, Fe). In Kerogen—Insoluble Organic Matter from Sedimentary Rocks, Durand, B., Ed. Editions Technip: Paris, 1980; pp 113-142. 112. Cheshire, S.; Craddock, P. R.; Xu, G.; Sauerer, B.; Pomerantz, A. E.; McCormick, D.; Abdallah, W., Assessing thermal maturity beyond the reaches of vitrinite reflectance and RockEval pyrolysis: A case study from the Silurian Qusaiba formation. International Journal of Coal Geology 2017, 180, 29-45. 113. Greenfield, M. L.; Byrne, M.; Mitra-Kirtley, S.; Kercher, E. M.; Bolin, T. B.; Wu, T.; Craddock, P. R.; Bake, K. D.; Pomerantz, A. E., XANES measurements of sulfur chemistry during asphalt oxidation. Fuel 2015, 162, 179-185. 114. Lewan, M. D., Sulphur-radical control on petroleum formation rates. Nature 1998, 391, 164-166. 115. Zhang, J.; Clennell, M. B.; Dewhurst, D. N.; Liu, K., Combined Monte Carlo and molecular dynamics simulation of methane adsorption on dry and moist coal. Fuel 2014, 122, 186-197. 116. Zhao, T.; Li, X.; Zhao, H.; Li, M., Molecular simulation of adsorption and thermodynamic properties on type II kerogen: Influence of maturity and moisture content. Fuel 2017, 190, 198207. 117. Guidry, K.; Luffel, D.; Curtis, J. Development of Laboratory and Petrophysical Techniques for Evaluating Shale Reservoirs, Final Report; Gas Research Institute, Chicago, Illinois: 1995; p 304. 118. Alfred, D.; Vernik, L., A new petrophysical model for organic shales. Petrophysics 2013, 54, 240-247. 119. Okiongbo, K. S.; Aplin, A. C.; Larter, S. R., Changes in Type II kerogen density as a function of maturity: Evidence from the Kimmeridge Clay Formation. Energy & Fuels 2005, 19, 2495-2499. 120. Pelet, R.; Behar, F.; Monin, J. C., Resins and asphaltenes in the generation and migration of petroleum. Organic Geochemistry 1986, 10, 481-498. 121. Dembicki Jr., H., Three common source rock evaluation errors made by geologists during prospecct or play appraisals. AAPG Bulletin 2009, 93, 341-356. 122. Lewan, M. D., Experiments on the role of water in petroleum formation. Geochimica et Cosmochimica Acta 1997, 61, 3691-3723.


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123. Le Doan, T. V.; Bostrom, N. W.; Burnham, A. K.; Kleinberg, R. L.; Pomerantz, A. E.; Allix, P., Green River oil shale pyrolysis: Semi-open conditions. Energy & Fuels 2013, 27, 74476459. 124. Buchardt, B.; Lewan, M. D., Reflectance of vitrinite-like macerals as a thermal maturity index for Cambrian-Ordovician Alum Shale, southern Scandinavia. AAPG Bulletin 1990, 74, 394-406. 125. Tissot, B.; Deroo, G.; Hood, A., Geochemical study of the Uinta Basin: formation of petroleum from the Green River formation. Geochimica et Cosmochimica Acta 1978, 42, 14691485. 126. Huss, E. B.; Burnham, A. K., Gas evolution during pyrolysis of various Colorado oil shales. Fuel 1982, 61, 1188-1196. 127. Burnham, A. K.; Clarkson, J. E.; Singleton, M. F.; Wong, C. M.; Crawford, R. W., Biological markers from Green River kerogen decomposition. Geochimica et Cosmochimica Acta 1982, 46, 1243-1261. 128. Sanderson, R. T., Chemical Bonds and Bond Energy. Academic Press: New York, N.Y., 1976. 129. Luo, Y.-R., Comprehensive Handbook of Chemical Bond Energies. CRC Press: Boca Raton, FL, 2007. 130. Valenza, J. J.; Drenzek, N.; Marques, F.; Pagels, M.; Mastalerz, M., Geochemical controls on shale microstructre. Geology 2013, 41, 611-614. 131. Bernard, S.; Wirth, R.; Schreiber, A.; Schulz, H.-M.; Horsfield, B., Formation of nanoporous pyrobitumen residues during maturation of the Barnett Shale (Fort Worth Basin). International Journal of Coal Geology 2012, 103, 3-11. 132. Milliken, K. L.; Rudnicki, M.; Awwiller, D. N.; Zhang, T., Organic matter-hosted pore system, Marcellus Formation (Devonian), Pennsylvania. AAPG Bulletin 2013, 97, 177-200. 133. Guidry, F. K.; Luffel, D. L.; Olszewskl, A. J., Devonian shale formation evaluation models based on logs, new core analysis methods, and production tests. In Transactions of the SPWLA 31st Annual Logging Symposium, June 24-27, Lafayette, Louisiana, USA, 1990.

973


Energy & Fuels

12

FeS2

9

Pyrite

S8

Elemental

S S

I/I0

Sulfide

6

Thiophene

S

O

O

OH H5C2O

Sulfinic

3

OH

S

Sulfoxide

NH2

O

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 36 of 49

OC2H5

S

O

O

Sulfite

H3C

S

O

O

O

Sulfone

H3C

Sulfonate

O-

S O S

CH3

CH3

O O-

O

Sulfate

2465

2470

2475

2480

Photon Energy, eV

2485

Fig. 1 1.5-column width (request figure caption right of figure, not below) ACS Paragon Plus Environment

Page 37 of 49

IR absorption, a.u.

0.8

FTIR structural indices

1.0

original acid-treated

1.0

0.8 0.6 0.4

0.6

0.2

0.4

0.0

CH3/CH2

0.2 0.0 500

1000

1500

2000

2500

Wavenumber, cm-1

3000

3.0

Aromaticity

C-Factor

3500

Organic sulfur species, mole percent

Acid-treated

2.0

0%

1.5

20%

elemental

1.0

sulfide

0.5 0.0 2460

A-Factor

Original

2.5

XANES I/I0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

2465

2470

2475

2480

2485

2490

Photon energy, eV

Fig. 2 Double-column width ACS Paragon Plus Environment

40%

60%

thiophene

80%

sulfoxide

100% sulfone

Energy & Fuels

480

Rock-Eval Tmax, oC

460 440 420 400 380 4

Rock-Eval VRe, %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 38 of 49

3 2 1 0

0

1

2

Ro, %

3

4

Fig. 3 Single-column width ACS Paragon Plus Environment

Page 39 of 49

type I kerogen

(Peters, 1986)

1.5

Atomic H/C

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

type II kerogen 0.55 %Ro 0.66 %Ro

1.0

type III kerogen 0.90 %Ro 1.16 %Ro 1.55 %Ro

0.5

1.82 %Ro 2.59 %Ro

0.0 0.0

0.1

0.2

Atomic O/C

0.3

Fig. 4 Single-column width

ACS Paragon Plus Environment

Energy & Fuels

1.75

Buchardt & Lewan 1990 Lis et al. 2005 Cheshire et al. 2017

1.5

Atomic H/C

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 40 of 49

1.25 1.0 0.75 0.5 0.5

1

1.5

2

2.5

Ro, %



Page 41 of 49

aliphatic CH2, CH3

aliphatic CH2, CH3

Absorption, a.u. (spectra offset)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

aliphatic CH3

aromatic CC aromatic CH

CO

0.55 %Ro 0.90 %Ro 1.16 %Ro 1.82 %Ro 2.59 %Ro

500

1000 1500 2000 2500 3000 3500

Wavenumber, cm-1



Energy & Fuels

CH3/CH2

1.25

2.59 %Ro 1.82 %Ro

1

1.16 %Ro 0.90 %Ro

0.75 0.55 %Ro

A-Factor

0.5 1 0.75 0.5 0.25

Aromaticity

0 0.5 0.4 0.3 0.2 0.1 0 0.6 0.5

C-Factor

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 42 of 49

0.4 0.3 0.2 0.1

0

1

2

Ro, %

3 1.2

1.0

0.8

0.6 0.4

Atomic H/C


Page 43 of 49

elemental

Ro, %

0.55 %Ro

Fluorescence, I/I0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

0.66 %Ro 0.90 %Ro 1.16 %Ro 1.82 %Ro

sulfide

thiophene

sulfoxide

sulfone

0.55 7 6

83

4

0.66 7 6

83

4

0.90 8

89

3

1.16 8

89

3

1.82 8

88

4

2.59 8

83

0

20

40

7

60

80

100

Organic sulfur species concentration, mol%

2.59 %Ro

2455 2460 2465 2470 2475 2480 2485 2490

Photon energy, eV

Fig. 8 Double-column width ACS Paragon Plus Environment

Energy & Fuels

Kerogen density, g/cm3

1.8

(a)

1.6 1.4 1.2

Guidry et al., 1995 Okiongbo et al., 2005 Alfred & Vernik, 2013 Cheshire et al., 2017

1.0 0.5

1.8

Kerogen density, g/cm3

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 44 of 49

1

1.5

Ro, %

2

2.5

(b)

1.6 1.4 1.2 1.0 1.2

1.0

0.8

Atomic H/C

0.6

0.4


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

Specific surface area, m2/g

Page 45 of 49

Valenza II et al. 2013 Cheshire et al. 2017

600

400

200

0

0.5

1

1.5

2

2.5

Ro, %


3

Energy & Fuels

5

S, wt%

4 3 2 1

Atomic S/C x 100

0 2.0 1.5 1.0 0.5 0 0.25

Saliphatic/Saromatic

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 46 of 49

0.2 0.15 0.1 0.05 0

0

1

2

Ro, %

3

1.2

1.0

0.8

0.6 0.4

Atomic H/C


Page 47 of 49 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

Table 1. Woodford shale samples, ordered by increasing vitrinite reflectance VRo OPL # County Type Depth TOC ft % N wt% 549G Garvin Cuttings 8,530-8,540 0.55 41 5.8 536G Carter Cuttings 7,690-7,700 0.57 55 6.0 576D Garvin Cuttings 10,970-10,990 0.63 44 4.3 787G McClain Cuttings 9,960-9,980 0.64 50 3.9 758M Garvin Cuttings 8,780-8,800 0.66 40 5.9 165 Blaine Core 8,414-8,437 0.71 145 6.6 150 Caddo Core 6,129-6,236 0.77 25 5.8 243M Grady Cuttings 12,120-12,130 0.83 40 2.2 569F Grady Cuttings 12,530-12,560 0.90 52 4.1 270H Blaine Cuttings 9,620-9,630 0.94 40 2.8 570E Stephens Cuttings 14,670-14,680 0.94 54 5.6 436U Stephens Cuttings 14,280-14,290 1.07 40 5.1 241 Grady Cuttings 12,960-12,970 1.16 65 8.3 241G Grady Cuttings 12,940-12,950 1.16 65 5.3 246D Canadian Cuttings 12,401-12,408 1.32 45 0.4 353H Blaine Cuttings 13,730-13,740 1.55 40 4.3 803M Beckham Cuttings 16,440-16,450 1.82 42 5.2 271D Roger Mills Cuttings 16,250-16,260 2.48 42 0.8 581E Roger Mills Cuttings 18,150-18,150 2.59 69 2.6 392E Beckham Cuttings 23,920-23,930 4.05 70 2.7

S1 mgHC/g 0.29 0.34 0.51 0.26 0.55 0.05 0.15 0.10 0.28 0.15 0.20 0.21 0.09 0.18 0.05 0.10 0.07 0.10 0.08 0.08

S2 mgHC/g 22.7 24.3 9.1 8.5 18.0 13.1 19.9 1.7 3.4 2.4 2.9 1.5 4.6 1.9 0.07 0.56 0.39 0.10 0.05 0.50


S3 mgHC/g 0.24 0.23 0.43 0.27 0.35 0.21 0.16 0.28 0.47 0.62 0.65 0.32 0.69 0.41 0.44 0.33 0.82 0.49 0.33 0.34

T max Calc. Ro H.I. O.I. °C % mgHC/g mgHC/g 434 0.65 392 4 440 0.76 402 4 440 0.76 213 10 437 0.71 215 7 435 0.67 308 6 444 0.83 200 3 441 0.78 343 3 452 0.98 78 13 446 0.87 82 11 438 0.72 83 22 442 0.80 52 12 465 1.21 29 6 462 1.16 55 8 452 0.98 36 8 474 1.37 17 107 456 1.05 13 8 444 0.83 7 16 436 0.69 13 65 414 0.29 2 13 397 0.00 19 13

Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Table 2. Definition of IR structural indices extracted from transmission FTIR spectra Parameter Description Formula Aliphatic CH3/CH2 ratio I 2962/I 2925 CH3/CH2 Aromaticity Aromatic CH/[Aromatic CH + Aliphatic CHx] I 3000-3100/[I 3000-3100 + I 2800-3000] A-factor Aliphatic CH2/[Aliphatic CH2 + Aromatic C=C] [I 2857 + I 2925]/[I 2857 + I 2925 + I 1600-1630] C-factor C=O/[C=O + Aromatic C=C] I 1650-1770/[I 1650-1770 + I 1600-1630]


Page 48 of 49

S aliphatic mol% 14 13 8 8 n.d. 8 8

S aromatic mol% 83 83 89 89 n.d. 88 83

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Table 3. Kerogen density, surface area, elemental analysis, FTIR, and XANES results Skeletal Surface Elemental Analysis Calc. IR Structural Indices XANES Ro Density Area C H N S O CAromatic CH3/CH2 Aromaticity A-Factor C-Factor Elemental Sulfide Thiophene Sulfoxide Sulfone Sulfonate Sulfate g/cm3 m2/g % wt% wt% wt% wt% wt% % mol% mol% mol% mol% mol% mol% mol% 0.55 1.203 54 80.2 7.3 2.0 3.5 5.9 46 0.59 0.01 0.74 0.45 7 6 78 3 0 3 2 0.66 1.228 57 80.0 6.9 2.4 3.1 5.4 49 n.d. n.d. n.d. n.d. 7 5 75 4 0 4 5 0.90 150 81.2 5.4 2.6 1.8 6.8 64 0.81 0.06 0.46 0.44 7 0 78 3 0 6 6 1.16 1.356 114 82.0 5.2 2.6 1.6 4.3 66 0.99 0.10 0.31 0.41 7 0 76 3 0 6 9 1.55 1.385 235 79.7 4.6 2.3 1.3 5.5 69 n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. 1.82 1.380 248 80.1 3.9 2.5 2.1 8.1 76 1.09 0.20 0.22 0.39 7 0 76 4 0 6 8 2.59 1.548 298 81.0 3.1 2.0 1.6 8.7 83 1.11 0.37 0.10 0.28 7 0 70 6 1 10 6 n.d. = no data acquired due to limited sample

Page 49 of 49 Energy & Fuels


Chemical, Molecular, and Microstructural Evolution of Kerogen during

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