Assessing thermal maturity beyond the reaches of vitrinite reflectance and Rock-Eval pyrolysis: A case study from the Silurian Qusaiba formation

Accepted Manuscript Assessing thermal maturity beyond the reaches of vitrinite reflectance and Rock-Eval pyrolysis: A case study from the Silurian Qusaiba formation

Stephen Cheshire, Paul R. Craddock, Guangping Xu, Bastian Sauerer, Andrew E. Pomerantz, David McCormick, Wael Abdallah PII: DOI: Reference:

S0166-5162(17)30114-3 doi: 10.1016/j.coal.2017.07.006 COGEL 2864

To appear in:

International Journal of Coal Geology

Received date: Revised date: Accepted date:

6 February 2017 29 May 2017 10 July 2017

Please cite this article as: Stephen Cheshire, Paul R. Craddock, Guangping Xu, Bastian Sauerer, Andrew E. Pomerantz, David McCormick, Wael Abdallah , Assessing thermal maturity beyond the reaches of vitrinite reflectance and Rock-Eval pyrolysis: A case study from the Silurian Qusaiba formation. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Cogel(2017), doi: 10.1016/j.coal.2017.07.006

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ACCEPTED MANUSCRIPT Assessing Thermal Maturity Beyond the Reaches of Vitrinite Reflectance and Rock-Eval Pyrolysis: A Case Study from the Silurian Qusaiba Formation

Stephen Cheshirea, Paul R. Craddockb,* , Guangping Xub, Bastian Sauererc, Andrew E.

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Pomerantzb, David McCormickb, and Wael Abdallahc

EXPEC Advanced Research Center, Saudi Aramco, Dhahran, Saudi Arabia

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Schlumberger-Doll Research Center, Cambridge, MA, USA

Schlumberger Dhahran Carbonate Research, Dhahran, Saudi Arabia

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* Corresponding author

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Keywords

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Thermal maturity; Vitrinite reflectance; Shale; Qusaiba

ACCEPTED MANUSCRIPT Abstract Thermal maturity assessment in pre-Devonian shales is challenging due to the absence of vitrinite macerals that form the basis for vitrinite reflectance petrography, the most-widely used technique for organic maturity assessment. This paper presents an integrated analysis of thermal

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maturity on the basis of alternative spectroscopic and geochemical techniques in lieu of

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conventional organic petrography, applied in four drilled wells in the pre-Devonian (Silurian)

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Qusaiba Member of the Qalibah Formation in northwestern Saudi Arabia. The techniques

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comprise both bulk sample (Rock-Eval pyrolysis, infrared and Raman spectroscopy), and kerogen isolate analysis (elemental, density, surface area, and X-ray absorption near edge

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structure), with each method calibrated to the vitrinite reflectance scale. Rock-Eval pyrolysis, a common alternative to vitrinite reflectance measurements, provided unreliable maturity estimates

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in the majority of samples because of low S2 signal. In contrast, the other techniques defined a

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consistent and narrow maturity range within each well, and revealed a wide range of maturity

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between the wells. On the basis of the spectroscopic and geochemical results, equivalent vitrinite reflectance for the Qusaiba Member in northwestern Saudi Arabia ranges from at least 0.9 ±0.1

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to 2.1 ±0.2 %Ro, demonstrating significant variation in its maturation history. The integrated

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assessment of maturity across multiple wells provides data that can be used to construct maturity maps for oilfield exploration and appraisal. More generally, the methods and calibrations for thermal maturity presented here can be used to establish vitrinite-reflectance-equivalent maturities in shales as a complement to or especially in the absence of conventional maturity estimates.

ACCEPTED MANUSCRIPT Highlights Thermal maturity is assessed in Silurian Qusaiba Member, Saudi Arabia

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Vitrinite reflectance analysis is precluded by absence of vitrinite in organic matter

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Study uses alternative spectroscopic and geochemical techniques

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Maturity estimates are consistent in a given well and range from 0.9–2.1 %Ro in the

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studied wells

ACCEPTED MANUSCRIPT 1. Introduction A key parameter in petroleum exploration of shale is thermal maturity that, together with organic matter type, controls the amount and type of hydrocarbon generated (e.g., Dow, 1977). Probably the most robust and widely used measure of thermal maturity is the vitrinite reflectance

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technique originally developed for the study of coals, in which the mean reflectivity of vitrinite

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macerals (expressed in units of %Ro) in sedimentary organic matter (OM, or kerogen) immersed

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in oil is determined by optical microscopy (Corcoran and Doré, 2006; Dembicki Jr., 2009;

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Dutcher et al., 1974; Hackley et al., 2015; Pawlewicz and King, 1992). A second common technique to estimate thermal maturity is programmed pyrolysis, also termed Rock-Eval

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pyrolysis (Behar et al., 2001; Clementz et al., 1979; Peters, 1986). In this method, powdered samples are heated in an inert atmosphere to quantify the abundance in the sample of existing,

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free hydrocarbons (S1 peak) and of potentially generative hydrocarbons after thermal maturation

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(S2 peak), among other hydrocarbon types. The temperature at which the maximum amount of

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S2 hydrocarbons is generated is called Tmax. Tmax has been calibrated to vitrinite reflectance %Ro for use as a thermal maturity indicator (e.g., Jarvie et al., 2001; Peters, 1986).

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These techniques, while generally robust, are not universally applicable. For example,

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marine shales that are the majority of the prolific petroleum-generating source rocks may contain little to no vitrinite, making the vitrinite reflectance technique challenging or impossible. In addition, the technique is limited to Devonian or younger sequences, because the first significant evolution of higher land plants that were built with lignin and cellulose occurred in the Devonian (Kenrick and Crane, 1997). Vitrinite, which is derived from diagenesis of these organic materials, therefore is absent in pre-Devonian formations. Interpretation of Rock-Eval Tmax must be done with caution. The Tmax of sedimentary OM is partly dependent upon kerogen type, can

ACCEPTED MANUSCRIPT be skewed by presence of additional organic phases such as migrated oil and indigenous solid bitumen, and Tmax values for samples containing little to no hydrocarbon-generative potential (low S2 signal) commonly are inaccurate and lead to erroneous maturity estimates (Clementz, 1979; Peters, 1986).

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Several petrographic methods have been offered as alternatives to vitrinite reflectivity for maturity determination, including zooclast (e.g., graptolite, chitinozoan, scolecodont) reflectivity

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(Bertrand, 1990; Bertrand and Héroux, 1987; Cole, 1994; Goodarzi, 1985; Goodarzi and

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Norford, 1989; Link et al., 1990; Tricker et al., 1992) and solid bitumen reflectivity (Gentzis and Goodarzi, 1990; Jacob, 1989; Petersen et al., 2013; Riediger, 1993). Zooclasts have been used

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for maturity determinations of pre-Devonian source rocks in particular. These alternative

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petrographic techniques are not without their challenges. Correct zooclast and maceral identification is critical for all petrographic studies. This is difficult for solid bitumen, for

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example, because it is known to have multiple origins (Curiale, 1986), and only indigenous

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source-rock bitumen is appropriate for maturity determinations, whose identification can be complicated by co-existence of migrated bitumen with nearly identical petrographic

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characteristics (Gentzis and Goodarzi, 1990). Chitinozoans may (Tricker et al., 1992) or may not

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(Cole, 1994; Goodarzi, 1985) be easily identified, but their abundance is low in pre-Devonian sedimentary formations, complicating chitinozoan reflectivity estimates. Graptolites are known to exhibit bireflectance making an objective analysis of reflectivity challenging, especially among different laboratories. Graptolite reflectivity also can be affected by factors other than thermal maturation, including their environment of deposition, rate of sedimentation, and degree of preservation (Cole, 1994). Probably the most acute impediment against zooclast and solid bitumen reflectance as maturity indicators is that multiple correlations between vitrinite

ACCEPTED MANUSCRIPT reflectance and both zooclast reflectance and solid bitumen reflectance have been proposed (Bertrand, 1990; Katz and Everett, 2016; Petersen et al., 2013), making it difficult to establish a consistent and absolute vitrinite reflectance from zooclast or solid bitumen inspection of unknown samples.

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The challenges of estimating thermal maturity exist for the Silurian Qusaiba Member of the Qalibah Formation in northwestern Saudi Arabia that is a case study for this paper. The

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Qusaiba Member is the principal source rock for Paleozoic petroleum reservoirs in the Central

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Province of Saudi Arabia (Cole et al., 1994; Mahmoud et al., 1992), and has recently been investigated as a potential unconventional petroleum target. The pre-Devonian age of the

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Qusaiba Member precludes vitrinite reflectivity. Cole (1994) presented an analysis of zooclast

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(graptolite, chitinozoan) reflectivity in the Qusaiba Member, but concluded these methods yielded maturity estimates inconsistent with those from Rock-Eval, biomarker, and thermal

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alteration index analysis done on the same samples and inconsistent with zooclast reflectivity

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studies done on other pre-Devonian formations. İnan et al. (2016) recently provided maturity estimates of the Qusaiba Member from graptolite reflectivity, the majority consistent with those

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from Rock-Eval run on the same samples. Graptolite reflectivity appears appropriate for the

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range of low maturities documented in İnan et al. (2016), but was not applied for higher maturity values examined in our study. In contrast to the studies of Cole (1994) and İnan et al. (2016), nearly every sample in our study have low Rock-Eval pyrolysis S2 yields below 1 mg hydrocarbon/g sample, making the resulting Tmax value unreliable for thermal maturity estimates. In lieu of conventional petrography and Rock-Eval measurements, we use several spectroscopic and geochemical alternatives to constrain thermal maturity of sedimentary OM in the Qusaiba Member of northwestern Saudi Arabia. The methods include infrared spectroscopy

ACCEPTED MANUSCRIPT (Christy et al., 1989; Craddock et al., 2017; Ganz and Kalkreuth, 1987; Lin and Ritz, 1993; Lis et al., 2005), Raman spectroscopy (Kelemen and Fang, 2001; Sauerer et al., 2017; Schmidt Mumm and İnan, 2016; Spötl et al., 1998; Wopenka and Pasteris, 1993), kerogen H/C composition (Buchardt and Lewan, 1990; Lis et al., 2005; Van Krevelen, 1950), kerogen sulfur speciation

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(Kelemen et al., 2012; Pomerantz et al., 2014), kerogen density (Alfred and Vernik, 2013; Jacob,

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1989; Ward, 2010), and kerogen surface area (Cao et al., 2015; Chalmers and Bustin, 2008;

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Valenza II et al., 2013). Each of these techniques provide complementary molecular and structural constraints on sedimentary OM, have been applied to maturity determinations, and are

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demonstrably correlated to absolute vitrinite reflectance. The techniques do not identify nor

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isolate the impact of original compositional variability among individual macerals, but do enable a large number of independent measurements. The measurements have been calibrated

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specifically for marine OM, which constrains the variability of the original bulk sedimentary OM

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while also encompassing the majority of globally important shale plays. The study of bulk OM composition makes this approach widely applicable to the study of any marine mudrock

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formation, unencumbered by the limitations based on alternative specific petrographic marker

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analyses as discussed above. Confident and consistent maturity estimates are provided for the Qusaiba Member by integrating numerous independent measurements. This integrated approach

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provides robust estimates of thermal maturity for exploration and assessment of petroleum systems in general, and is especially useful in shale plays where conventional maturity techniques are precluded or fail.

ACCEPTED MANUSCRIPT 2. Geology 2.1. Regional setting and generalized stratigraphy The Qalibah Formation comprises Silurian-age marine sediments that are distributed throughout much of present-day Saudi Arabia and that extend into neighboring Jordan and Iraq

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(Cole et al., 1994; Fox and Ahlbrandt, 2002; Husseini, 1991; Jones and Stump, 1999; Mahmoud

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et al., 1992; Pollastro, 2003; Stump and Van Der Eem, 1995). The sequence was deposited

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during the deglaciation of Gondwanaland, which led to a major period of coastal onlap

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represented as shallow- marine to mid-shelf sediments. Accumulation of the Qalibah Formation was synchronous across at least two, rapidly subsiding depocenters: the Taymah trough

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(northwestern Saudi Arabia, from which drill core samples for this study were obtained) and Qalibah trough (central and southern Saudi Arabia). The depocenters began their subsidence in

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the latest Ordovician or earliest Silurian (Hayton et al., 2017; Jones and Stump, 1999).

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A generalized stratigraphy of Late Ordovician-Silurian sedimentary succession comprising the Qalibah Formation in Saudi Arabia is presented in Fig. 1. The base of the Qalibah

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Formation overlies a variety of rocks ranging in age from the Precambrian Arabian shield to Late

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Ordovician Sarah Formation (Mahmoud et al., 1992; Stump and Van Der Eem, 1995). The Qalibah Formation is unconformably overlain by the sandstones of the Tawil Formation and

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correlative units, comprised of coarse and medium-grained sands (Mahmoud et al., 1992; Stump and Van Der Eem, 1995). The Qalibah Formation comprises the Qusaiba and Sharawa Members (Husseini, 1991; Mahmoud et al., 1992; Stump and Van Der Eem, 1995) and is contemporaneous with and equivalent to the Mudawwara Formation in Jordan (Fox and Ahlbrandt, 2002; Naylor et al., 2013), which contain comparable fossil graptolite successions (Williams et al., 2016).

ACCEPTED MANUSCRIPT The Qusaiba Member comprises predominantly mudstone with thin interbeds of sandstone and is informally subdivided into “lower” and “upper” units (Stump and Van Der Eem, 1995). Hayton et al. (2017) present a recent and detailed biostratigraphic interpretation of the lower Qusaiba Member in northwestern Saudi Arabia, representing the area and sequences

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examined in this study. The lower Qusaiba encompasses a 1–30 m thick basal sandstone

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immediately overlain by a series of variably thick, highly fossiliferous, pyritic, organic-rich

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mudstones that are interbedded with upward-coarsening sandstones (Stump and Van Der Eem, 1995). Hayton et al. (2017) refer to the Qusaiba Transgressive Siltstone (“QTSS”) which may be,

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in part, a lateral equivalent of the coastal sandstones observed in outcrop. The lowermost

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mudstone unit in the lower Qusaiba Member is generally referred to in the literature as "basal Qusaiba hot shale" (sensu Mahmoud et al., 1992), defined by the high gamma-ray response on

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downhole logs (“BQHS” in the interpretation of Hayton et al., 2016). The hot shale is generally

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10–30 m thick, but up to 70 m thick locally (Cole et al., 1994; Hayton et al., 2017; Jones and Stump, 1999; Mahmoud et al., 1992; Stump and Van Der Eem, 1995). The hot shale unit is less

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bioturbated than overlying shale sequences and, together with high present-day total organic

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carbon content [average TOC ~ 5 wt% (Cole et al., 1994; Jones and Stump, 1999)], suggests a period of high primary productivity and anoxia in the deposited muds (Hayton et al., 2017; Jones

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and Stump, 1999). In addition to the organic-rich mudstones of the BQHS, Hayton et al. (2017) identified two special lithofacies, the Qusaiba Black Chert (“QBCH”) and Qusaiba Chert-Shale Transition (“QCST”). The QBCH comprises a range of pervasively silica-cemented organic-rich mudstones lacking terrigenous material. The chert developed diagenetically as fine-grained, biogenic-sourced silica from dissolution and re-precipitation of radiolarian tests soon after their sedimentation. The QCST is a sequence of alternating layers of silica-cemented and non-silica-

ACCEPTED MANUSCRIPT cemented mudstone. Unlike the QBCH, the QCST contains varying abundance of fine-grained terrigenous material (Hayton et al., 2017). The basal hot shale of the Qusaiba Member is interpreted broadly to be the principal hydrocarbon source rock for oil that migrated into upper Paleozoic successions in the Central Province of Saudi Arabia (Cole et al., 1994; Jones and

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Stump, 1999; Mahmoud et al., 1992), although the interpretations differ as to which

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paleogeographic region served as the principal hydrocarbon source. Overlying the BQHS in the

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lower Qusaiba Member, mudstones are interbedded with detrital silts/sands, with the detrital component becoming progressively more common up-section (Husseini, 1991; Stump and Van

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Der Eem, 1995). This succession is generally termed the Qusaiba Grey Shale by Hayton et al.

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(2017). Upward-coarsening of the succession is interpreted as regression that caused an influx of terrigenous detrital material after the initial massive transgression associated with the Late

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Ordovician deglaciation (Husseini, 1991).

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The upper Qusaiba Member consists mostly of varicolored mudstones, with thin (1–3 m thick) interbeds of siltstones and fine-grained sandstones. The mudstone sequences generally are

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finely laminated and bioturbated. The TOC content of these shales averages ~ 1 wt% (Cole et al.,

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1994), although there are more organic-rich intervals within this succession (Hayton et al., 2017). The interbedded sandstones in the upper Qusaiba Member are similar to those in the

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lower, but define an upward-fining sequence. The Sharawa Member overlies the upper Qusaiba Member and is composed mostly of micaceous sandstone, with subordinate siltstone and mudstone and generally low TOC content [average < 1 wt% (Cole et al., 1994)].

2.2. Burial History The depositional and burial history of the Qusaiba Member is presented in earlier publications (e.g., Abu-Ali et al., 1999; Cole et al., 1994; Pollastro, 2003; Wender et al., 1998).

ACCEPTED MANUSCRIPT Most studies focus on reconstructing the burial history of the Qusaiba Member in central and southern Saudi Arabia, where petroleum generation peaked in the Mesozoic and continues in places to the present. The maturation history for northwestern Saudi Arabia, from which core samples for this study were obtained (Fig. 2), is less understood. The present-day depth of burial

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of the Qusaiba Member in the northwest is generally less than 2500 m, substantially less than in

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the central and southern regions. The Qusaiba Member in northwest Saudi Arabia is interpreted

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to have remained at approximately this depth since the Late Paleozoic after the Hercynian orogeny uplifted the deposits to shallow depths (Hadley et al., 1991), indicating that petroleum

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generation here peaked and terminated significantly earlier compared to central and southern

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Saudi Arabia. Naylor et al. (2013) using drill core samples studied the depositional and burial history of the Silurian Mudawwara Formation in Jordan (well RH-19 in Fig. 2), which is close

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to, and a stratigraphic equivalent of, the Qalibah Formation. The reconstructed burial history of

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the Mudawwara Formation here is similar to that interpreted for the Qalibah Formation in northwestern Saudi Arabia. It was rapidly buried until approximately 300 Ma, and reached a

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depth of about 3000 m and a temperature close to 200°C at this time, which represented the

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interval of maximum hydrocarbon generation. Following maximum burial, the Mudawwara Formation was uplifted between 300 and 265 Ma to depths less than 1000 m, which stopped

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further hydrocarbon generation and established the maximum thermal maturity of the source rock. The Mudawwara Formation in this area was buried again to current depths between the Eocene and Miocene epochs.

ACCEPTED MANUSCRIPT 3. Materials and Methods 3.1. Qusaiba Member drill core samples The samples selected for thermal maturity analysis are a subset of whole core taken for a broader petrophysical and geochemical study of the Qusaiba Member in northwestern Saudi

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Arabia. For this study, whole core samples are from four wells (wells AA, BB, EE, and GG; Fig.

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2), predominantly from within the organic-rich basal hot shale of the lower Qusaiba Member.

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Core was sampled as ~ 25 mm to 50 mm-thick sections from selected depths, and broken

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down to 10 mm to 20 mm size chips, each being representative of the core at that depth. A minimum of 50 g of chips were crushed to fine powder (~ 10 µm) using an auto-mortar. The

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powders were cleaned with dichloromethane in a Soxhlet extractor to remove soluble

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hydrocarbons (e.g., bitumen) and residual drilling fluid, although in nearly all samples (wells AA, BB, and EE) no solvent-extractable organic phases were collected, indicating their absence

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in the cores. After air drying, the cleaned powders were split into homogenous fractions using an

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auto-splitter in preparation for physical and geochemical analysis.

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Kerogen isolates were obtained from splits of selected bulk powders using chemical treatment procedures described in Ibrahimov and Bissada (2010). A series of concentrated HF

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and HCl acid additions were made to remove inorganic silicates and carbonates, followed by the addition of acidic CrCl2 (chromium chloride) to demineralize metal sulfides (in particular pyrite) that are otherwise difficult to remove. Kerogen is the insoluble organic concentrate remaining after the chemical treatment. Kerogen isolates were suspended in either water or ethanol prior to drying and analysis. Kerogen purity was confirmed by ashing and chemical analysis, and shown to be between 96 and 99 wt% in all except one sample (well EE, X498.2 ft, which contained ~ 20 wt% residual pyrite).

ACCEPTED MANUSCRIPT A subset of the kerogen isolates were dried using critical point drying (CPD) procedures described in Suleimenova et al. (2014), instead of using routine oven-drying methods. The technique was previously developed for the preservation of microscopic clay mineral structures in formation samples for permeability measurements (Heaviside et al., 1983). Here, the

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procedure was used to preserve the microstructure of kerogen enabling specific surface area to be

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measured. The procedure employs liquid (ethanol)-gas phase transition without crossing the

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phase boundary. The CPD used a 2400 s (40 min) flush of the kerogen isolate with liquid CO 2 at 5.5 ×106 Pa (800 psi) and 0 °C to remove miscible ethanol, followed by vaporization of the CO 2

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at 8.3 ×106 Pa (1200 psi) and 30 °C, slightly above its critical point. The cycle was then repeated

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using a 1200 s (20 min) flush.

3.2. Reference shale samples

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A set of 21 shale samples with known vitrinite reflectance were used as part of this study

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to provide a reference calibration against which to interpret the Qusaiba Member data. The

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reference bulk samples and kerogen isolates were prepared using the same procedures as used for the Qusaiba Member samples, described above. Table 1 reports the relevant geochemical and

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physical parameters of the reference shales and kerogen isolates used for thermal maturity

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interpretations. The majority of these reference samples have been described in other publications (Craddock et al., 2017; Sauerer et al., 2017). Briefly, all of the formations are paleomarine, with the majority deposited in the late-Devonian to early-Mississippian. Vitrinite reflectance of the of majority samples was obtained by one or more external laboratories prior to their study following established vitrinite reflectivity measurements by immersion of polished sections in oil. Rock-Eval pyrolysis was run on powders for the same samples to complement vitrinite reflectance estimates, yielding consistent maturity results for those samples with Rock-

ACCEPTED MANUSCRIPT Eval S2 peaks above 1 mg HC/g; samples with S2 < 1 mg HC/g returned unreliable Tmax and did not allow a vitrinite reflectance determination. We recognize the age difference between the Silurian Qusaiba Member and the Devonian or younger shales used as the reference. Sedimentary OM in the Qusaiba Member includes graptolites and palynomorphs (Hayton et al.,

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2017), whereas that in Devonian and younger deposits generally is derived from a broad group of

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microscopic zooplankton and phytoplankton, and their specific distributions in the samples

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studied in bulk here are unknown. Nonetheless, the classification for sedimentary OM in both sets of samples is type II, and their compositional differences will be small when compared to

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non-marine type I and III kerogen.

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3.3. Measurements on bulk shales

3.3.1. Total organic carbon and Rock-Eval pyrolysis

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Total organic carbon (TOC) determinations were made on bulk sample powders using

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routine coulometry methods (Jackson and Roof, 1992). Total carbon content was determined by coulometric titration of total evolved CO 2 following sample combustion. Total carbonate

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(inorganic carbon) was determined by coulometric titration of acid-evolved CO 2 . The total

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organic carbon content was calculated as the difference between the total carbon and inorganic

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carbon concentrations.

Rock-Eval pyrolysis was run to provide independent assessment of hydrocarbon generation potential and thermal maturity to compare to the other methods described here. RockEval involves temperature-programmed heating of the powdered sample in an inert atmosphere (Behar et al., 2001). The amount of free hydrocarbon in the bulk sample is determined from the intensity of the S1 peak (units of mg hydrocarbon per g of rock) during pyrolysis. Because the samples were Soxhlet cleaned prior to analysis, the amount of free hydrocarbon is negligible as

ACCEPTED MANUSCRIPT demonstrated by their low S1 values. The amount of potentially generative hydrocarbon during pyrolysis of organic matter is quantified by the intensity of the S2 peak (units of mg hydrocarbon per g of rock). Hydrogen index (H.I.) is a common metric from Rock-Eval, where H.I. = 100 × S2/TOC (units of mg hydrocarbon per g of total organic carbon). One indicator for thermal

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maturity from Rock-Eval is Tmax, which is the temperature at which the maximum amount of

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hydrocarbon is generated from organic matter decomposition (i.e., temperature of the S2 peak

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maximum). One method to correlate Tmax (°C) to the vitrinite reflectance scale is using the equation from Jarvie et al. (2001): %VRe = [0.018 × Tmax] – 7.16. Our estimate of the uncertainty

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on Tmax, based upon duplicate analysis of identical sample splits, is 5–7 °C for samples with S2

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values greater than 10 mg HC/g rock, equivalent to an internal precision of ±0.10–0.13 in %VRe

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estimate.

3.3.2. Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS)

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Infrared spectroscopy has been used to measure the composition of coal, kerogen,

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bitumen, and asphaltenes, including their variation with thermal maturity (Barth et al., 1995; Borrego et al., 1996; Chen et al., 2012; Christy et al., 1989; Christy et al., 1991; Craddock et al.,

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2015; Ganz and Kalkreuth, 1987; Guo and Bustin, 1998; Ibarra et al., 1996; Iglesias et al., 1995; Lin and Ritz, 1993; Lis et al., 2005; Painter et al., 1981; Pomerantz et al., 2016; Rouxhet and

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Robin, 1978; Rouxhet et al., 1980). Here, diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) measurements were made following the procedures described by Charsky and Herron (2012) and Herron et al. (2014). Samples were scanned as bulk powders without dilution. Measurements were performed on a Bruker Optics Alpha-R spectrometer operating in reflectance mode. Spectra were collected over the mid-IR range from 375 to 4000 cm-1 with 4 cm-1 resolution. For each sample, 180 scans were collected, and the DRIFTS

ACCEPTED MANUSCRIPT spectrum was obtained from the average of the scans and quantified in Kubelka-Munk (KM) intensities (Kubelka and Munk, 1931). Thermal maturity estimates from the DRIFTS spectra were obtained based on spectrum fitting procedures described by Craddock et al. (2017). Kerogen spectra were mathematically extracted between 2800 and 3100 cm-1 from the measured

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bulk-powder spectra by solving for and stripping the mineral contributions. The sum absorption

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area under the mathematically extracted kerogen spectrum was normalized to unity. The

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intensities of the absorption bands for the well-characterized aliphatic CH2 , aliphatic CH3 , and aromatic CH functional groups in kerogen between 2800 and 3100 cm-1 were quantified by

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fitting polynomials through pre-defined spectral windows representing each of the absorption

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bands. Fig. 3 illustrates the polynomial fitting of the normalized spectra of two type II kerogens with known and different vitrinite reflectance. The vastly different spectral band intensities

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reflects their distinct composition and structure because of their different thermal maturities. The

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mean thermal maturity Qusaiba Member samples was estimated by computing vitrinite reflectance equivalence from the fitted intensities of the aliphatic CH2 , aliphatic CH3 , and

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aromatic CH absorption bands, based on a stochastic multiple linear regression model (Craddock

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et al., 2017) calibrated using the reference set of paleo-marine shales. The model takes the form, (Eq. 1)

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%VRe =  0  1  CH 2, AL   2  CH 3,AL   3  CH AR ,

where CH2, AL, CH3, AL, and CHAR are fitted spectral intensity values for the aliphatic CH2 , aliphatic CH3 , and aromatic CH bands, respectively, in the unknown sample, and the α coefficients are fixed values optimized in the model. The reported uncertainty on the mean vitrinite reflectance estimate for each sample was calculated from the standard deviation of the residuals between the polynomial fits and the normalized, measured spectrum.

ACCEPTED MANUSCRIPT 3.3.3. Raman spectroscopy Raman spectra were acquired using a Thermo Scientific DXR Raman Microscope on intact chips taken from the whole core, following the procedures described in Sauerer et al. (2017). Sedimentary OM (kerogen) was visually identified by microscopic inspection, and upon

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which was focused the laser with a 10× magnification objective and an estimated spot size of 2.1

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µm. Monochromatic excitation was performed with a 532 nm wavelength laser. Laser power was

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set to 10 mW. A 25 µm slit was used as aperture. The grating was set to 1800 lines/mm. Automated fluorescence correction was applied. Scans were recorded from 1877 to 200 cm-1 .

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Each spectrum is the average of 16 scans. Four spectra were acquired for each sample on

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randomly selected areas of the sample surface in which OM was identified. The Raman spectra were analyzed using OMNIC v9.3.03 software. Curve fitting and band deconvolution of the

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spectra were performed using mixed Gaussian/Lorentzian profiles applied to five peaks with a

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linear baseline (Beyssac et al., 2003). The Raman band separation (RBS) was calculated as the difference between the mean G band (~ 1600 cm-1 ) and D1 band (~ 1350 cm-1 ; Fig. 4) positions

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of the four spectra taken on each sample. Vitrinite reflectance was computed from the RBS value

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using the regression established from Raman measurements of the reference shales with known vitrinite reflectance (Sauerer et al., 2017). The quoted uncertainty is the standard error

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propagated from the uncertainty on the RBS estimate from four spectra for each sample and the uncertainty of the regression, and ranges from ±0.1 to ±0.4 %VRe.

3.4. Measurements on Kerogen isolates 3.4.1. Elemental analysis Kerogen elemental concentrations were determined using routine laboratory techniques (Durand and Monin, 1980). Carbon, hydrogen, nitrogen, and sulfur were quantified by flash combustion at ~ 1000 °C under a stream of oxygen. Oxygen was determined by flash pyrolysis at

ACCEPTED MANUSCRIPT ~ 1000 °C in an inert atmosphere. The data validate the purity of the kerogen isolates and were used to calculate atomic H/C ratios of the kerogen isolates. 3.4.2. Helium pycnometry Helium pycnometry was used to determine the skeletal density of kerogen isolates. The

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measurements were made using a Micromeritics AccuPyc II 1340 gas pycnometer, which

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determines the volume of a sample based on Boyle’s Law and computes the density from the

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volume and known mass of the sample. Ten volumetric measurements were made for each kerogen isolate and the volume computed as the mean of the replicates. The kerogen skeletal

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density is reproducible within ±0.02 g/cm3 . Gas pycnometry densities were corrected for the

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presence of residual inorganic minerals in the kerogen isolate assuming the ash was entirely pyrite (ρ = 5.0 g/cm3 ). The correction was typically less than 0.01 g/cm3 , and always less than

3.4.3. BET specific surface area

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0.05 g/cm3 .

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Specific surface area (SSA) measurements were made on a subset of kerogen isolates that were dried using critical point drying procedures to preserve their physical microstructure

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(Suleimenova et al., 2014). The analyses were done on a Micromeritics ASAP 2420 surface area

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analyzer using multi-stage nitrogen adsorption pressure measurements. The SSA for each kerogen isolate was computed from the resulting Brunauer-Emmett-Teller (BET) adsorption isotherm. Several hundred milligrams of sample was precisely weighed into a surface area tube followed by a fill-rod to eliminate head space. The pre-measurement mass of the sample plus tube was recorded. The samples were then degassed for 6 hr at programmed temperature and pressure (60 °C, < 5 µm Hg) to eliminate surface-adsorbed atmospheric gases. The mass of sample plus tube was recorded again to confirm adsorbed gas loss.

ACCEPTED MANUSCRIPT The surface area tube was wrapped in a thermal jacket, loaded onto the SSA analyzer inside of a dewar, and immersed in liquid nitrogen. The jacket ensures thermal equilibrium inside of the surface area tube during the measurement. The measurement consisted of sequential quantification of adsorbed nitrogen as a function of relative pressure, p/p0 , between 0 and 1

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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

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p/p0 of 0.012 and 0.286. The estimated uncertainty of the analysis from replicate analysis of

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kerogen specific surface area is ±15 m2 /g.

3.4.4. Sulfur K-edge X-ray absorption near edge structure (XANES)

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Sulfur K-edge XANES is an X-ray synchrotron method to measure the molecular distribution of sulfur-containing functional groups in nonvolatile organic compounds including

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kerogen (Kelemen et al., 2007; Kelemen et al., 2012; Pomerantz et al., 2014; Wiltfong et al.,

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2005), coal (Gorbaty et al., 1992; Huffman et al., 1991; Spiro et al., 1984), bitumen (Bolin et al.,

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2016; Kelemen et al., 2010; Pomerantz et al., 2014), asphalts (Greenfield et al., 2015), and asphaltenes (George and Gorbaty, 1989; Mitra-Kirtley et al., 1998; Pomerantz et al., 2013;

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Waldo et al., 1992). XANES measures the electronic transitions from 1s orbitals to vacant molecular orbitals with 3p character. The energy of the transition is a function of the oxidation

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state of sulfur (increasing energy of transition for sulfur with higher oxidation state), which correspond to different sulfur moieties (Fig. 5). Our procedure to acquire and analyze the sulfur K-edge spectra of kerogen isolates in this study followed that described by previous publications (Bolin, 2010; Pomerantz et al., 2014; Pomerantz et al., 2013). XANES spectra were acquired in fluorescence mode at the Advanced Photon Source beamline 9-BM using a Lytle detector. The detector housing was connected directly to and separated from a He-purged sample chamber by a

ACCEPTED MANUSCRIPT 2.5 μm thick aluminized-Mylar window. Energy was calibrated against a Na-thiosulfate pre-edge feature at 2469.20 eV. Quantitative XANES spectra were obtained in the absence of selfabsorption, which required total sulfur concentrations in the analyzed sample to be less than ~ 2 wt% (Prietzel et al., 2011). Pomerantz et al. (2014) developed a dilution procedure for kerogen

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for XANES analysis to eliminate self-absorption artifacts in kerogen isolates with high sulfur

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content. However, elemental analysis of Qusaiba kerogen isolates shows that their sulfur content

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generally is less than 1 wt%, such that the samples were scanned successfully without dilution. Measured XANES spectra are plotted as the ratio of I, the intensity of the total fluorescence

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

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energy. Spectra were baseline-corrected by setting I/I0 to zero in the pre-edge region (<2465 eV) and normalized by setting I/I0 to unity in the post-edge region (>2490 eV). The organic sulfur

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species concentrations in the Qusaiba kerogens were quantified by fitting the resulting spectra to

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a linear combination of model sulfur compounds (Fig. 5). Inorganic sulfur forms such as pyrite, if present in the isolate phase, can overlap the XANES spectra of kerogen and so were included

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in the spectrum fitting. The kerogen isolates were treated specifically with CrCl2 to remove metal

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sulfides (predominantly pyrite) and minimize its contribution to the XANES spectra. The kerogen isolation procedure has been shown previously not to alter the composition or

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abundance of organic sulfur species in organic matter (Pomerantz et al., 2014).

4. Results and Discussion 4.1. Thermal maturity from Rock-Eval pyrolysis Tmax Table 2 reports the total organic carbon (TOC), and Rock-Eval pyrolysis data from the analysis of 39 core samples from the Qusaiba Member. All samples from wells AA, BB, and EE have an S2 peak less than 1 mg HC/g rock. The average hydrogen index (H.I.) values for the

ACCEPTED MANUSCRIPT samples in wells AA, BB, and EE are 20 mg HC/g TOC (range 12–28), 12 mg HC/g TOC (range 8–18), and 4 mg HC/g TOC (range 0.3–6.5), respectively (Fig. 6). The low S2 signal precluded reliable Tmax determinations, so no equivalent vitrinite reflectance could be established for these samples based on Tmax. However, the low H.I. values indicate that OM in the Qusaiba Member

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intersected by these three wells is relatively thermally mature. The two samples from well GG

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have higher S2 peaks of 13 and 16 mg HC/g rock, and a higher H.I. of 137 mg HC/g TOC (Fig.

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6). The Tmax values from the S2 peaks are 446 and 448 °C. Vitrinite reflectance equivalence from

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Tmax is 0.9 ± 0.1 %VRe.

Fig. 6 overlays the Rock-Eval S2 and H.I. data from this study with those for the Qusaiba

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Member published by Cole et al. (1994) and İnan et al. (2016). Note that the H.I. values are

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present-day and not original values at the time of OM deposition. Peters (1986) showed that original hydrogen (and oxygen) index varies with the type of OM. The broad range of high

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present-day H.I. values reported by Cole et al. (1994) may reflect, in part, compositional

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differences in original OM (e.g., zooclast or maceral type), because these differences are maximally expressed at low thermal maturity, and their distribution is unknown in the studied

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samples. However, the process of thermal maturation significantly reduces compositional

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variability within sedimentary OM, such that hydrogen (and oxygen) index values converge toward zero for all kerogen types (Peters, 1986). Samples from the Qusaiba Member from wells AA, BB, and EE clearly are concentrated toward near-zero S2 and H.I. values. These data strongly support the interpretation that thermal maturity of sedimentary OM in the Qusaiba Member intersected by wells AA, BB, and EE is substantially higher than for OM in samples from well GG, and in the majority of samples studied by Cole et al. (1994) and İnan et al. (2016).

ACCEPTED MANUSCRIPT 4.2. Thermal maturity from DRIFTS Fig. 7 compares the DRIFT spectra of the Qusaiba Member core samples (spectra are vertically offset for clarity). The two samples from well GG show spectra dominated by aliphatic C—H vibrational modes, whereas samples from wells AA, BB, and EE show substantial or

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dominant contribution from aromatic C—H vibrational modes (Table 2). These DRIFT spectra

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are indicative of relatively immature OM in samples from well GG, and thermally mature OM in

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samples from wells AA, BB, and EE.

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The vitrinite-reflectance-equivalent maturities derived from the DRIFTS measurements are reported in Table 2 and shown in Fig. 8, plotted as a function of depth in each well, with

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biostratigraphic correlation between the wells indicated. Kerogen in the two samples from well GG in the basal Qusaiba hot shale have low %VRe estimates of 0.8 and 0.9 %, consistent with

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the estimates from Rock-Eval Tmax. The maturity of kerogen from well AA in the interval

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immediately overlying the basal Qusaiba hot shale clusters between 1.5 and 1.8 %VRe, with a

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mean of 1.7 ± 0.2 %VRe (n = 10). It was not possible to mathematically extract robust kerogen IR spectra from the measured bulk shale spectra of samples from well AA in the basal hot shale

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unit due to a poorly quantified mineral absorption band overlapping those of kerogen. Kerogen

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in samples from well BB in the basal Qusaiba hot shale have individual estimates of maturity between 1.5 and 1.8 %VRe, with a mean of 1.7 ± 0.2 %VRe (n = 13), indistinguishable from that from nearby well AA. Kerogen in samples from well EE overlying the basal Qusaiba hot shale have the highest estimated vitrinite reflectance values between 1.8 and 2.5 %VRe, with a mean of 2.0 ± 0.2 %VRe (n = 9). The higher thermal maturity for well EE from the DRIFTS analysis is consistent with these samples having the lowest Rock-Eval H.I. values.

ACCEPTED MANUSCRIPT 4.3. Thermal maturity from Raman spectroscopy The Qusaiba bulk samples show Raman spectra typical for kerogen. Estimates of vitrinite reflectance equivalence in the Qusaiba samples were made from the Raman band separation (RBS; G band shift minus D1 band shift) that has been previously demonstrated to correlate with

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thermal maturity (e.g., Kelemen and Fang, 2001; Sauerer et al., 2017; Schito et al., 2017;

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Schmidt Mumm and İnan, 2016). Here, vitrinite reflectance was determined using the vitrinite

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reflectance versus RBS correlation developed by Sauerer et al. (2017) from Raman measurements of the reference set of shales containing type II kerogen (Table 1; Fig. 9). In well

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AA, maturity estimates range from 1.2 to 1.7 %VRe, with a mean of 1.5 ± 0.2 %VRe. For well

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BB, maturity estimates range from 1.6 to 1.9 %VRe, with a mean of 1.7 ± 0.2 %VRe. Vitrinite reflectance equivalent estimates in well EE are between 1.6 and 2.2 %VRe, with a mean of 1.9 ±

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0.2 %VRe. Again, well GG has the lowest maturity estimates of the samples studied, with values

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of 0.6 and 1.1 %VRe. Within error, the Raman data support the same absolute vitrinite

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reflectance as the DRIFTS estimates.

4.4. Thermal maturity from kerogen isolate analysis

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4.4.1. Atomic H/C

Figure 10 plots on the Van Krevelen diagram, the atomic H/C versus atomic O/C

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contents of the Qusaiba kerogen isolates. Kerogen isolates from well GG plot at higher atomic H/C (0.74–0.78) than kerogen isolates from the other wells, which cluster at low atomic H/C (0.48–0.63; Table 3). Overlain on the plot are maturation trends for type I, II, and III kerogen (Peters, 1986), and contours of approximate %Ro (Waples, 1985). The Van Krevelen diagram qualitatively shows that OM in the Qusaiba Member from well EE is the most mature, OM from wells BB and AA has practically the same maturity and slightly lower than well EE, and that OM in well GG is least mature of all the wells in this study.

ACCEPTED MANUSCRIPT Quantitative determination of thermal maturity from atomic compositions can be challenging because different OM types can have different initial compositions. However, thermal maturation substantially reduces these compositional differences (i.e., atomic H/C and O/C trends converge toward zero; Fig. 10). For sedimentary OM with vitrinite reflectance values

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~1.2 %Ro or higher, present-day atomic H/C and O/C compositions reflect primarily thermal

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maturity state, whereas initial spread in H/C and O/C due to compositional variability is largely

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eliminated. Fig. 11 provides a quantitative estimate of vitrinite reflectance equivalence from the measured atomic H/C ratios of the Qusaiba Member kerogens, based on the correlation between

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atomic H/C and %Ro constructed from the analysis of kerogen isolates from the reference shales.

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The inverse relationship reflects preferential loss of hydrogen-rich (methyl and methylene) functional groups from kerogen during thermal maturation, leaving behind a kerogen residue that

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is increasingly hydrogen-depleted, as demonstrated for example by X-ray, magnetic resonance,

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and IR spectroscopy studies (Cao et al., 2013; Craddock et al., 2015; Durand and Espitalié, 1976; Ganz and Kalkreuth, 1987; Kelemen et al., 2007; Mao et al., 2010; Rouxhet and Robin, 1978).

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Shown for comparison are atomic H/C data from Lis et al. (2005) for the paleo-marine New

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Albany and Exshaw Formations (Devonian-Mississippian), and from Buchardt and Lewan (1990) for the paleo-marine Alum Shale (Cambrian-Ordovician), confirming nearly identical

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trends for type II kerogen despite their different age and source. Applying the calibration to the Qusaiba Member samples gives the following maturity estimates (Fig. 11b): Well AA shows a mean of 1.6 ± 0.4 %VRe, with samples clustering in two populations of ~ 1.5 and 1.8 %VRe. Well BB shows a mean of 1.9 ± 0.2 %VRe (range 1.9–2.0 %VRe). Well EE shows a mean of 2.2 ± 0.2 %VRe (range 2.1–2.3 %VRe). Well GG shows a single estimate of 1.0 ± 0.1 %VRe.

ACCEPTED MANUSCRIPT Maturity estimates from H/C ratios for each of the wells are essentially the same as those obtained from DRIFTS and Raman analyses. 4.4.2. Kerogen skeletal density The loss of hydrogen-rich functional groups from kerogen during thermal maturation

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leads to increasing kerogen skeletal density. Plotted in Fig. 12 as the reference for interpreting

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the Qusaiba Member results are skeletal densities of kerogen from reference shales against their

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corresponding vitrinite reflectance, with the regression indicated. Kerogen density data from

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earlier studies (Alfred and Vernik, 2013; Guidry et al., 1995; Okiongbo et al., 2005) are shown for comparison, and demonstrate the robust and consistent correlation between kerogen density

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and thermal maturity for paleo-marine shales spanning a range of age and source.

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Overlain on the regression in Fig. 12b are the skeletal densities of the Qusaiba Member kerogen isolates (Table 3). The measured density of the kerogen isolate from well GG (1.11

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g/cm3 ) indicates a low thermal maturity ~ 0.5 ± 0.2 %VRe. This vitrinite reflectance estimate is

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somewhat lower than from other measures above, but is consistent with the distinctly lower thermal maturity of sedimentary OM in the Qusaiba Member intersected by this well. The

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densities of kerogens from wells AA (1.38–1.51 g/cm3 ), BB (1.45–1.46 g/cm3 ), and EE (1.47–

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1.50 g/cm3 ) are substantially higher. For well AA, vitrinite reflectance equivalent estimates from skeletal densities range individually from 1.4 to 2.2 %VRe, but mostly cluster between 1.6 and 1.8 %VRe, with a mean value of 1.7 ± 0.2 %VRe. Thermal maturity estimates for samples from well BB are identical within error, with a mean of 1.8 ± 0.1 %VRe. Two isolates from well EE have vitrinite reflectance equivalent maturity estimates of 1.8 ± 0.2 and 2.1 ± 0.2 %VRe, corresponding to a mean of 2.0 ± 0.2 %VRe. Within the uncertainties, maturity estimates from kerogen density measurements are the same as from the other measurements.

ACCEPTED MANUSCRIPT 4.4.3. Specific surface area Organic matter-hosted pores develop, in part, during thermal maturation as generated hydrocarbons are cracked and expelled from solid OM in shale (e.g., Bernard et al., 2012; Curtis et al., 2012; Loucks et al., 2012; Milliken et al., 2013; Valenza II et al., 2013). Therefore,

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specific surface area (SSA) of kerogen should increase with thermal maturity. Fig. 13

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demonstrates this correlation for sets of reference shales both measured directly as part of this

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study (Table 1), as well as calculated from gas sorption measurements of intact shale (Valenza II et al., 2013). The scatter in the available data result in larger uncertainty in maturity estimates

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from SSA measurements compared to other techniques. The scatter likely reflects confounding

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factors independent of maturity, such as differences in primary porosity and OM source (Löhr et al., 2015; Loucks et al., 2012), lithofacies variability (Löhr et al., 2015), and potential

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compaction that occurred after development of secondary OM porosity. Specific surface area is

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here taken to be a semi-quantitative indicator of thermal maturity. The two kerogens from well GG have low measured SSA less than 40 m2 /g, those from well AA have measured SSA of 257

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and 315 m2 /g, and those from well EE have the highest measured SSA of 325 and 349 m2 /g.

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Applying the correlation from the set of reference shales (Table 1) to the surface areas of the Qusaiba Member kerogen yields vitrinite reflectance estimates of < 1 %VRe in well GG, ~ 1.6–

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1.9 %VRe in well AA, and ~ 2.0 %VRe in well EE, recognizing uncertainties of at least 0.3 %VRe on these estimates. Despite lower confidence on these individual estimates, these estimates in aggregate show the same maturity trend in the Qusaiba Member samples as other techniques. 4.4.4. Sulfur K-edge XANES Figure 14 shows the sulfur K-edge XANES spectra and fitting results for Qusaiba kerogen isolates. The spectra are dominated by a peak corresponding to thiophene, but show

ACCEPTED MANUSCRIPT contributions from inorganic sulfur in the form of residual pyrite and sulfate ± sulfonate. The absolute abundance of inorganic phases in the kerogen isolates is low (1–5 wt%) based on kerogen purity analysis, but they are expressed prominently in the XANES spectra owing to their high sulfur content. Inorganic sulfur standards were used in the spectral fitting, but eliminated

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from the reported organic sulfur speciation results. The kerogens have high relative abundance of

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thiophene (aromatic sulfur; 70–90 mol%; Table 3, Fig. 14b), together with minor amounts of

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aliphatic sulfur (5–22 mol%) identified as sulfide (thioether) and elemental sulfur. The relative abundance of sulfide and elemental sulfur is difficult to quantify because their peaks nearly

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entirely overlap (Waldo et al., 1991), but the sum of their abundance is robust. Sulfoxide (sulfur

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sulfur species are largely or entirely absent.

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double-bonded to oxygen) is a low-abundance organic-sulfur species (< 7 mol%). Other organic-

Previous studies of sulfur speciation in kerogen by XANES (Kelemen et al., 2007;

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Pomerantz et al., 2014) point to an inverse correlation between thermal maturity and the ratio of

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aliphatic sulfur to aromatic (thiophenic) sulfur content (SAliphatic/SAromatic; Fig. 15). Note that, in constructing this correlation, the maturity estimates for the samples reported by Kelemen et al.

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(2007) are not from direct vitrinite reflectance measurements, but calculated from other

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geochemical parameters (Rock-Eval Tmax and atomic H/C). Much of the change in sulfur speciation (SAliphatic/SAromatic ratio from 2 to 0.5) occurs at maturities less than 1 %Ro, which is expected because sulfur-containing bonds (e.g., S—S, S—C) in kerogen are energetically weaker than carbon-carbon bonds in aliphatic chains and aromatic rings, and are labile during kerogen decomposition (Lewan, 1998; Orr, 1986; Sinninghe Damsté et al., 1990; Tomić et al., 1995). The observed trend in sulfur speciation parallels an analogous trend for carbon during maturation recognized by magnetic resonance and IR spectroscopy studies, revealing a shift from

ACCEPTED MANUSCRIPT predominantly aliphatic to aromatic carbon moieties (Cao et al., 2013; Christy et al., 1991; Craddock et al., 2015; Ganz and Kalkreuth, 1987; Kelemen et al., 2007; Lis et al., 2005; Mao et al., 2010; Miknis et al., 1982; Rouxhet and Robin, 1978; Witte et al., 1988). The SAliphatic/SAromatic ratio of the Qusaiba kerogen isolates fall within a narrow range of

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low values between 0.05 and 0.30. Most of these values are outside the range of XANES sulfur species abundances reported by earlier publications, which focused on immature samples,

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making it difficult to extrapolate the estimate of vitrinite reflectance for the Qusaiba Member

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samples to their low measured SAliphatic/SAromatic ratios. Instead, vitrinite reflectance estimates of the Qusaiba Member samples from the other spectroscopic and geochemical measurements were

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used as a benchmark to extend the correlation between vitrinite reflectance and ratio of aliphatic

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sulfur to aromatic (thiophenic) sulfur content beyond published data (Fig. 15). It is clear that the SAliphatic/SAromatic ratios in kerogen from immature well GG are only marginally higher than those

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in the majority of samples from wells AA, BB, and EE, which are shown by the other techniques

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to be of substantively higher maturity. The similarity of the sulfur speciation and low aliphatic S versus aromatic S content of the Qusaiba Member samples are at least consistent with previously

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observed changes in sulfur bonding occurring at the early stages of thermal maturation of

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sedimentary OM, owing to the labile nature of sulfur bonds in kerogen. For the Qusaiba Member samples, because of their generally high thermal maturity, the XANES sulfur speciation results show little discriminatory value to establish vitrinite reflectance estimates.

4.5. Discussion Table 4 and Fig. 16 summarize the vitrinite-reflectance-equivalent maturity estimates from integrated spectroscopic and geochemical measurements of Qusaiba bulk shales and corresponding kerogen isolates, excluding only the XANES results because of their limited

ACCEPTED MANUSCRIPT maturity determinations. The complementary techniques yield consistent thermal maturity estimates for each sample. The independent measurements are based on different principles, and together provide robust maturity estimates with greater confidence than from any individual measurement. A best estimate of vitrinite reflectance for each sample was calculated from the

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mean of the %VRe estimates for the individual measurements, each weighted by their associated

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uncertainties (Table 4). In aggregate, the two core samples from the lower Qusaiba Member in

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well GG have a mean vitrinite reflectance equivalence of 0.9 ± 0.1 %VRe (2σ). Core samples from the lower Qusaiba Member intersected by wells AA, BB, and EE to the north of well GG

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have systematically and substantially higher thermal maturity. The mean vitrinite reflectance

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equivalence of the Qusaiba Member intersected by well AA is 1.7 ± 0.2 %VRe (2σ). Well BB is nearby to well AA, and has a nearly identical vitrinite reflectance equivalence of 1.8 ± 0.1 %VRe

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(2σ). Thermal maturity in well EE is shown by nearly all measures to be the highest of all four

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wells, with a mean vitrinite reflectance equivalence of 2.1 ± 0.2 %VRe (2σ). The data demonstrate that the thermal maturity of the lower Qusaiba Member in the Qalibah Formation in

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northwestern Saudi Arabia ranges from at least 0.9 to 2.1 %Ro. This range reflects varying sites

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of initial deposition and varying maturation history of the Qusaiba Member across at least the Taymah basin in northwestern Saudi Arabia. The maturity interpretations from this study are

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consistent with those published by Naylor et al. (2013), who presented a regional burial history model for the nearby and stratigraphically equivalent Mudawwara Formation in eastern Jordan. The equivalent vitrinite reflectance for the Qusaiba Member from the study of wells AA, BB, and EE, ranging from 1.7 to 2.1 %VRe, is similar to that determined for the Mudawwara Formation from a nearby drilled well, RH-19 (1.6 to 2.1 %Ro; Naylor et al., 2013). These data

ACCEPTED MANUSCRIPT show that thermal maturity for the Qusaiba Member in northwestern Saud Arabia reached the dry-gas window. İnan et al. (2016) recently published an assessment of thermal maturity in the lower Qusaiba Member from a different set of wells in northwestern and central Saudi Arabia. They

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based their study on zooclast (graptolite) reflectance, together with Rock-Eval pyrolysis of bulk samples, Raman spectroscopy of graptolites, and chemical and spectroscopic analysis of

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extractable bitumen. Equivalent vitrinite reflectance estimated from graptolite reflectance from

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thirteen samples from the lower Qusaiba Member range from 0.54 to 1.76 %VRe. The results from our study extend these maturity estimates of the Qusaiba Member in northwestern Saudi

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Arabia to at least 2 %VRe. In general, the two studies together indicate the Qusaiba Member in

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northwestern Saudi Arabia to be more thermally mature than its stratigraphic equivalent in central and south Saudi Arabia. The contrast in thermal maturity is consistent with the different

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interpreted burial histories of the Qusaiba Member in these regions.

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The integrated spectroscopic and geochemical maturity assessment in this study provides

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a consistent framework for interpreting thermal maturity in shales, especially in those formations for which conventional vitrinite reflectance assessment is not possible. Rock-Eval Tmax can be a

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reliable indicator of thermal maturity for relatively immature OM, but it is unreliable for the majority of samples in this study. Infrared (DRIFTS) and Raman spectroscopy on bulk shale samples can provide rapid and robust estimates of vitrinite reflectance values up to at least 3 %Ro. Thermal maturity estimation based upon compositional analysis of kerogen isolates, including atomic H/C, skeletal density, and surface area, further provide consistent vitrinite reflectance estimates. A disadvantage of approaches based on the analysis of kerogen isolates is the need to separate kerogen from the bulk sample. Methods for evaluation of kerogen

ACCEPTED MANUSCRIPT composition and structure applied here, including DRIFTS, elemental analysis, and XANES, indicate increasing abundance of aromatic forms of both carbon and sulfur with increasing thermal maturity, an observation that is consistent with previous structural analysis of sedimentary OM in other sedimentary basins. The analysis of bulk sedimentary OM does not

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allow for identification of compositional or structural variability among individual macerals, but

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does not rely on specialized separation or identification of macerals within the bulk OM as is

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required by conventional petrographic techniques.

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5. Summary and Conclusions

Integration of complementary, repeatable, and quantitative spectroscopic and

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geochemical methods permits a robust assessment of thermal maturity of the Silurian Qusaiba

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Member of the Qalibah Formation in northwestern Saudi Arabia, which lacks vitrinite. The techniques here are based upon analysis of both bulk shale as well as organic matter (kerogen)

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isolates. The analyses comprise Rock-Eval pyrolysis, infrared and Raman spectroscopy,

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elemental analysis, density, surface area, and X-ray absorption near edge structure spectroscopy. Rock-Eval pyrolysis, a common alternative method to vitrinite reflectance petrography for

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maturity determinations, was not useful for the majority of samples studied here because of low S2 signal and unreliable Tmax, as may be expected from the relatively high maturities of these samples. In contrast, maturity methods based on chemical and structural characterization of kerogen—each method based on different analytical principles—provided robust and consistent estimates of thermal maturity in the Qusaiba Member.

ACCEPTED MANUSCRIPT In drill core recovered from four wells in the Qusaiba Member, these maturity methods demonstrate a narrow and consistent estimate of thermal maturity within each well. However, thermal maturity ranges substantially between the wells, from 0.9 to 2.1 %VRe. The trend established in thermal maturity for the lower Qusaiba Member between the four wells is GG (0.9

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± 0.1 %Ro) << AA (1.7 ± 0.2 %Ro) ~ BB (1.8 ± 0.1 %Ro) < EE (2.1 ± 0.2 %Ro). These data

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increase the range of thermal maturity in the Qusaiba Member documented by earlier studies

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(Cole, 1994; Cole et al., 1994; İnan et al., 2016). The aggregate data demonstrate significant variation in thermal maturity for the Qusaiba Member across Saudi Arabia, from at least the

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early-oil window into the dry-gas window. Thermal maturity of the Qusaiba Member in

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northwestern Saudi Arabia appears generally to be more thermally mature than its stratigraphic equivalents in central and southern Saudi Arabia. This is consistent with a very different burial

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history in the northwest compared with central and south. The integrated maturity assessment

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across multiple wells is useful for appraisal and prospecting of the amount, type, and distribution of generated petroleum. More generally, the methods and calibrations for thermal maturity

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presented here can be used to establish vitrinite-reflectance-equivalent maturities in shales as a

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complement to or especially in the absence of conventional maturity estimates.

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Acknowledgments

The authors gratefully acknowledge Saudi Aramco and Schlumberger for support and permission to publish this work. We thank two anonymous reviewers and the associate editor for their constructive comments on our original submission. Shaun Hayton (Saudi Aramco EXPEC Advanced Research Center) provided expert sampling of core and technical discussions throughout. We thank Andrew Judd, Kyle Bake, MaryEllen Loan, and Nathan Curtis

ACCEPTED MANUSCRIPT (Schlumberger-Doll Research) for careful core sample preparation and analysis. Khalid L. Alsamadony and Mohammed D. AlJohani (Schlumberger Dhahran Carbonate Research) provided assistance with Raman spectroscopy measurements. This research used resources of the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science User Facility

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operated for the DOE Office of Science by Argonne National Laboratory under Contract DE-

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AC02-06CH11357.

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Raman spectrometric study. Organic Geochemistry 28, 535-542. http://dx.doi.org/10.1016/S01466380(98)00021-7. Stump, T.E., Van Der Eem, J.G., 1995. The stratigraphy, depositional environments and periods of deformation of the Wajid outcrop belt, southwestern Saudi Arabia. Journal of African Earth Sciences 21, 421-441. http://dx.doi.org/10.1016/0899-5362(95)00099-F. Suleimenova, A., Bake, K.D., Ozkan, A., Valenza II, J.J., Kleinberg, R.L., Burnham, A.K., Ferralis, N., Pomerantz, A.E., 2014. Acid demineralization with critical point drying: A method for kerogen isolation that preserves microstructure. Fuel 135, 492-497. http://dx.doi.org/10.1016/j.fuel.2014.07.005. Tomić, J., Behar, F., Vandenbroucke, M., Tang, Y., 1995. Artificial maturation of Monterey kerogen (Type II-S) in a closed system and comparison with Type II kerogen: implication for the fate of sulfur. Organic Geochemistry 23, 647-660. http://dx.doi.org/10.1016/0146-6380(95)00043-E. Tricker, P.M., Marshall, J.E.A., Badman, T.D., 1992. Chitinozoan reflectance: a Lower Palaeozoic thermal maturity indicator. Marine and Petroleum Geology 9, 302-307. Valenza II, J.J., Drenzek, N., Marques, F., Pagels, M., Mastalerz, M., 2013. Geochemical controls on shale microstructre. Geology 41, 611-614. http://dx.doi.org/10.1130/G33639.1. Van Krevelen, D.W., 1950. Graphical-statistical method for the study of structure and reaction processes of coal. Fuel 29, 269-283. Waldo, G.S., Carlson, R.M.K., Moldowan, J.M., Peters, K.E., Penner-Hahn, J.E., 1991. Sulfur speciation in heavy petroleums: Information from X-ray absorption near-edge structure. Geochimica et Cosmochimica Acta 55, 801-814. http://dx.doi.org/10.1016/0016-7037(91)90343-4. Waldo, G.S., Mullins, O.C., Penner-Hahn, J.E., Cramer, S.P., 1992. Determination of the chemical environment of sulphur in petroleum asphaltenes by X-ray absorption spectroscopy. Fuel 71, 53-57. http://dx.doi.org/10.1016/0016-2361(92)90192-Q. Waples, D.W., 1985. Geochemistry in Petroleum Exploration. D. Reidel Publishing Corp., Dordecht, Netherlands. Ward, J.A., 2010. Kerogen density in the Marcellus shale, Paper SPE-131767 presented at the SPE Annual Technical Conference and Exhibition, Pittsburgh, Pennsylvania, USA. Wender, L.E., Bryant, J.W., Dickens, M.F., Neville, A.S., Al-Moqbel, A.M., 1998. Pre-Khuff (Permian) hydrocarbon geology of the Ghawar area, eastern Saudi Arabia. GeoArabia 3, 273-301. Williams, M., Zalasiweicz, J., Boukhamsin, H., Cesari, C., 2016. Early Silurian (Llandovery) graptolite assemblages of Saudi Arabia: biozonation, palaeoenvironmental significance and biogeography. Geological Quarterly 60, 3-25. http://dx.doi.org/10.7306/gq.1270. Wiltfong, R., Mitra-Kirtley, S., Mullins, O.C., Andrews, B., Fujisawa, G., Larsen, J.W., 2005. Sulfur speciation in different kerogens by XANES spectroscopy. Energy & Fuels 19, 1971-1976. http://dx.doi.org/10.1021/ef049753n. Witte, E.G., Schenk, H.J., Müller, P.J., Schwochau, K., 1988. Structural modifications of kerogen during natural evolution as derived from 13 C CP/MAS NMR, IR spectroscopy and Rock-Eval pyrolysis of Toarcian shales. Organic Geochemistry 13, 1039-1044. http://dx.doi.org/10.1016/0146-6380(88)90286-0. Wopenka, B., Pasteris, J.D., 1993. Structural characterization of kerogens to granulite-facies graphite: Applicability of Raman microprobe spectroscopy. American Mineralogist 78, 533-557.

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Figure Captions

Fig. 1. Generalized stratigraphic column of lower Paleozoic rocks of northern Saudi Arabia,

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modified after Williams et al. (2016), and northeast Jordan, modified after Naylor et al. (2013).

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Fig. 2. Location map of four wells (AA, BB, EE, and GG) from which samples of Qusaiba

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Member, Qalibah Formation were extracted. Well RH-19 contains core samples in the

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stratigraphically equivalent Mudawwara Formation in Jordan, which Naylor et al. (2013) used to

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construct a regional burial history. Their work is a proxy to evaluate thermal maturity estimates

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of the Qusaiba Member in our study.

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Fig. 3. Mathematically extracted DRIFTS spectra of immature (Eagle Ford, Ro = 0.65 %) and

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mature (Woodford, Ro = 1.55 %) kerogen in reference shale samples (Craddock et al., 2017).

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The gray bars indicated the spectral windows within which polynomial functions (red lines) were fit to the spectrum. The intensity of three absorption bands calculated from the polynomial fits are used in a regression model to quantify thermal maturity.

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Fig. 4. Raman spectrum of sedimentary OM in core from well GG showing the measured

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spectrum (blue) and curve fits (G and D bands, black) to the spectrum. Thermal maturity is

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computed from the separation between the G and D1 bands.

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Fig. 5. Sulfur K-edge XANES spectra of model compounds, plotting normalized fluorescence

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intensity versus incident photon energy. Spectra are arranged vertically by sulfur oxidation state

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and are offset for clarity.

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Fig. 6. Rock-Eval hydrogen index (100 × S2/Corg ) versus Rock-Eval S2 from analysis of Qusaiba

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core samples. Samples from well GG have relatively high S2 and hydrogen index values, compared with other samples in our study. Samples from wells AA, BB, and EE cluster at low

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S2 and hydrogen index values. Literature data for the Qusaiba Member (Cole et al., 1994; İnan et

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al., 2016) are shown for comparison.

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Fig. 7. Stacked DRIFTS spectra for Qusaiba core samples from wells AA, BB, EE, and GG. Principal IR absorption bands associated with aliphatic and aromatic C—H vibrational modes in sedimentary OM are indicated, and are the same as shown in Fig. 3. Individual spectra are vertically offset for clarity.

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Fig. 8. Thermal maturity estimates from DRIFTS, plotted as a function of depth in the Qusaiba Member wells in this study. The stratigraphic correlation between wells is based on graptolite biozones identified by Hayton et al. (2016). Stars (white) indicate depths from which kerogen was isolated from bulk samples. Note the vertical scales are the same for wells AA, BB, and CC, but expanded for well GG.

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Fig. 9. Correlation between vitrinite reflectance and Raman band separation, RBS. (a) Reference

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calibration constructed using Raman data from reference shales (Table 1; Sauerer et al., 2017). (b) The calibration is used as the basis to estimate the vitrinite reflectance equivalence from RBS of samples in the Qusaiba Member.

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Fig. 10. Van Krevelen diagram for kerogens from the Qusaiba Member. Maturation trends for

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kerogen types I, II, and III are shown by grey solid lines (Peters, 1986). Contours of approximate

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vitrinite reflectance are shown by black dashed lines (Waples, 1985). Kerogens from well GG

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have high atomic H/C ratios (0.74–0.78) compared to those from wells AA, BB, and EE (0.48–

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0.63).

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Fig. 11. Correlation between vitrinite reflectance and atomic H/C ratio. (a) Reference calibration

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constructed using atomic H/C data from reference shales (Table 1, this study). Published data (Buchardt and Lewan, 1990; Lis et al., 2005) shows nearly identical trends. (b) The calibration is used as the basis for estimating vitrinite reflectance equivalence from atomic H/C ratios of kerogens from the Qusaiba Member.

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Fig. 12. Correlation between vitrinite reflectance and kerogen skeletal density. (a) Reference calibration constructed using data from reference shales (this study; Table 1). Published data

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(Alfred and Vernik, 2013; Guidry et al., 1990; Okiongbo et al., 2005) show nearly identical trends. The anomalously heavy density for sample with Ro ~ 2 %, published by Guidry et al. (1990) may reflect mineral contamination. (b) The calibration is used as the basis to estimate vitrinite reflectance equivalence from skeletal densities of kerogens from the Qusaiba Member.

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Fig. 13. Vitrinite reflectance versus kerogen specific surface area. The calibration through reference shales from this study (Table 1) is used as the basis on which to estimate vitrinite

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reflectance equivalence for kerogen in the Qusaiba Member. Data for U.S. shales from Valenza

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II et al. (2013) show a trend between kerogen specific surface area and thermal maturity

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consistent with that for the set of reference shales.

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Fig. 14. (a) Sulfur XANES spectra of kerogens from the Qusaiba Member. All spectra show abundant thiophene, and minor abundance of inorganic sulfur forms corresponding to pyrite and sulfate ± sulfonate. Inorganic sulfur forms were included in the peak fitting, but excluded from the displayed sulfur speciation. (b) Organic sulfur speciation, showing the similar distribution of sulfur moieties dominated by thiophene (aromatic sulfur) with minor abundance of aliphatic sulfur (sulfide and elemental sulfur) and sulfoxide (sulfur bonded to oxygen).

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Fig. 15. Ratio of aliphatic sulfur (elemental sulfur + sulfide) to aromatic sulfur (thiophene) in

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kerogen plotted versus vitrinite reflectance. Data from this and earlier studies (Kelemen et al., 2007; Pomerantz et al., 2014) define a broad, inverse correlation between thermal maturity and

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the aliphatic vs. aromatic sulfur content.

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Fig. 16. Comparison of vitrinite reflectance equivalence estimates for Qusaiba Member samples.

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The bounding bars around each data point are 2σ uncertainties on the vitrinite reflectance estimate. The multiple techniques demonstrate robust and consistent maturity estimations, within

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the uncertainties of the measurements.

ACCEPTED MANUSCRIPT Table 4. Summary of maturity estimates from analysis of Qusaiba shales and corresponding kerogen isolates*

AA AA

-

± ± ± ±

AA

X256

-

-

AA

X261. 7

-

-

AA

X264

-

-

BB

X725. 5

-

BB

X751

-

BB

X767

-

GG GG

-

0. 9 0. 9

± ±

± ± ± ±

0. 1 0. 1 0. 1 0. 2 0. 1 0. 2 0. 1 0. 1

PT

EE

-

CE

EE

X280. 4 X350. 7 X498. 2 X905. 4 X908. 6

0. 1 0. 1

AC

EE

1. 7 1. 7 1. 6 1. 8 1. 9 2. 1 0. 8 0. 9

± ± ± ±

1. 4 1. 3 1. 5 1. 5 1. 7 1. 6 1. 7 1. 7 1. 5 1. 7 1. 7 1. 6

± ± ± ± ± ± ± ± ± ±

0. 1 0. 2 0. 2 0. 1 0. 1 0. 2 0. 2 0. 2 0. 1 0. 3 0. 2 0. 1

± ± -

2. 1 1. 8 1. 1 0. 6

± ± ± ±

1. 8 1. 8 1. 5 1. 4 1. 4 1. 8

0. 4 0. 1 0. 2 0. 1

± ± ± ±

0. 3 0. 3 0. 3 0. 3 0. 3 0. 3

1. 8 1. 9 2. 0 1. 9 2. 2 2. 1 2. 3 1. 0 1. 0

1. 6 1. 8

± ± -

2. 2

±

± -

±

±

-

± ± ± ± ± ± ± ± ±

0. 3 0. 3 0. 3 0. 3 0. 3 0. 3 0. 3 0. 2 0. 2

0. 2 0. 2 0. 2

-

1. 8 1. 4 1. 7 1. 6 1. 8 1. 8 1. 8 1. 9 2. 1

±

± ± ± ± ± ± ±

± -

-

1.9 -

0. 2 0. 2 0. 2 0. 2 0. 2 0. 2 0. 2 0. 2 0. 2

0. 5

% VRe (Surfac e Area)

T

-

±

0. 1 0. 1 0. 1 0. 1 0. 1 0. 2

IP

-

±

% VRe (Density)

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AA

1. 7 1. 5 1. 7 1. 6 1. 6 1. 5

-

% VRe (Raman)

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AA

% VRe (Atomic H/C)

% VRe (DRIFTS)

AN

AA

X170. 7 X176. 5 X179. 3 X184. 3 X189. 1 X191. 2

% VRe (T max)

M

AA

Depth , ft

Kerogen: % VRe and 1σ uncertainty

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Well Nam e

Bulk shale: % VRe and 1σ uncertainty

1.6 2.1 2.0 -

0. 2

0.5 0.6

Mean % VRe and 95% c.i.

1. 6 1. 5 1. 6 1. 7 1. 6 1. 7 1. 5 1. 7 1. 6 1. 7 1. 8 1. 7 1. 9 2. 0 1. 9 0. 8 0. 9

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

* Quoted uncertainties on individual % VRe estimates for each technique are 1σ values. The calculated uncertainties on the weighted-mean for each sample are 95 % confidence interval (2σ) values.

0. 1 0. 2 0. 2 0. 1 0. 1 0. 2 0. 2 0. 2 0. 2 0. 1 0. 1 0. 1 0. 2 0. 2 0. 2 0. 2 0. 2

ACCEPTED MANUSCRIPT Table 2. Geochemical and physical characteristics of Qusaiba bulk samples

A A A A A A A A

-

2. 9

0 .3

0 .5

-

4. 0

0 .4

0 .7

-

4. 8

0 .5

0 .8

-

4. 7

0 .4

1 .0

-

2. 4

0 .3

0 .5

-

2. 2

0 .2

0 .5

-

4. 6

0 .4

0 .6

-

4. 3

0 .4

0 .7

6. 6

0 .4

2 0 1 9 1 8 1 7 2 1 2 1 2 3 1 4

-

0 .8

-

A A

X2 56

3. 9

0 .3

0 .9

-

A A

X2 59

3. 0

0 .3

0 .8

-

A A

X2 61. 7

3. 5

0 .3

0 .5

-

A A

X2 64

3. 5

0 .4

0 .9

-

A A

X2 66.

3. 5

0 .5

0 .9

-

1

6

1 2 2 4 2 8 1 5 2 5 2 6

C HA

, AL

, AL

R

0. 00 7 0. 00 6 0. 00 7 0. 00 7 0. 00 8 0. 00 6 0. 00 6 0. 00 8 0. 00 7 0. 00 8

0. 00 7 0. 00 6 0. 00 7 0. 00 7 0. 00 7 0. 00 6 0. 00 6 0. 00 7 0. 00 7 0. 00 7

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

0. 00 6 0. 00 6 0. 00 6 0. 00 5 0. 00 5 0. 00 6 0. 00 6 0. 00 5 0. 00 5 0. 00 5

% VRe

1 . 7 1 . 7 1 . 7 1 . 6 1 . 5 1 . 7 1 . 8 1 . 6 1 . 6 1 . 5

0 ± . 1 0 ± . 1 0 ± . 1 0 ± . 1 0 ± . 1 0 ± . 1 0 ± . 1 0 ± . 1 0 ± . 1 0 ± . 2

G shi ft, cm

D shi ft, cm

-1

-1

T

0 .6

° C

% VRe (T max)

C H3

16 00. 0 16 02. 6 16 02. 8 16 03. 0 16 03. 0 16 02. 8 16 02. 7 16 03. 4 16 03. 7 16 03. 3 16 02. 5 16 00. 9 16 04. 2 16 00. 2 16 01.

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0 .6

x,

C H2

Raman spectroscopy

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2. 8

ma

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A A

S2, mg HC/ g

M

A A

S1, mg HC/ g

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A A

H. I., m g H C/ g

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A A

T

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A A

X1 64. 5 X1 67. 7 X1 70. 7 X1 74. 8 X1 76. 5 X1 79. 3 X1 82. 5 X1 84. 3 X1 89. 1 X1 91. 2

T O C, wt %

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A A

De pth , ft

DRIFTS

13 60. 0 13 60. 3 13 59. 6 13 60. 0 1 36 1.7 1 35 9.2 1 35 7.8 1 35 9.3 1 35 7.3 1 35 7.7 13 56. 2 13 56. 5 13 57. 1 13 55. 6 13 58.

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Rock-Eval W ell N a m e

(G - D1), cm-1

24 0. 0 24 2. 3 24 3. 2 24 3. 0 24 1. 3 24 3. 6 24 4. 8 24 4. 1 24 6. 4 24 5. 6 24 6. 3 24 4. 4 24 7. 1 24 4. 6 24 2.

3 ± . 8 1 ± . 6 1 ± . 0 0 ± . 6 2 ± . 1 1 ± . 8 1 ± . 0 0 ± . 8 0 ± . 7 2 ± . 1 1 ± . 9 1 ± . 1 1 ± . 7 1 ± . 1 2 ± .

% VRe (RBS)

1 . 2 1 . 4 1 . 4 1 . 4 1 . 3 1 . 5 1 . 5 1 . 5 1 . 7 1 . 6 1 . 7 1 . 5 1 . 7 1 . 5 1 .

0 ± . 3 0 ± . 1 0 ± . 1 0 ± . 1 0 ± . 2 0 ± . 2 0 ± . 1 0 ± . 1 0 ± . 1 0 ± . 2 0 ± . 2 0 ± . 1 0 ± . 2 0 ± . 1 0 ± .

ACCEPTED MANUSCRIPT

B B B B B B B B B B

X7 51 X7 54. 5 X7 58. 7 X7 59. 7

0 .4

0 .9

- 8

-

8. 1

0 .4

0 .6

- 8

-

7. 5

0 .6

0 .6

- 8

-

6. 6

0 .6

0 .9

-

5. 6

0 .3

0 .6

-

5. 4

0 .2

0 .7

-

4. 7

0 .4

0 .6

-

5. 2

0 .2

0 .6

-

4. 5

1 .0

0 .8

-

5. 1

0 .5

0 .8

-

0 .6

B B

X7 63

4. 4

0 .2

B B

X7 67

4. 3

0 .2

5. 2

0 .2

6. 1

E E E E E E E E

X2 80. 4 X3 00. 1 X3 50. 7 X3 53. 2 X3 75. 3

1 0 1 2 1 2 1 1 1 8 1 5

-

-

1

5

1

-

0 .7

-

0 .1

- 2

-

0 .2

0 .2

- 3

-

5. 1

0 .3

0 .2

- 4

-

4. 7

0 .3

0 .3

- 6

-

3. 0

0 .1

0 .0

- 1

-

AC

E E

1 3

6

-

0. 00 6 0. 00 7 0. 00 6 0. 00 6 0. 00 7 0. 00 6 0. 00 7 0. 00 6 0. 00 6 0. 00 7 0. 00 7 0. 00 6 0. 00 6 0. 00 5 0. 00 5 0. 00 5 0. 00 5 0. 00 5

0. 00 6 0. 00 6 0. 00 6 0. 00 5 0. 00 6 0. 00 6 0. 00 5 0. 00 6 0. 00 5 0. 00 5 0. 00 5 0. 00 5 0. 00 5 0. 00 6 0. 00 6 0. 00 6 0. 00 6 0. 00 7

1 . 7 1 . 8 1 . 7 1 . 5 1 . 7 1 . 7 1 . 6 1 . 7 1 . 6 1 . 5 1 . 6 1 . 6 1 . 6 1 . 8 1 . 8 1 . 9 1 . 8 2 . 1

0 ± . 1 0 ± . 2 0 ± . 1 0 ± . 1 0 ± . 2 0 ± . 1 0 ± . 1 0 ± . 1 0 ± . 1 0 ± . 1 0 ± . 1 0 ± . 1 0 ± . 1 0 ± . 2 0 ± . 2 0 ± . 1 0 ± . 2 0 ± . 2

7

5

4

2

16 02. 5 15 98. 8 16 01. 9 16 00. 8 16 02. 6

13 54. 6 13 52. 9 13 55. 6 13 52. 1 13 54. 6

24 7. 8 24 5. 9 24 6. 4 24 8. 7 24 8. 1

1 ± . 8 1 ± . 0 2 ± . 7 1 ± . 8 2 ± . 1

1 . 8 1 . 7 1 . 7 1 . 9 1 . 8

0 ± . 2 0 ± . 1 0 ± . 3 0 ± . 2 0 ± . 3

-

-

16 01. 5 16 02. 0 16 01. 8 16 01. 1 16 00. 7

13 56. 2 13 56. 3 13 54. 4 13 54. 9 13 55. 1

-

-

16 00. 2

13 54. 6

-

-

16 02. 1 16 03. 0 16 04. 3 16 01. 8

13 53. 1 13 52. 9 13 53. 8 13 53. 2

IP

10 .8

0. 00 8 0. 00 8 0. 00 7 0. 00 8 0. 00 8 0. 00 8 0. 01 0 0. 00 9 0. 00 9 0. 01 0 0. 01 0 0. 01 0 0. 01 0 0. 00 8 0. 00 7 0. 00 6 0. 00 7 0. 00 6

5

CR

X7 35. 9 X7 48. 3

-

US

B B

X7 31

- 8

AN

B B

0 .8

M

B B

0 .8

ED

B B

11 .2

PT

B B

X7 20. 8 X7 21. 9 X7 25. 5 X7 27. 7

CE

B B

3

T

1

24 5. 3 24 5. 7 24 7. 4 24 6. 1 24 5. 7

2 ± . 2 1 ± . 5 0 ± . 5 0 ± . 4 1 ± . 2

1 . 6 1 . 7 1 . 8 1 . 7 1 . 7

24 5. 6

0 ± . 5

1 . 6

24 9. 0 25 0. 2 25 0. 5 24 8. 6

2 ± . 2 2 ± . 7 1 ± . 4 0 ± . 8

0 ± . 2 0 ± . 2 0 ± . 1 0 ± . 1 0 ± . 1

0 ± . 1 -

1 . 9 2 . 1 2 . 1 1 . 9

0 ± . 3 0 ± . 4 0 ± . 2 0 ± . 1

ACCEPTED MANUSCRIPT

0 .0

- 6

-

0. 4

0 .1

0 .0

- 6

-

3. 2

0 .1

0 .0

- 0

-

9. 6

3 .1

1 3.2

11 .4

3 .2

1 5.6

4

1 0 . 9 4 1 0 4 3 . 6 7 9 3 7

0 ± . 1 0 ± . 1

2 . 0 2 . 3 2 . 5 2 . 1 0 . 8 0 . 9

0 ± . 2 0 ± . 2 0 ± . 2 0 ± . 2 0 ± . 1 0 ± . 1

US

4 8

0. 00 7 0. 00 8 0. 00 8 0. 00 7 0. 00 2 0. 00 2

16 04. 5 16 05. 9 16 05. 1 16 03. 6 16 00. 1 15 91. 1

13 53. 5 13 57. 8 13 59. 3 13 55. 8 13 61. 0 13 64. 2

T

0 .0

0. 00 5 0. 00 4 0. 00 4 0. 00 4 0. 00 9 0. 01 0

IP

0. 3

0. 00 5 0. 00 3 0. 00 3 0. 00 5 0. 01 3 0. 01 2

CR

-

AN

G G

- 4

M

G G

0 .1

ED

E E

0 .1

PT

E E

1. 7

CE

E E

X3 90. 8 X4 15. 2 X4 88. 2 X4 98. 2 X9 05. 4 X9 08. 6

AC

E E

25 1. 0 24 8. 1 24 5. 8 24 7. 8 23 9. 2 22 6. 9

0 ± . 9 1 ± . 2 0 ± . 8 0 ± . 3 2 ± . 6 4 ± . 4

2 . 2 1 . 8 1 . 6 1 . 8 1 . 1 0 . 6

0 ± . 2 0 ± . 2 0 ± . 1 0 ± . 1 0 ± . 2 0 ± . 1

ACCEPTED MANUSCRIPT Table 3. Geochemical and physical characteristics of kerogen isolates from Qusaiba core samples

AA AA

X2 56

AA

X2 61. 7

AA

X2 64

BB

X7 25. 5

BB

X7 51

BB

X7 67

EE EE EE G G

X2 80. 4 X3 50. 7 X4 98. 2 X9 05.

8. 3

0 .54

3. 7

2. 2

1. 7

8. 4

0 .54

4. 1

2. 2

0. 9

6. 6

0 .59

4. 3

2. 3

0. 7

6. 1

0 .60

4. 4

2. 2

0. 8

6. 6

0 .63

3. 7

2. 2

1. 5

8. 0

0 .54

4. 0

2. 1

0. 8

1 0. 8

0 .65

3. 9

2. 2

1. 4

7. 6

0 .59

3. 8

2. 3

1. 0

7. 2

0 .54

3. 6

2. 2

1. 2

8. 5

0 .52

3. 5

2. 2

1. 0

8. 9

0 .51

3. 6

2. 2

0. 9

8. 7

0 .52

3. 5

2. 1

1. 0

6. 9

0 .49

3. 5

2. 2

0. 6

6. 8

0 .50

2. 2

1. 4

4. 8

0 .48

5. 4

3. 0

6. 1

0 .78

2 0. 0 1. 0

0 .07 5 0 .07 8 0 .05 9 0 .05 3 0 .05 8 0 .07 3 0 .11 0 0 .07 2 0 .06 4 0 .07 6 0 .08 1 0 .07 9 0 .06 1 0 .06 0 0 .06 7 0 .05

9 8.1 9 7.4 9 7.6 9 8.7 9 8.6 9 7.7

0 1.42 .6 4

-

0 1.44 .3 5

-

-

-

0 1.50 .3 9 1 .2

Ye s

-

0 1.45 .7 2

9

0 1.37 .9 7

9

1 1.43 .1 6

9

1.4 4.2 8.1

9 8.6 9 8.4 9 8.4 9 8.7 9 8.1 8 2.8 9 8.5

1 1

Thio phen e 0

T

1. 0

Sul fid e

Elem ental

1

IP

2. 2

g/c m3

Sulfur K-edge XANES

5

-

8

3

3 15

0

7

6

CR

3. 7

#

Sur fac e Are a,* m2/ g

US

% O

Crit ical Poi nt Dry

AN

AA

% S

% As h

Skel etal den sity,

-

M

AA

% N

% Ke roge n

ED

AA

8 2. 9 8 1. 4 8 3. 6 8 5. 3 8 4. 6 8 2. 3 7 3. 7 7 9. 1 8 3. 9 8 3. 2 8 2. 7 8 3. 0 8 5. 3 8 5. 0 5 4. 4 8 3.

% H

Ato mic O/ C

PT

AA

X1 70. 7 X1 76. 5 X1 79. 3 X1 84. 3 X1 89. 1 X1 91. 2

% C

Ato mic H/ C

CE

AA

De pth , ft

Elemental Analysis

AC

W ell Na m e

1 0 1 1

2 0

Sulf oxid e 8

3 7 2 7 0 8 4 8 2 8 2

Sul fini c

Su lfit e

6

0

0

6

0

0

6

1

6

3

0

6

0

0

6

0

0

1 1

-

-

-

-

-

-

-

2 57

-

-

-

-

-

-

0 1.42 .3 3

-

8

0

5

0

0

0 1.45 .7 5

-

7

0

6

0

0

0 1.45 .6 6

-

5

0

6

0

0

1 1.44 .0 9

-

6

0

5

0

0

7

0

0

6

0

0

-

-

-

4

0

0

Ye s

4 1.46 .8 5

Ye s

3 50

6

0

2 1.50 .1 3

Ye s

3 25

7

0

-

-

-

2 0. 8 2 1.11 .3 3

Ye s

3 0

1 2

2

8 7 8 8 9 0 8 9 8 7 8 6 8 2

ACCEPTED MANUSCRIPT

G G #

4

1

X9 08. 6

8 0. 3

5 5. 0

2. 0

3. 3

6. 0

0 .74

0 .05 6

9 6.5

7 .4

-

Ye s

3 8

1 1

5

7 5

6

Insufficient mass of kerogen was recovered for density analysis of four kerogen isolates

AC

CE

PT

ED

M

AN

US

CR

IP

T

* Specific surface area measurements performed only on kerogen isolates that were critical point dried and does not include sampels from well BB

0

2

ACCEPTED MANUSCRIPT Table 4. Summary of maturity estimates from analysis of Qusaiba shales and corresponding kerogen isolates*

AA AA

-

± ± ± ±

AA

X256

-

-

AA

X261. 7

-

-

AA

X264

-

-

BB

X725. 5

-

BB

X751

-

BB

X767

-

GG GG

-

0. 9 0. 9

± ±

± ± ± ±

0. 1 0. 1 0. 1 0. 2 0. 1 0. 2 0. 1 0. 1

PT

EE

-

CE

EE

X280. 4 X350. 7 X498. 2 X905. 4 X908. 6

0. 1 0. 1

AC

EE

1. 7 1. 7 1. 6 1. 8 1. 9 2. 1 0. 8 0. 9

± ± ± ±

1. 4 1. 3 1. 5 1. 5 1. 7 1. 6 1. 7 1. 7 1. 5 1. 7 1. 7 1. 6

± ± ± ± ± ± ± ± ± ±

0. 1 0. 2 0. 2 0. 1 0. 1 0. 2 0. 2 0. 2 0. 1 0. 3 0. 2 0. 1

± ± -

2. 1 1. 8 1. 1 0. 6

± ± ± ±

1. 8 1. 8 1. 5 1. 4 1. 4 1. 8

0. 4 0. 1 0. 2 0. 1

± ± ± ±

0. 3 0. 3 0. 3 0. 3 0. 3 0. 3

1. 8 1. 9 2. 0 1. 9 2. 2 2. 1 2. 3 1. 0 1. 0

1. 6 1. 8

± ± -

2. 2

±

± -

±

±

-

± ± ± ± ± ± ± ± ±

0. 3 0. 3 0. 3 0. 3 0. 3 0. 3 0. 3 0. 2 0. 2

0. 2 0. 2 0. 2

-

1. 8 1. 4 1. 7 1. 6 1. 8 1. 8 1. 8 1. 9 2. 1

±

± ± ± ± ± ± ±

± -

-

1.9 -

0. 2 0. 2 0. 2 0. 2 0. 2 0. 2 0. 2 0. 2 0. 2

0. 5

% VRe (Surfac e Area)

T

-

±

0. 1 0. 1 0. 1 0. 1 0. 1 0. 2

IP

-

±

% VRe (Density)

CR

AA

1. 7 1. 5 1. 7 1. 6 1. 6 1. 5

-

% VRe (Raman)

US

AA

% VRe (Atomic H/C)

% VRe (DRIFTS)

AN

AA

X170. 7 X176. 5 X179. 3 X184. 3 X189. 1 X191. 2

% VRe (T max)

M

AA

Depth , ft

Kerogen: % VRe and 1σ uncertainty

ED

Well Nam e

Bulk shale: % VRe and 1σ uncertainty

1.6 2.1 2.0 -

0. 2

0.5 0.6

Mean % VRe and 95% c.i.

1. 6 1. 5 1. 6 1. 7 1. 6 1. 7 1. 5 1. 7 1. 6 1. 7 1. 8 1. 7 1. 9 2. 0 1. 9 0. 8 0. 9

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

* Quoted uncertainties on individual % VRe estimates for each technique are 1σ values. The calculated uncertainties on the weighted-mean for each sample are 95 % confidence interval (2σ) values.

0. 1 0. 2 0. 2 0. 1 0. 1 0. 2 0. 2 0. 2 0. 2 0. 1 0. 1 0. 1 0. 2 0. 2 0. 2 0. 2 0. 2

ACCEPTED MANUSCRIPT

Highlights Thermal maturity is assessed in Silurian Qusaiba Member, Saudi Arabia



Vitrinite reflectance analysis is precluded by absence of vitrinite in organic matter



Study uses alternative spectroscopic and geochemical techniques



Maturity estimates are consistent in a given well and range from 0.9–2.1 %Ro in the studied wells

AC

CE

PT

ED

M

AN

US

CR

IP

T

