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Marine and Petroleum Geology 31 (2012) 70e89

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Marine and Petroleum Geology

journal homepage: www.elsevier .com/locate/marpetgeo

Geochemical evolution of organic-rich shales with increasing maturity: A STXMand TEM study of the Posidonia Shale (Lower Toarcian, northern Germany)

Sylvain Bernard a,b,*, Brian Horsfield a, Hans-Martin Schulz a, Richard Wirth a, Anja Schreiber a,Neil Sherwood c

aGFZ German Research Centre for Geosciences, Telegrafenberg, 14473 Potsdam, Germanyb Laboratoire de Minéralogie et de Cosmochimie du Museum (LMCM), MNHN, CNRS, 57 rue Cuvier, 75231 Paris Cedex 5, FrancecCSIRO Earth Science and Resource Engineering, P.O. Box 136, North Ryde, NSW 1670, Australia

a r t i c l e i n f o

Article history:Received 23 February 2011Received in revised form12 May 2011Accepted 23 May 2011Available online 13 June 2011

Keywords:Shale gasKerogenBitumenPyrobitumenPorosityNanoscaleFIBSTXMXANES/NEXAFSTEM

* Corresponding author. GFZ German ResearcTelegrafenberg, 14473 Potsdam, Germany.

E-mail address: sbernard@mnhn.fr (S. Bernard).

0264-8172/$ e see front matter � 2011 Elsevier Ltd.doi:10.1016/j.marpetgeo.2011.05.010

a b s t r a c t

Hydrocarbon generation and retention processes occurring within gas shales as a response to increasesin thermal maturation are still poorly constrained. While efforts have been directed at unravelling theresource potential, composition and textures of these economically important unconventional systems,their spatial variability in chemistry and structure is still poorly documented at the sub-micrometerscale. Here, we have characterized samples of the Lower Toarcian Posidonia Shale samples fromnorthern Germany at varying stages of thermal maturation using a combination of compositional organicgeochemistry and spectromicroscopy techniques, including synchrotron-based scanning transmissionX-ray microscopy (STXM). We document geochemical and mineralogical heterogeneities down to thenanometer scale within the investigated samples as a function of their level of thermal maturity. Inparticular, authigenic albite crystals containing nanometric halite inclusions have been documentedwithin the investigated mature and overmature samples. The presence of such tracers of palaeobrineecarbonate interactions supports a maturation scenario for the Lower Toarcian Posidonia Shale inti-mately related to ascending brine fluids rather than a maturation scenario solely resulting from high heatflows. In addition, various types of asphaltene- and NSO-rich bitumen have been detected within thesame samples, very likely genetically derived from thermally degraded organic precursors. Furthermore,the formation of nanoporous pyrobitumen has been inferred for samples of gas window maturity, likelyresulting from the formation of gaseous hydrocarbons. By providing in-situ insights into the fate ofbitumen and pyrobitumen as a response to the thermal evolution of the macromolecular structure ofkerogen, the results reported here constitute an important step towards better constraining hydrocarbongeneration processes during natural shale gas maturation.

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1. Introduction

As a new and abundant energy resource, unconventional gasshales have already impacted the US energy supply. Understandingthe geological and geochemical nature of gas shale formation andimproving the gas recovery have thus been at the heart ofmillions ofdollars worth of research. Currently, worldwide exploration activi-ties are drastically increasing (e.g., Curtis, 2002; Jenkins and Boyer,2008). A wide range of bulk analyses have been routinely used inorder to estimate the level of maturity, the content and type ofkerogen and bitumen present, as well as how much and what typeof hydrocarbons may potentially have been generated within

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numerous formations worldwide (e.g., Tissot and Welte, 1984;Curtis, 2002; Jarvie et al., 2007). However, in spite of recent prog-ress (e.g., Curtis, 2002; Jarvie et al., 2007; Ross and Bustin, 2009;Loucks et al., 2009; Bernard et al., 2010a), detailed elucidation ofhydrocarbon generation and retention processes occurring withinsuch complex self-contained source-reservoir systems remainsa poorly constrained subject of organic geochemical research.

Four distinct processes may result in the formation of thermo-genic gas (e.g., Lewan et al., 1979; Burnham and Braun, 1990;Pepper and Dodd, 1995; Pepper and Corvi, 1995; Behar et al.,1997; Schenk et al., 1997; Lorant and Behar, 2002; Erdmann andHorsfield, 2006; Jarvie et al., 2007; Tian et al., 2008; Guo et al.,2009 and references therein): (1) the decomposition of kerogento gas and bitumen, (2) the secondary decomposition of bitumen tooil and gas, (3) the secondary cracking of oil to gas and a carbon-rich residue such as pyrobitumen, and (4) the degradation of

S. Bernard et al. / Marine and Petroleum Geology 31 (2012) 70e89 71

mature kerogen within the metagenesis zone leading to theformation of late gas. Generated gas can be stored in gas shales asfree gas in intergranular pores and natural fractures, adsorbed ontokerogen and clay particle surfaces or dissolved in kerogen andbitumen (e.g., Curtis, 2002; Montgomery et al., 2005; Jarvie et al.,2007; Jenkins and Boyer, 2008; Ross and Bustin, 2009; Louckset al., 2009 and references therein). The volume of gas generatedby secondary cracking has been said to largely and directly dependon oil retention in the system, i.e. on the nature of organic andinorganic phases and their specific adsorption capabilities as wellas on organic/inorganic structural relationships, pores and fracturenetworks. Almost all currently producing shale gas reservoirs areovermature oil-prone source rocks (e.g., Curtis, 2002; Jarvie et al.,2007; Jenkins and Boyer, 2008 and references therein). Throughheating and/or burial, these reservoirs have evolved from marineorganic-rich muds to highly heterogeneous (both vertically andlaterally) organic-rich shales containing various remnants oforganic precursors and products associated with various inorganicphases (e.g., Ross and Bustin, 2009; Loucks et al., 2009; Bernardet al., 2010a). While the basic stoichiometries and source-sinkrelationships are understood, the chemical and structural vari-ability of gas shales at the sub-micrometer scale is still poorlydocumented (e.g., Ross and Bustin, 2009; Loucks et al., 2009;Bernard et al., 2010a).

Improvement in our knowledge of the geochemical evolution ofsuch systems with increasing maturity appears necessary toimprove the reliability of the kinetic models used for predictingthe amount and composition of generated hydrocarbons. Artificialmaturation and pyrolysis experiments performed in laboratoryunder well-constrained physical and chemical conditions havebeen extensively used for decades to better constrain hydrocarbongeneration processes occurring in source rocks (e.g., Lewan et al.,1979; Lewan, 1985, 1997; Horsfield and Dueppenbecker, 1991;Horsfield et al., 1992; Larter and Horsfield, 1993; Vandenbrouckeet al., 1993; Kruge et al., 1997; Schenk et al., 1997; Stasiuk, 1997;Lorant et al., 1998; Seewald et al., 1998; Putschew et al., 1998;Hill et al., 1996, 2003; Seewald, 2001a, 2001b; Tiem et al., 2008;Pan et al., 2009; Behar et al., 1992, 2008a, 2008b, 2010; Lewanand Roy, 2011). However, temperatures and heating rates consid-erably higher than those encountered in nature have to be appliedto the rock samples to compensate for long geological timeperiods. Extrapolating laboratory results to natural hydrocarbongeneration processes might thus sometimes be perilous (e.g.,Snowdon, 1979). The chemical and structural characterization ofnatural samples thus appears essential to provide additionalinsights on hydrocarbon generation processes and improve ourability to extrapolate laboratory results to natural conditions ina realistic manner.

Classical analytical methods such as total organic carboncontent (TOC) estimation, RockeEval pyrolysis, open systempyrolysis e gas chromatography and vitrinite reflectancemeasurements are most frequently used for evaluating the poten-tial and producibility of natural gas shale systems (e.g., Tissot andWelte, 1984; Horsfield, 1989; Jarvie et al., 2007; Jenkins andBoyer, 2008; Dembicki, 2009). In parallel, non-destructive spec-troscopic analyses, such as solid-state nuclear magnetic resonance(NMR) and Fourier transform infrared (FTIR) spectroscopy, areincreasingly used for the bulk chemical and structural character-ization of natural kerogen and study their degradation processes(e.g., Patience et al., 1992; Wei et al., 2005; Lis et al., 2005; Smerniket al., 2006; Kelemen et al., 2007; Petersen et al., 2008; Mao et al.,2010). However, better constraining thermal maturation processescrucially requires spatially-resolved geochemical information inaddition to bulk information (e.g., Bernard et al., 2010a). Forinstance, elucidating the pore structure of organic-rich shales and

identifying the organic phases displaying nanoscale porosity arefundamentally important to better understand the processesoccurring in coal or shale gas reservoirs during maturation (e.g.,Clarkson and Bustin, 1996, 1999; Mastalerz et al., 2008; Nelson,2009; Ross and Bustin, 2009; Loucks et al., 2009; Strapoc et al.,2010). Transmission electron microscopy (TEM), which providesspatially-resolved information on organic constituent texture at thesub-nanometer scale, and synchrotron-based scanning trans-mission X-ray microscopy (STXM), which provides spatially-resolved information on organic constituent speciation at the20 nm scale (e.g., Bluhm et al., 2006), appear intrinsicallycomplementary and well suited for the nanoscale characterizationof heterogeneous and organic-rich samples such as gas shales (e.g.,Bernard et al., 2010a).

In the present contribution, we report the multiscale charac-terization of core samples of the Lower Toarcian Posidonia Shalefrom the Hils half-graben in northern Germany. The investigatedsamples are taken from a progressive maturation profile fromimmature organic-rich mudstones to overmature gas shales. Theexceptional geological situation of the investigated samples,combining awidematuration range togetherwith facies continuity,offers an excellent opportunity to precisely study the generation(and retention) of (gaseous) hydrocarbons that naturally occurredwithin gas shales. First, we present the compositional organicgeochemical data available on these samples. Then, using TEM andSTXM, we document geochemical and mineralogical heterogene-ities at themicro- and nanoscale within the investigated samples asa function of their maturity level. We finally discuss all theseobservations in the light of thermal history constraints and gaseoushydrocarbon generation/retention processes.

2. Materials and methods

2.1. Maturation series of the Posidonia Shale

The samples investigated in this study come from the Lias e levelof the Lower Toarcian Posidonia Shales from the Hils half-graben inthe Lower Saxony Basin, northern Germany. This formation is seenas a reference of source rock kerogens of Type II elementalcomposition with high oil-generating potential and is regarded asone of the most widespread and economically important petro-leum source rocks of Western Europe (Tissot and Welte, 1984;Horsfield, 1989; Vandenbroucke et al., 1993; Vandenbroucke andLargeau, 2007). These black shales were deposited in an epiconti-nental sea of moderate depth extending from the Yorkshire Basin(England) over the Lower Saxony Basin and the Southwest GermanBasin into the Paris Basin during the Lower Toarcian (e.g., Rullköteret al., 1988; Littke et al., 1991a). Deposition probably occurred inrestricted environments where water stratification leading tooxygen depleted bottomwaters could easily develop (e.g., Rullköteret al., 1988; Littke et al., 1991a). The study reported here focuses onsamples from three boreholes, the Wickensen, the Harderode andthe Haddessen boreholes, which were drilled along the westernflank of the Hils half-graben, following a line of progressive increasein thermal maturation towards the NW from the Wickensen to theHaddessen well.

2.2. Sample preparation procedures and analytical methods

2.2.1. Organic petrology and vitrinite reflectance measurementsStandard methods of petrographic sample preparation were

used to prepare polished sample blocks for reflectance measure-ments and maceral analyses. Reflectance values were taken onrandomly oriented phytoclasts in non-polarised light (Ro%) in oilimmersion having a refractive index of 1.518 at 23 � 1 �C. Zeiss

S. Bernard et al. / Marine and Petroleum Geology 31 (2012) 70e8972

equipment was employed, using a �40 objective with a measuringdiaphragm giving a 3 mm diameter spot size and an interferencefilter having a passband peak of 546 nm. A minimum of 25 reflec-tance measurements are made on vitrinite/vitrinite-like matter inorder to achieve a stable mean. A smaller number of measurementswas also made on other macerals for comparative purposes. Thephotometer was calibrated against a synthetic yttrium-aluminium-garnet standard of 0.92% reflectance. Maceral analyses were doneusing a stepping stage and point-counter, with the steppinginterval set to enable complete coverage of the sample in 10traverse lines and a minimum of 500 points counted.

2.2.2. Extraction of soluble organic compoundsPowdered rock samples (10e20 g) were extracted for 48 h using

an azeotropic solvent system (chloroform (32%), acetone (38%),methanol (30%)) in a Soxhlet apparatus. Asphaltenes were precip-itated from an aliquot of the total extract using the method ofTheuerkorn et al. (2008). The maltenes, remaining in solution, wereconcentrated then separated into saturate, aromatic and NSOfractions using Medium Pressure Liquid Chromatography (Radkeet al., 1980). The XANES spectra of all four fractions weremeasured to provide calibration standards thereby allowing FIBsections to be scanned to define the distribution of organic speciesaccording to differing polarity and affinity. The remaining insolubleorganic fraction, mainly consisting of kerogen (associated withpyrobitumen in Haddessen samples), has also been investigated byC-XANES spectroscopy.

2.2.3. Sample preparation by focused ion beam (FIB) millingTEM and STXMare transmission techniques and thus require the

samples to be electron and X-ray transparent, respectively, i.e.,thinner than 200e250 nm. Focused ion beam (FIB)millingwas usedto prepare such ultrathin samples. This sample preparation tech-nique preserves microtextural information while avoiding harshchemical extraction or staining. Ultrathin foils, typicallyw15� 5� 0.10 mm,were prepared using the FIB single beam device(FEI FIB 200 TEM) operating at GFZ Potsdam. This sample prepara-tion has become a standard sample preparation technique in Earthsciences (e.g., Benzerara et al., 2007; Schiffbauer and Xiao, 2009;Lepot et al., 2009; Papineau et al., 2010; Bernard et al., 2007, 2010b).

The FIB milling procedure is described in detail in Heaney et al.(2001), Benzerara et al. (2005) and Wirth (2004, 2009). Basically,a thin layer of platinum was deposited on the sample surface inorder to protect it during the milling process. A 30 kV Gaþ beamemitted from a Ga liquid metal ion source operating at w20 nAallows excavating the sample from both sides of the Pt layer toa depth ofw5 mm. Before removal of the thin slide, the sample wasfurther thinned to w100 nm with a glancing angle beam at muchlower beam currents of w100 pA to prevent any artifact during themilling procedure. Finally, the foil was transferred at room pressurewith a micromanipulator onto the membrane of a carbon-coated200 mesh copper grid for subsequent TEM and STXM analyses.

Milling at low Gaþ currents has been convincingly shown toallow reducing potential artifacts like local Gallium implantation,mixing of components, creation of vacancies or interstitials, andalso, depending on the material, creation of a several nm-thickamorphous layer covering the surface of the specimen (e.g., Obstet al., 2005; Langford, 2006; Drobne et al., 2007). This prepara-tion technique has thus been proved valuable to prepare FIB foilsfrom micro-to nanoporous and/or highly sensitive materials as itprevents shrinkage as well as micro- to nanopore deformation (e.g.,Smith et al., 2001; Thompson et al., 2006). In addition, Bernard et al.(2009) have recently shown that FIB milling does not inducesignificant changes in the speciation of carbon in model polymersas measured by STXM-based C-XANES spectroscopy.

2.2.4. Scanning transmission X-ray microscopy (STXM)STXM is a synchrotron-based transmission spectromicroscopy

technique using a monochromated X-ray beam produced bysynchrotron radiation. This technique allows both microscopicobservations - i.e. imaging at the 25-nm scale with speciationsensitivity e and spectroscopic measurements - i.e. recording X-rayabsorption near edge structure (XANES) spectra which provideinformation on the bonding environment of carbon in organiccompounds at the same spatial scale. For STXM imaging, the X-raybeam is focused on the sample using a zone plate, and a 2-D imageis collected by scanning the sample at a fixed photon energy ata spatial resolution of about 20 nm. The image contrast results fromdifferential absorption of X-rays, which depends, at the CarbonK-edge for instance, on the speciation of the organic particlespresent within the investigated samples. XANES spectra can beobtained by collecting image stacks with energy increments of0.1 eV over the energy range of interest (280e320 eV at the C K-edge). Counting times are of the order of a few milliseconds or lessper pixel. Absorption peaks constitute sensitive indicators of thelocal chemical bonding environment surrounding the carbon atomsin question and correspond to transitions from inner shell 1selectrons to both unoccupied p* (antibonding) and low lying s*orbitals.

Similarly to all electricedipole transition spectroscopies,XANES spectroscopy adheres to Beer’s law. Variations in peakintensity thus directly result from variations in concentration ofthe functional groups absorbing at these specific energies.However, the intensity of a given peak is a function of the specificoscillator strength of the transition (Ishii and Hitchcook, 1988;Francis and Hitchcock, 1992). Without a precise estimation ofthese oscillator strengths, the respective concentrations of thedifferent functional groups cannot be quantitatively estimated.Additional experimental and theoretical studies, as well as a reli-able calibration based on reference materials similar to theinvestigated samples, are needed in order to better understandthe physics beyond XANES data and provide quantitative esti-mations of the contribution of the different moieties within theorganic compounds. Nevertheless, given that the C-XANES spectrapresented here have been normalized to the total carbon amount(corresponding to the absorption at 320 eV) and assuming that theoscillator strength of a given functional group is essentially thesame in organic compounds of similar chemistry, the relativeconcentrations of the different functional groups can be discussedqualitatively.

Measurements of the present study were done using the STXMlocated on beamline 5.3.2.2 (STXM Polymer beamline - Kilcoyneet al., 2003) at the Advanced Light Source (ALS) and on beamline10ID-1 (SM beamline - Kaznatcheev et al., 2007) at the CanadianLight Source (CLS). Beamline 5.3.2.2. (ALS) uses soft X-rays(250e800 eV) generated via a bending magnet while the electroncurrent in the storage ring is held constant in top-off mode at500 mA at a storage ring energy of 1.9 GeV. Beamline 10ID-1 (CLS)uses soft X-rays (130e2500 eV) generated with an ellipticallypolarized undulator (EPU) inserted in the 2.9 GeV synchrotronstorage ring (250e100 mA). On both beamlines, the microscopechamber was evacuated to 100 mTorr after sample insertion andback-filled with He. Energy calibration was accomplished using thewell-resolved 3p Rydberg peak at 294.96 eV of gaseous CO2 for theC K-edge. The numerous images of each stack were aligned usingthe automated image alignment routine of the a Xis2000 software(ver2.1n - available on http://unicorn.mcmaster.ca/aXis2000.html)which was also used to extract XANES spectra from image stackmeasurements. Spectral peak positions, intensities and widthswere determined using the Athena software package (Ravel andNewville, 2005).

S. Bernard et al. / Marine and Petroleum Geology 31 (2012) 70e89 73

The C-XANES data shown here have been performed followingthe procedures for X-ray microscopy studies of radiation sensitivesamples recommended by Wang et al. (2009), although radiationdamage per unit of analytical information has been shown to betypically 100e1000 times lower in STXM-based XANES spectros-copy than in TEM-based EELS (Rightor et al., 1997; Braun et al.,2005, 2009; Hitchcock et al., 2008). Extensive databases of refer-ence XANES spectra measured on hundreds of C-containingcompounds at the C K-edge, sometimes supported by theoreticalcalculations using multiple scattering approaches are available(e.g., http://unicorn.mcmaster.ca/corex/cedb-title.html; Myneni,2002; Solomon et al., 2009). Examples of STXM applications invarious fields can be found in Hitchcock (2001), Bluhm et al. (2006)and Thieme et al. (2010).

2.2.5. Transmission electron microscopy (TEM)Transmission electron microscopy was performed at the GFZ-

Potsdam using a TECNAI F20 XTWIN TEM operated at 200 kVwith a field emission gun (FEG) as the electron source. The TEM isequipped with a Gatan Tridiem� energy filter, an EDAX Genesis�X-ray analyzer and a Fishione high angle annular dark filed detector(HAADF) allowing image acquisition in the scanning transmissionelectron microscopy (STEM) mode. Energy dispersive X-ray spec-troscopy (EDXS) element maps are always displayed as backgroundsubtracted intensity maps. STEM images have been collected at thesub-nanometer spatial resolution on the FIB sections extractedfrom the investigated samples.

Figure. 1. Top: Mean vitrinite reflectance values (Ro), TOC content and RockeEval parameHaddessen wells. Bottom: Hydrogen Index vs Oxygen Index diagram (left) and Hydrogen Indethe Wickensen well, the yellow to the investigated samples from the Harderode well and thShale samples appear to follow the mean evolution pathways of kerogen type II on the Hydrlegend, the reader is referred to the web version of this article.)

3. Results

3.1. Bulk and micro-scale observations

3.1.1. Bulk organic geochemical analysesBulk organic geochemical data (TOC content and RockeEval

parameters) of the investigated samples are given in Figure 1. TheTOC content of immature shale samples from theWickensenwell ishigher (>10 w%) than the TOC values of the more mature equiva-lent shale samples from the Harderode and Haddessen wells(w6.5 w%). The remaining hydrocarbon generating potential value(S2) as well as the free hydrocarbon content (S1) decreasedramatically between Wickensen and Haddessen. Based onhydrogen index (HI) vs. oxygen index (OI) and HI vs. Tmax diagrams(Fig. 1), the kerogen type and petroleum potential of the investi-gated shale samples can be evaluated. The maturation seriesinvestigated here appears to follow the mean evolution pathwaysof kerogen type II, leading to the onset of gaseous hydrocarbongeneration for Haddessen samples, which is consistent with theirTmax value of 470 �C. According to the guidelines of Jarvie et al.(2007), all the geochemical parameters of the investigated Had-dessen samples are therefore consistent with a high shale gaspotential (Bernard et al., 2010a). More detailed organic geochemicalcharacterizations of the Lias e sediments from the Hils synclinehave been previously presented (Leythaeuser et al., 1988; Littkeet al., 1988, 1991a; Rullkötter and Marzi, 1988; Mann and Müller,1988; Rullkötter et al., 1988; Vandenbroucke et al., 1993).

ters of the investigated Posidonia Shale samples from the Wickensen, Harderode andx vs Tmax diagram (right). The green dot corresponds to the investigated samples frome red to the investigated samples from the Haddessen well. The investigated Posidoniaogen Index vs Tmax diagram. (For interpretation of the references to color in this figure

S. Bernard et al. / Marine and Petroleum Geology 31 (2012) 70e8974

3.1.2. Organic petrology and vitrinite reflectance measurementsThe macerals appear to be uniformly disseminated within the

rock matrices of the investigated samples. Organic constituents ofthe investigated immature samples (Wickensen) mainly comprisealginites largely derived from dinoflagellate/acritarch and prasi-nophyte cysts, along with lesser amounts of liptodetrinite, bitu-minite, bitumen, vitrinite and inertinite (Fig. 2). Brightly fluorescingalginites are the most abundant maceral in Wickensen samples(Littke and Rullkötter, 1987; Littke et al., 1988). Alginites, togetherwith liptodetrinites, account for more w90% of the recognizableorganic material in the samples analyzed in the present study.Inertinite group macerals account for about 1.5% of the sampleswhile zooclasts (faunal relics) and vitrinite account for 0.5 andw0.2%, respectively. The vitrinite and most of the inertinite arederived from terrigenous sources.

With increasing maturation, the fluorescence characteristics ofthe liptinites change. The prasinophyte-derived alginites in the

Figure. 2. Photomicrographs (oil immersion) of the investigated Posidonia Shale samples frowhite light (black and white photos) and fluorescence mode (color photos) illumination. (Fothe web version of this article.)

Harderode samples have weaker fluorescence than in the Wick-ensen samples (Fig. 2). Although the fluorescence has been lost andthe reflectances of the various organic matter types have convergedfor the Haddessen samples, the different macerals can commonlybe recognized on the basis of morphology (Fig. 2). The maceralcomposition of the Wickensen and Harderode samples are similarbut the Haddessen samples contain more finely disseminatedmicrinite, most likely as a residual of other macerals after hydro-carbon generation. More details on the uniformity of the PosidoniaShale organofacies can be found in the literature (e.g., Littke andRullkötter, 1987; Littke et al., 1988).

The maturation levels of the investigated Posidonia Shalesamples estimated from reflectance measurements performed onvitrinite and vitrinite-like components range from 0.50% Ro for theWickensen samples, to 0.85% Ro for the Harderode samples and1.45% Ro for the Haddessen samples (Fig. 3). These values are inagreement with the RockeEval data and the previous vitrinite

m the Wickensen (top), Harderode (middle) and Haddessen (bottom) wells in reflectedr interpretation of the references to color in this figure legend, the reader is referred to

Figure. 3. Reflectance histograms for liptinite, bituminite, bitumen, inertinite andvitrinite-like organic matter of the investigated Posidonia Shale samples from theWickensen (top), Harderode (middle) and Haddessen (bottom) wells.

S. Bernard et al. / Marine and Petroleum Geology 31 (2012) 70e89 75

reflectance measurements reported in the literature (e.g., Mann,1987; Littke et al., 1988, 1991a; Mann et al., 1991). Two pop-ulations of bitumen can be identified on the basis of reflectancemeasurements within the investigated Haddessen samples, onehaving reflectances below 1.4% and the other up to 1.65% (Fig. 3).

3.1.3. Micro-scale mineralogical characterizationMineralogically, the w30 m thick sediments of the Lias e Pos-

idonia Shale mainly consist of clay (illite, kaolinite, smectite,chlorite), calcite, pyrite, quartz and some feldspars (Littke et al.,1988) (Fig. 2). Except for the slightly higher content of calcite inthe marlstone unit (w5 m thick) and of clay minerals and pyrite inthe overlying calcareous shale facies (w25 m thick), there is noappreciable mineralogic compositional variability. Both units aretexturally extremely fine-grained with porosities well below 10%(Mann, 1987; Mann and Müller, 1988; Littke et al., 1988). Even on

a micro-scale, no obvious primary pathways might act aspermeable conduits for the expulsion of fluids (Littke et al., 1988)(Fig. 2).

Within all the investigated samples, silicates mostly comprisequartz and clay minerals, among which illite and kaolinite prevail(Mann, 1987). In Wickensen samples, calcite mainly consists ofcoccoliths and other plankton-derived microfossils. A subordinatefraction of the carbonate is recrystallized. In the northern Hils half-graben, however, where thermal maturity is much greater (i.e., inHarderode and Haddessen samples), recrystallized carbonatepredominates overmicrofossil remains. Pyrite is themajor sulphur-containing compound in this sedimentary sequence. Framboidalpyrite is the most prominent microscopic variety in all the inves-tigated samples, although isolated euhedral grains are ubiquitous.Measurements of sulphur and iron contents of immature samples(Wickensen) reveal a slight excess of sulphur compared to thequantity needed to form pyrite (Littke et al., 1991a). This excess ofsulphur has been previously assumed to reside in organic matter(Littke et al., 1991a).

In addition, sparsely disseminated albite crystals have beendocumented within the investigated Harderode and Haddessensamples. No K- or Ca-feldspars have been observed.

3.2. C-XANES spectroscopy analyses of extracted soluble fractionsand isolated insoluble fractions

3.2.1. Fractions of extractable soluble organic matter (bitumen)The C-XANES spectra of the soluble fractions (aliphatics,

aromatics, polar NSOs and asphaltenes) extracted from the inves-tigated Wickensen, Harderode and Haddessen samples are repor-ted in Figure 4 (yields are reported in Table 1). These fractionsconstitute internal references and allow documenting the forma-tion of organic compounds with increasing maturity.

The C-XANES spectra of the aliphatic fractions display a strongabsorption feature at 287.7 eV with a shoulder at 288 eV (Fig. 4).Such absorption at 288 eV is unambiguously indicating the pres-ence of aliphatic carbon (1s / 3p/s*) (CeH1e3) (Cody et al., 1998).Although the absorption at 287.7 eV is sometimes attributed to thepresence of unsaturated bonds between carbon and heteroatomssuch as oxygen (Braun et al., 2005, 2009), nitrogen (Shi et al., 2005),or sulphur (Liu et al., 2005), it is most commonly ascribed to the1s / 3p/s* transitions of CeH1e3 (aliphatic carbon bonded to one,two, or three hydrogens) (Cody et al., 1998; Boyce et al., 2002;Hitchcock et al., 2007). The very low intensity features observedat 285.3 eV can be interpreted as 1s/ p* transitions in aromatic orolefinic carbon (C¼C) (Cody et al., 1996, 1998; Boyce et al., 2002,2004). As C-XANES spectroscopy gives no precise information onthe carbon chain length, no particular evolution of the C-XANESsignature of the aliphatic fraction can be observed with increasingmaturity (Fig. 4).

The C-XANES spectra of the aromatic fractions extracted fromthe investigated samples exhibit a wide and intense peak at285.3 eV, corresponding to the specific absorption of aromatic orolefinic carbon (1s / p* transitions). Wide absorption features at287.7e288 eV attributed to the 1s / 3p/s* transitions of CeH1e3are also present. Similarly to the aliphatic fraction, the C-XANESsignature of the aromatic fraction does not vary significantly withincreasing maturity (Fig. 4).

Although indicating higher concentrations of aromatic groups,the C-XANES spectra of polar NSO compounds (hydrocarbonheterocyclic compounds containing variable amounts of nitrogen,sulphur, and oxygen) appear very similar to the reference spectrumof fracture bitumen, probably indicating a major contribution ofpolar NSO compounds to the Posidonia Shale bitumen (Fig. 4). TheXANES signature of these compounds clearly evolves with

Figure. 4. C-XANES spectra of the extracted aliphatic, aromatic and polar (NSO) fractions of the investigated Posidonia Shale samples from the Wickensen, Harderode and Had-dessen wells and C-XANES spectra of asphaltenes and insoluble fractions (kerogens and pyrobitumen) from the same samples. The C-XANES spectrum of bitumen from a fracture(bottom) is shown as reference. Absorption features at 285.3, 286.3, 286.7, 287.7 (and 288), 288.6, 290.3 and 290.6 eV are attributed to electronic transitions of aromatic groups,unsaturated CeS bonds, ketonic or phenolic groups, aliphatic groups, carboxylic groups, carbonate groups and alkyl carbon, respectively. See text for more details.

Table 1Yields of products obtained from column fractionation of the extracted solubleproducts from the investigated Wickensen, Harderode and Haddessen samples.

Totalextraction(mg)/(gsample)

Asphaltene(mg)/(gextract)

Aliphatic(Maltenewt. %)

Aromatic(Maltenewt. %)

Polar NSO(Maltenewt. %)

Wickensen 9.95 235.6 17.0 16.9 66.1Harderode 7.65 169.3 21.1 23.7 55.2Haddessen 2.25 353.4 49.4 15.2 35.4

Figure. 5. Fitting procedure of the C-XANES spectra of the Wickensen, Harderode andHaddessen kerogens (top). After normalization to the absorption at 320 eV, theabsorption edge is modeled by an arctangent function which center position, ampli-tude and width are fixed to 291.5 eV, 1.0 and 0.4 eV, respectively. The C-XANES signal isthen deconvolved into Gaussian functions at fixed position and constant width (0.4 eVbelow 295 eV and 2 eV above). The table below reports the values of the fiveparameters extracted from the C-XANES spectra of Wickensen, Harderode and Had-dessen kerogens. SA values are defined as the area of the three Gaussian functionsaccounting for the aromatic contribution (peak at w 285 eV). The four other param-eters correspond to the ratios between the areas of specific Gaussian functions and SA.Absorption features at 285.3, 286.3, 286.7, 287.7 (and 288), 288.6, 290.3 and 290.6 eVare attributed to electronic transitions of aromatic groups, unsaturated CeS bonds,ketonic or phenolic groups, aliphatic groups, carboxylic groups, carbonate groups andalkyl carbon, respectively. See text for more details.

S. Bernard et al. / Marine and Petroleum Geology 31 (2012) 70e89 77

increasing maturity (Fig. 4). The intensity of the peak correspond-ing to the aromatic groups (1s / p* transitions at 285.3 eV)increases from Wickensen to Harderode and then decreases fromHarderode to the Haddessen, contemporaneously with theappearance of a peak at 288.6 eV indicative of the presence ofcarboxylic functional groups (1s / p*) (Cody et al., 1996).

All these spectra also exhibit a broad region of absorptionaround 292.5 eV which principally corresponds to the ionizationedge of carbon modulated by a superposition of broad 1s / s*transitions (Fig. 4).

3.2.2. Isolated insoluble fractions (kerogen and pyrobitumen)The C-XANES spectra of the insoluble fractions, likely corre-

sponding to the kerogens of the investigated Wickensen, Harder-ode and Haddessen samples, are reported on Figure 4. Thesespectra have been deconvolved following the procedure describedin Bernard et al. (2010c) (Fig. 5).

The spectrum of the insoluble fraction of the Wickensensamples (kerogen) displays two main absorption features at 285.3and 288.6 eV, interpreted as electronic transitions of aromatic orolefinic groups (C¼C) (1s / p*) and carboxylics groups (COOH)(1s / p*), respectively (Cody et al., 1996; Boyce et al., 2002). Theshoulder at 287.7 eV is indicative of aliphatic carbon (1s / 3p/s*)(CeH1e3). The absorption feature at 286.3 eV has been previouslyattributed to the presence of unsaturated bonds between carbonand heteroatoms such as oxygen (Braun et al., 2005, 2009), nitrogen(Shi et al., 2005) or sulphur (Liu et al., 2005). The more gentleabsorption feature observed at 290.3 eV likely results from theclose association with carbonates which specifically absorb at290.3 eV (Benzerara et al., 2004; Brandes et al., 2010).

Compared to the Wickensen kerogen, the Harderode kerogenappears to be more aromatic and to contain less aliphatic andoxygenated functional groups: the C-XANES spectrum of the Har-derode kerogen displays a broader and more intense absorptionpeak at 285.3 eV (aromatic or olefinic groups, C¼C, 1s / p*) andless important absorption features at 287.7 eV and 288.6 eV(aliphatics, 1s / 3p/s*, and carboxylics groups, COOH, 1s / p*,respectively) compared to the C-XANES spectrum of theWickensenkerogen. Similarly to above, the more gentle absorption featureobserved at 290.3 eV in the C-XANES spectrum of the Harderodekerogen likely results from the close association with carbonates.

The same differences can be observed when comparing theC-XANES spectra of the Harderode and Haddessen kerogens, thelatter clearly appearing more aromatic and containing less aliphaticand oxygenated functional groups than the former. The importantsurface of the aromatic peak (285.3 eV) relatively to the absorptionfeature at 288.6 eV ostensibly related to the presence of carboxylicgroups is consistent with an overmature organic material (e.g.,Cody et al., 1996, 1998). Similarly to the Wickensen and Harderodekerogens, a more gentle absorption feature likely resulting from theclose association with carbonates can be observed at 290.3 eV.

Surprisingly, some isolated organic particles present within theinsoluble fraction of the investigated overmature Haddessensamples do not display C-XANES spectra of overmature kerogen

particles. These spectra do not exhibit any absorption feature at287.7 eV (aliphatics, 1s / 3p/s*) (CeH1e3) but a clear absorptionpeak at 286.7 eV likely corresponding to the presence of ketonic orphenolic (C¼O) functional groups (1s / 2p*) (Cody et al., 1996;Solomon et al., 2009). An additional absorption feature can beobserved at 290.6 eV in this C-XANES spectrum which mightcorrespond either to additional 1s / p* transitions of carboxylicsor peroxide groups (Cody et al., 1996), or, more likely, to 1s / 4ptransitions of alkyl carbon (Cody et al., 1998). This is surprising asfar as such highly aromatized residual solid is not expected tocontain that much oxygen, the most labile oxygen functions fromthe original kerogen having been removed during maturation.Based on their optical characteristics and their C-XANES signature,

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these oxygen-rich compounds found within the insoluble fractionof the investigated overmature Haddessen samples have beenidentified as pyrobitumen. This is discussed in more detail below.

3.2.3. Deconvolution procedure of kerogen C-XANES spectraTo obtain a more quantitative insight on the evolution of

kerogen C-XANES spectra with increasing temperature, thecollected spectra have been normalized to the absorption at 320 eV,which is proportional to the total amount of carbon, and decon-volved using an arctangent function to model the absorption edgeitself and Gaussian functions following the procedure described inBernard et al. (2010c) based on the recommendations of Carravettaet al. (1998) and Braun et al. (2006) (Fig. 5). The center position, theamplitude and the width of this arctangent function are fixed to291.5 eV, 1.0 and 0.4 eV, respectively. Each Gaussian function hasa fixed energy position and a constant width (0.4 eV below 295 eVand 2 eV above). Although the natural lineshape of a XANES peak isa Lorentzian function, we used Gaussian functions to take inaccount the contribution to the linewidth produced by instru-mental broadening effects (Braun et al., 2006).

It is emphazised here that most of the Gaussian functions usedfor such deconvolutions do not account for the presence of func-tional groups but for the overlapping contribution of wide andstructured continuum steps resulting from multiple diffusionprocesses (Bernard et al., 2010c). The broad peak at 285 eV wasdeconvolved into three Gaussian functions, accounting for reso-nances from different multiple carbonecarbon bond species aspreviously done by Braun et al. (2006). The parameter SA is definedas the contribution of the areas of Gaussian functions accountingfor the aromatic contribution (peak at w 285 eV) and can thus beseen as a parameter directly related to the aromaticity. Four otherparameters have been estimated (Fig. 5): (1) the ratio Aliphatics/Aromatics (RAliph) corresponding to the ratio between the area ofthe Gaussian function at 287.7 eV (specific absorption energy of thealiphatic groups) and SA, (2) the ratio (Phenols þ Ketones)/Aromatics corresponding to the ratio between the area of theGaussian functions at 286.7 and 287.2 eV (specific absorptionenergy of phenolic and ketonic groups) and SA, (3) the ratioCarboxylics/Aromatics corresponding to the ratio between the areaof the Gaussian function at 288.6 eV (specific absorption energy ofcarboxylic groups) and SA, and (4) the ratio Oxygen/Aromatics(ROxygen) corresponding to the sum of the two preceding ratios.

It should be kept in mind that these ratios do not correspond toabsolute concentration. They are only used here to qualitativelyfollow the evolution of the C-XANES signature of the PosidoniaShale kerogen with increasing maturity. The values estimated fortheWickensen, Harderode and Haddessen kerogens are reported inFigure 5: all the parameters decrease from Wickensen to Haddes-sen, with the exception of the SA values.

3.3. Nanoscale observations performed on FIB sections

Ultrathin FIB foils have been extracted from polished sections ofWickensen, Harderode and Haddessen samples for nanoscalecharacterization using STXM and TEM. Energy-filtered STXMimaging, coupled with C-XANES spectroscopy, have allowedlocating and identifying the various organic compounds presentwithin the FIB sections, while STEM imaging together with EDXSmapping have allowed locating and identifying the associatedinorganic phases at the nanoscale.

3.3.1. Wickensen samplesThe TEM and STXM observations performed on the FIB foils

extracted from the investigated Wickensen samples reveal thestrongly heterogeneous nature of these immature shales at the

nanometer scale (Fig. 6). Numerous inorganic phases have beenidentified within these samples: biogenic calcium carbonates(CaCO3) and phosphates together with detrital quartz (SiO2), Al-richsilicates and K-rich clays. Some nanometric pyrites (FeS2) and Tioxides, sparsely disseminated within the matrix, have also beenobserved. The total porosity of this heterogeneous mineral matrixappears to be filled with organic material, within which no signifi-cant nanoporosity can be observed (no intra-particle pores of1e50 nm can be seen on STEM images). Spectroscopically, thisorganic matter seems more or less homogeneous and consistentwith an immature kerogen. The collected C-XANES spectra displaytwo main absorption features at 285.3 eV and 288.6 eV, interpretedas electronic transitions of aromatic or olefinic groups (C¼C)(1s / p*) and carboxylics groups (COOH) (1s / p*), respectively(Cody et al., 1996; Boyce et al., 2002). Gentle absorption features canbe observed at 290.3 eV, likely resulting from the close associationwith carbonates (Benzerara et al., 2004; Brandes et al., 2010).Interestingly, a kind absorption feature at 286.3 eV can only beobserved on some C-XANES spectra. The organic compounds dis-playing this peak appear to be enriched in sulphur as indicating bythe EDXSmaps (Fig. 6). This peak can thus undoubtedly be attributedto the presence of unsaturated bonds between carbon and sulphur aspreviously proposed by Liu et al. (2005) and Lepot et al. (2009).Additionally, a shift of the aromatic peak from 285.3 to 285.1 eV anda decrease of the 287.7 shoulder intensity occur when a significantabsorption is measured at 286.3 eV. Such modificationmight also berelated to the presence of thiophenic groups (Lepot et al., 2009).Consistently with previous results (Horsfield, 1997), Wickensenorganic matter appears to mainly consist of immature kerogencontaining variable amount of organic sulphur moieties. Suchsulphur incorporation into organic matter is believed to rapidlyoccur during bacterial sulphate reduction at the early stages ofsediment burial (Sinninghe Damsté; and de Leeuw, 1990; Hebtinget al., 2006; Vandenbroucke and Largeau, 2007; Bottrell et al.,2009; Lepot et al., 2009), on time-scales that can be of less thana year (Bartlett et al., 2009). Reactive iron is usually considered tooutcompete organic matter for sulphur incorporation, due to fasterformation of pyrite (Berner, 1984). Accordingly, the important sul-phurization ofWickensen kerogen likely indicates that the Posidoniasediments only contained a low concentration of reactive iron.

3.3.2. Harderode samplesSimilarly to the Wickensen samples, the STEM images and the

elemental EDXS maps of the FIB foils extracted from the Harderodesamples clearly evidence their strong mineralogically heteroge-neous nature at the nanoscale (Fig. 7). The mineral matrix of theHarderode samples consists of calcium carbonates, detrital andautomorphous quartz crystals, isolated euhedral and framboidalpyrite, Al-rich silicates, K-rich clays, as well as nanometric Ti oxidessparsely disseminated within the matrix. In addition, disseminatedapatite and albite crystals can be observed within the Harderodesample matrix. These albite crystals appear to contain very little Kand Ca and display a very restricted compositional and structuralvariability, their composition being consistent with a 99 mol%stoichiometric end-member albite component (Fig. 8). Althoughrarely abundant volumetrically, these albite crystals arewidespreadwithin the investigated samples and no K- or Ca-feldspars havebeen observed. Noteworthy, these albite crystals display nano-metric halite inclusions (Fig. 8).

The porosity of the mineral matrix is almost totally filled withorganic matter (Fig. 7). Kerogen particles found within the inves-tigated Harderode samples (Fig. 7 - Spectra 2 and 3) appear morearomatic than the Wickensen kerogen particles (the peak at285.3 eV is broader and more intense), contain less aliphatic (theabsorption features at 287.7 and 288 eV are less important) and

Figure. 6. TEM and STXM characterization of two FIB sections extracted from Wickensen samples. STEM images (HAADF mode) of the FIB foils (top and bottom left) in whichorganic matter appears in dark while silicates and carbonates appear in bright. The red continuous square indicates the location of the elemental EDXS maps of the FIB foils shownon the right (carbon, oxygen, sulphur, phosphorus, potassium, sodium, silicium, aluminium, calcium, iron, magnesium and titanium respectively), while the red dashed squareindicates the location of the higher magnification STEM images (HAADF mode) shown on the left side of the figure. C-XANES spectra (1e4) of the different organic compoundsencountered within these FIB foils are show on the right side of the figure. All these spectra are consistent with immature kerogen particles. C-XANES spectra of calcite and K-richparticles are shown as references. Absorption features at 285.3, 286.3,0.7, 288.6 and 290.3 eV are attributed to electronic transitions of aromatic groups, unsaturated CeS bonds,carboxylic groups and carbonate groups, respectively. See text for more details. (For interpretation of the references to color in this figure legend, the reader is referred to the webversion of this article.)

S. Bernard et al. / Marine and Petroleum Geology 31 (2012) 70e89 79

Figure. 7. TEM and STXM characterization of two FIB sections extracted from Harderode samples. STEM images (HAADF mode) of the FIB foils (top and bottom left) in which organicmatter appears in dark while silicates and carbonates appear in bright. The red continuous square indicates the location of the elemental EDXS maps of the FIB foils shown on theright (carbon, oxygen, sulphur, phosphorus, potassium, sodium, silicium, aluminium, calcium, iron, magnesium and titanium respectively), while the red dashed square indicatesthe location of the higher magnification STEM images (HAADF mode) shown on the left side of the figure (the red dashed square indicates the location of the STEM image shownFig. 8). C-XANES spectra of the different organic compounds encountered within these FIB foils are show on the right side of the Figures 1e5. C-XANES spectra 2, 3 and 4 areconsistent with mature kerogens while the organic compounds displaying the spectra 1 and 5 have been identified as asphaltene-rich bitumen, characterized by a variable amountof aromatic and oxygen-containing functional groups. C-XANES spectra of bitumen from a fracture, calcite and K-rich particles are shown as references. Absorption features at 285.3,286.7, 287.7, 288.6, 289, 289.5 and 290.3 eV are attributed to electronic transitions of aromatic groups, ketonic or phenolic groups, aliphatic groups, carboxylic groups, aldehydegroups or aromatic carbon, alcoholic groups and carbonate groups, respectively. See text for more details. (For interpretation of the references to color in this figure legend, thereader is referred to the web version of this article.)

S. Bernard et al. / Marine and Petroleum Geology 31 (2012) 70e8980

Figure. 8. High magnification STEM image (HAADF mode) of the top left corner of the FIB foil extracted from a Posidonia Shale sample from the Harderode well shown on top ofFigure 7 and EDXS analyses of the Albite crystal and its nanoscale NaCl inclusions.

S. Bernard et al. / Marine and Petroleum Geology 31 (2012) 70e89 81

oxygenated functional groups (the peak at 288.6 eV is less intense)and seem to contain only trace amounts of sulphur (no peak at286.3 eV can be observed). These particles are closely associatedwith carbonates as shown by the absorption peak at 290.3 eV.

In addition to these kerogen particles, some different organicmacromolecules in close association with the mineral matrix havebeen documented by STXM within the Harderode samples, fillingsubmicrometric fractures (Fig. 7). These compounds are charac-terized by different C-XANES spectra (Fig. 7 - Spectra 1, 4 and 5),showing a variable amount of aromatic, aliphatic and oxygen-containing functional groups.

Similarly to the C-XANES spectra of kerogen (Fig. 7 - Spectra 2and 3), the C-XANES spectrum 4 displays a broad and intenseabsorption peak at 285.3 eV, indicative of a similar concentration ofaromatic or olefinic groups (C¼C), as well as a distinct absorptionfeature at 287.7 eV, indicative of the presence of aliphatic groups(CeH1e3). In addition to these features, this spectrum also exhibitsan absorption peak at 289 eV, previously ascribed to either 1s/ p*transitions of aldehyde groups or 1s / 2p* transitions of aromaticcarbon bonded to either carbon or hydrogen (CAR-C or CAR-H)(Francis and Hitchcock, 1992). The spectrum 4 thus also appearsconsistent with a mature kerogen enriched in oxygen-containingfunctional groups.

Although intensities are different, the C-XANES spectra 1 and 5exhibit absorption peaks at the same energies (285.3, 286.7 and288.6 eV), as well as a gentle shoulder at 287.7 eV (Fig. 7 - Spectra 1and 5). The main peaks at 285.3, 286.7 and 288.6 eV are attributedto 1s / p* transitions of aromatic or olefinic groups (C¼C),1s / 2p* transitions of ketonic or phenolic functional groups(C¼O) and 1s / p* transitions of carboxylics groups (COOH),respectively, while the shoulder at 287.7 eV can be ascribed to1s / 3p/s* transitions of aliphatic carbon (CeH1e3). These twotypes of aliphatic and oxygen-rich organic compounds are boththought likely to correspond to NSO- and asphaltene-rich bitumen.

The Harderode samples thus clearly appear geochemicallyheterogeneous at the submicrometric scale. Various organicmacromolecules are coexisting within very small areas, none ofthem displaying significant intra-particle porosity at the nanoscale(no intra-particle pores of 1e50 nm can be seen on STEM images).

3.3.3. Haddessen samplesAgain, the nanoscale observations performed using TEM and

STXM on FIB foils extracted from Haddessen samples clearlyevidence the strong heterogeneous nature of these samples(Fig. 9). Based on STEM images and EDXS maps, it can be statedthat the mineralogy of the Haddessen samples is similar to the

mineralogy of the Harderode samples: calcium carbonates,detrital and automorphous quartz crystals, isolated euhedral andframboidal pyrites, Al-rich silicates, K-rich clays, as well asnanometric Ti oxides and disseminated apatite and pure albitecrystals containing nanometric halite inclusions. Similarly to theHarderode samples, the porosity of the mineral matrix is almosttotally filled with organic matter (Fig. 9), although some free porespaces can be observed.

From an organic geochemical point of view, the Haddessensamples are also very heterogeneous at the nanometer scale.Organic compounds displaying different C-XANES signatures havebeen found within the investigated FIB foils (Fig. 9). Kerogenparticles (Fig. 9 - Spectra 2 and 3) are more aromatic than theHarderode kerogen particles (the peak at 285.3 eV is broader andmore intense) and contain even less aliphatic and oxygenatedfunctional groups (the absorption features at 287.7, 288 and288.6 eV are less intense). It can be noted that these kerogenparticles do not show any visible porosity at the nanoscale (nointra-particle pores of 1e50 nm can be seen on STEM images) norcontain any sulphur. The Haddessen kerogen appears closelyassociated with carbonates similarly to the Harderode or Wick-ensen kerogens (a gentle absorption peak at 290.3 eV can be seenon the C-XANES spectra).

Similar to the Harderode samples, additional organic macro-molecules have been documented by STXM within the Haddessensamples, filling submicrometric fractures (Fig. 9). The C-XANESspectra of these compounds (Fig. 9 - Spectra 1, 4, 5 and 6) are quitedifferent from the Haddessen kerogen spectra and also differ fromthe C-XANES spectra of the organic molecules found within theHarderode samples. First, the presence of aromatic-rich organiccompounds, enriched in oxygen-containing functional groups(Fig. 9 - Spectrum 1), has been evidenced in some FIB foils extractedfromHaddessen samples such as the one shown in Figure 10. Theseorganic compounds exhibiting a network of nanopores of about1e50 nm, which are clearly visible on the STEM image shown inFigure 10, have been identified as pyrobitumens (cf discussion).

In addition to these pyrobitumens, three different types ofaliphatic-rich organic macromolecules have been identified usingXANES spectroscopy at the C K-edge (Fig. 9 - Spectra 4, 5 and 6). TheC-XANES spectrum of the first type of aliphatic-rich organiccompounds (Fig. 9 - Spectrum 4) displays small absorption peaks at285.1 and 285.3 eV, corresponding to electronic transitions ofaromatic or olefinic carbon (C¼C) (1s / p*), as well as clearlyindividualized peaks at 286.6, 288 and 289.5 eV (Fig. 8). These peaksrespectively indicate the presence of phenolic groups (AreOH)(1s / p*), aliphatic groups CeH1e3 (1s / 3p/s*) and alcoholic or

Figure. 9. TEM and STXM characterization of two FIB sections extracted from Haddessen samples. STEM images (HAADF mode) of the FIB foils (top and bottom left) in whichorganic matter appears in dark while silicates and carbonates appear in bright. The red continuous square indicates the location of the elemental EDXS maps of the FIB foils shownon the right (carbon, oxygen, sulphur, phosphorus, potassium, sodium, silicium, aluminium, calcium, iron, magnesium and titanium respectively), while the red dashed squareindicates the location of the higher magnification STEM images (HAADF mode) shown on the left side of the figure. C-XANES spectra of the different organic compoundsencountered within these FIB foils are show on the right side of the figure. Spectra 2 and 3 are consistent with overmature kerogen particles while spectra 4, 5 and 6 correspond toasphaltene- and NSO-rich bitumens. The spectrum 1 corresponds to the nanoporous pyrobitumen of the FIB section shown in Figure 10. C-XANES spectra of bitumen from a fracture,calcite and K-rich particles are shown as references. Absorption features at 285.1 (and 285.3), 286.7, 287.7 (and 288), 288.6, 289, 289.5, 290.3 and 290.6 eV are attributed toelectronic transitions of aromatic groups, ketonic or phenolic groups, aliphatic groups, carboxylic groups, aldehyde groups or aromatic carbon, alcoholic groups, carbonategroups and alkyl carbon, respectively. See text for more details. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of thisarticle.)

S. Bernard et al. / Marine and Petroleum Geology 31 (2012) 70e8982

Figure. 10. Left: High magnification STEM image (HAADF mode) of a FIB foil extracted from an overmature Haddessen sample. Organic matter appears in dark while silicates andcarbonates appear in bright. This organic matter displays intra-particle nanoporosity. The C-XANES spectrum collected on this nanoporous pyrobitumen is reported in Figure 9(Spectrum 1). Right: Elemental EDXS maps of the FIB foil (carbon, oxygen, sulphur, calcium, silicium, aluminium, sodium, chlorin and potassium respectively). See text for moredetails.

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ether groups (CeOH or CeOeC) (1s / 3p/s*) (Cody et al., 1996,1998; Boyce et al., 2002). These organic molecules are also closelyassociated with carbonates and K-bearing phases as evidenced bythe presence of gentle absorption features at 290.3, 297.3 and300 eV (de Groot et al., 1990; Benzerara et al., 2004; Brandes et al.,2010). The C-XANES spectra of the two other types of aliphatic-rich organic compounds are different (Fig. 9 - Spectra 5 and 6):the aromatic peak at 285.3 eV is not divided into twopeaks, and onlytwo absorption features can be observed at 287.7 and 288.6,traducing the presence of aliphatic and carboxylic functional groups(Cody et al., 1996). These two compounds, displaying C-XANESspectra very similar to the C-XANES spectrum of reference fracture-filling bitumen, can thus be identified as slightly oxidized bitumen.

4. Discussion

The lower and upper boundaries of the Posidonia Shale aregeochemically and petrographically abrupt (e.g., Rullkötter et al.,1988; Littke et al., 1991a). Within a given well, the PosidoniaShale displays considerable consistency in geochemical andmineralogical characteristics, suggesting uniform depositionalsettings as well as similar processes of organic matter produc-tion and incorporation within the sediments (e.g., Rullkötteret al., 1988; Littke et al., 1991a). Therefore, the geochemicaland mineralogical differences observed between the samplesfrom the three investigated wells likely largely result, althoughnot exclusively, from the maturation level locally reached bythis formation, i.e., from the kerogen thermal cracking processesassociated with sediment burial and geothermal heating. In thissection, we thus discuss the observations reported here in thelight of the progressive increase of thermal maturation.

4.1. Implications of the mineral assemblage for the thermal historyof the Posidonia Shale

From theWickensen to the Haddessen well, the Posidonia Shaledisplays progressive levels of increasing thermal maturation. Suchtrend in maturity distribution more or less coincides with thedistance from gravity highs and magnetic anomalies (e.g.,Rullkötter et al., 1988; Littke et al., 1991a; Vandenbroucke et al.,1993). The high maturity of this area has thus been interpreted as

resulting from accelerated coalification caused by lateral heattransfer from a large intrusive body of presumed Turonian age, the“Vlotho Massif”. This intrusive body is believed to have beenemplaced at the beginning of the inversion of the Lower SaxonyBasin and to be at a present depth of about 5e6 km (e.g., Rullkötteret al., 1988; Jochum et al., 1995a, 1995b).

However, based on numerical simulations, Petmecky et al.(1999) have proposed that a stable high temperature regimesince the Cretaceous, coupled with ascending brine fluids, might bethe ultimate cause for the maturation anomalies in major parts ofthe southern Lower Saxony Basin, rather than a relatively shallow(6 km) magmatic intrusion. Evidence for this high temperatureregime are, according to Petmecky et al. (1999) and referencestherein, the elevated maximum present-day heat flows, hydro-thermal mineralization and the hot springs at several locationsalong the southern rim of the Lower Saxony Basin. This long-lastingregime of elevated temperatures would have been controlled bythe tectonic situation of the area (Petmecky et al., 1999).

Here, we were able to document the occurrence of sparselydisseminated albite crystals within the investigated Harderode andHaddessen samples (Figs. 7 and 9). These crystals contain very littleK and Ca and display a very restricted compositional and structuralvariability, their composition being consistent with a 99 mol%stoichiometric end-member Ab component. Although rarelyabundant volumetrically, these albite crystals are widespreadwithin the investigated Harderode and Haddessen samples and noK- or Ca-feldspars have been observed. Based on these character-istics, following Kastner and Siever (1979), Spötl et al. (1999), Raiset al. (2008) and Bernard et al. (2010a), the albite crystalsdescribed here might be identified as authigenic feldspars. Notstrictly synonymous with diagenesis, authigenesis is commonlyused in the literaturewhen referring tominerals that formed in-situduring either high-grade diagenesis or low-grade metamorphism(Kastner and Siever, 1979; Spötl et al., 1999).

Interestingly, these authigenic albite crystals contain nanometrichalite inclusions (Fig. 8), which may result from the participation ofevaporite-derived brines in the crystallization of albite. Spötl et al.(1999) have previously identified authigenic albite as a potentiallydiagnostic tracer of palaeobrine-carbonate-(shale) interactions.NaCl-type brines have been shown to thermodynamically favoralbite over K-feldspar (Aagaard et al., 1990; Bazin et al., 1997). Such

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saline fluidmay have constituted the source of Na and Si needed foralbite crystallization while Al may have come from the alterationand/or breakdown of clay minerals in the matrix. Once initiated,such feldspar crystallizationmight havebeenmaintainedby thepH-buffering capacity of carbonates as long as the ionic supply lasted(e.g., Spötl et al., 1999 and references therein).

As the investigated shale formation displays very low porosity(the investigated samples are texturally extremely fine-grainedwith porosities well below 10% - Mann, 1987; Mann and Müller,1988; Littke et al., 1988), fluid access has most likely been limited.Therefore, the crystallization of authigenic albite from brineecar-bonate interactions likely needed higher temperatures than thoseprevailing in shallowly buried settings. Previous studies have sug-gested that albite authigenesis is retarded in low porositycarbonate-rich shales relatively to feldspar-producing reactions insandstones which usually display higher porosity (Crampon, 1973;Spötl et al., 1999). The scarcity and nanometric size of haliteinclusions evidenced within the authigenic albite described hereprecludes a precise determination of their temperature of crystal-lization. Nevertheless, crystallization of authigenic albite incarbonate-rich shales has only been reported in deep burialsettings (e.g., Crampon, 1973; Spötl et al., 1999) at temperaturesranging from high-grade diagenesis (150e200 �C) to incipientmetamorphism (200e250 �C).

Based on the study of primary fluid inclusions and numericalsimulations, Jochum et al. (1995a) have previously estimated thatthe Harderode samples have experienced a temperature of about150 �C. Consistently, a maximum temperature of about 170e180 �Chas been qualitatively estimated based on the characteristics of theRaman spectrum of Haddessen organic matter (Bernard et al.,2010a), in good agreement with the temperature of 170 �C thatwould be anticipated based on kinetic analysis of secondary gasgeneration in the Posidonia Shale (Dieckmann et al., 1998). Thematuration of the Lower Toarcian Posidonia Shale would thus notsolely result from high heat flows related tomagmatic intrusions ordeep burial, but also originate from interactions with hydrothermalfluids, either associated with late stages of cooling of an intrusivebody or in a deep burial setting.

4.2. Chemical and structural evolution of Posidonia Shale kerogenwith increasing maturity

Although kerogen, as the main precursor of petroleum, haslong drawn attention from organic geochemists, the chemical andstructural evolution of its component macerals as a function ofmaturity in natural series is still poorly documented. Here, theevolution of the C-XANES spectra of the immature (Wickensen),mature (Harderode) and overmature (Haddessen) kerogensclearly documents the chemical evolution of macromolecularstructures as a response to the progressive increase of thermalmaturation (Figs. 4 and 5). It is worth noting that such changesare accompanied by net losses of carbon from Wickensen toHarderode, as shown by the drastic decrease in TOC content, butnot from Harderode to Haddessen (Fig. 1) (e.g., Rullkötter et al.,1988). Based on the evolution of the parameters reported inFigure 5, the principal changes in the molecular structure of thisPosidonia Shale kerogen with increasing maturity can besummarized as follows:

1. The relative abundance of aromatic structures within kerogenparticles, i.e., the aromaticity of the kerogen, increases. Thecontinuous increase of SA values, from 1.25 for Wickensensamples to 1.90 for Haddessen samples, is related to theincreasing intensity of the peak at 285.3 eV, attributed to1s / p* electronic transitions in aromatic or olefinic groups.

C-XANES spectroscopy has been recently proved valuable toprovide a reliable proxy for quantifying the in-plane extent ofthe aromatic sheets of graphitic compounds in metamorphicrocks based on the intensity of the sharp 1s / s* exciton at291.7 eV (Bernard et al., 2010c). However, such a sharpabsorption feature, related to the presence of extensive planardomains of highly conjugated aromatic layers (Ahuja et al.,1996; Brandes et al., 2008; Bernard et al., 2010c), cannot beseen in the C-XANES spectra of the Posidonia Shale kerogen.The absence of this exciton suggests that, despite an aroma-ticity increase, the aromatic cluster size of the Posidonia Shalekerogen does not significantly increase within the studiedrange of thermal maturation.

2. The relative concentration of aliphatic carbon in the macro-molecular kerogen structure drastically decreases, as clearlyillustrated by the evolution of RAliph values from 1 for theWickensen kerogen to 0.5 for the Haddessen kerogen (Fig. 5).Such a decrease, consistent with the evolution of the HydrogenIndex as shown in the HI vs. OI diagram (Fig. 1), supports thequantitative infrared spectroscopic results of Schenk et al.(1986). Together with the absence of 1s / s* exciton at291.7 eV, such evolution of RAliph values indicates that thearomaticity increase with thermal maturation likely resultsfrom the reduction of the alkyl chains attached to the aromaticcores rather than from a pronounced growth of the size of thefused aromatic ring clusters. Thus, the thermal maturation ofPosidonia kerogen appears to be primarily a process of alkyldegradation, presumably by b-cleavage, concomitant with anincrease of aromaticity.

3. Sulphur and oxygen-containing functional groups areprogressively lost during thermal maturation. ROxygen valuesdecrease from 1.25 for the Wickensen kerogen to 0.7 for theHaddessen kerogen (Fig. 5), indicating the progressive loss ofketonic, phenolic and carboxylic groups. Both Harderode andHaddessen kerogens do not contain any significant amount oforganic sulphur, in contrast to the Wickensen kerogen(Horsfield, 1997). Such loss of organic sulphur during thethermal decomposition of kerogen has been known since the1970s (e.g., Orr, 1974, 1986). More recently, kerogen thermaldegradation has been suggested to release sulphur radicals,which activity has been recognized to play an integral role inregulating the production of hydrocarbons by initiating thebreaking of CeC bonds within kerogen (Lewan, 1998). Therelease of sulphur radicals during the Posidonia Shale kerogendegradationmay thus have favored the diagenetic precipitationof pyrite, which may in turn have enhanced hydrocarbongeneration processes (Bakr et al., 1991) as well as hydrothermaloxidation of hydrocarbons during bitumen maturation(Seewald, 2001a, 2001b).

In summary, the XANES signatures of the Posidonia Shalekerogens from Wickensen to Haddessen clearly document thestructural changes taking place with increasing maturity: animmature kerogen with a significant aliphatic component,a considerable concentration of oxygen and sulphur-containingfunctional groups and with only a few isolated aromatic struc-tures, evolves into an overmature kerogen containing feweraliphatic chains and dominated by poorly condensed aromaticstructures. These conclusions conform to those previously reportedfor naturally buried as well as laboratory-heated sediments basedon micro-scale sealed vessel (MSSV) pyrolysis, gas chromatographymass spectrometry (GCeMS), Fourier transform infrared (FTIR)spectroscopy, solid-state nuclear magnetic resonance (NMR) andX-ray photoelectron spectroscopy (XPS) investigations (Schenket al., 1986; Behar and Vandenbroucke, 1987; Requejo et al., 1992;

S. Bernard et al. / Marine and Petroleum Geology 31 (2012) 70e89 85

Patience et al., 1992;Wei et al., 2005; Lis et al., 2005, 2008; Smerniket al., 2006; Kelemen et al., 2007; Mao et al., 2010; Horsfield andDueppenbecker, 1991; Vandenbroucke et al., 1993; Kruge et al.,1997; Putschew et al., 1998).

The present study thus demonstrates that STXM-basedC-XANES spectroscopy provides useful geochemical informationon kerogen maturation in good agreement with more conventionalgeochemical techniques. In addition, as discussed below, byproviding spatially resolved information on organic constituentspeciation at the 20 nm scale, STXM may definitely help to betterconstrain hydrocarbon generation and retention processes occur-ring with complex systems such as gas shales.

4.3. Hydrocarbon generation as a response to the thermalmaturation of the Posidonia Shale

In this section, we first discuss the chemical evolution of theextracted soluble fractions and isolated insoluble fractions of theinvestigated samples with increasing maturity. Then, we interpretthe spatially-resolveddataobtainedonFIB sectionsbasedonthisbulkcompositional information. Inparticular,we focus on the significanceof the appearance of intra-pyrobitumen nanoporosity (intra-particlepores of 1e50 nm) within the investigated overmature samples.

4.3.1. Chemical evolution of NSO compounds and asphaltenes informing pyrobitumen

The geochemical significance and the particular role of polarNSO compounds and asphaltenes in hydrocarbon generationprocesses remain constrained. Using C-XANES spectroscopy, weshow here that the aromatic content of the extracted polar NSOfraction increases from Wickensen to Harderode and decreasesfrom Harderode to Haddessen (Fig. 4). Additionally, the presence ofoxygen-containing functional groups has been evidenced withinthe polar NSO compounds extracted from the investigated Had-dessen samples (Fig. 4). Similarly, the chemical signature of theextractable asphaltene fraction drastically evolves fromWickensento Haddessen (Fig. 4): while the Wickensen asphaltenes mostlyconsist of aromatic and aliphatic functional groups, Harderode andHaddessen asphaltenes are characterized by an important contri-bution of oxygen absorbances.

Based on confined and hydrous pyrolysis experiments, changesin asphaltene geochemistry have been attributed to a combinationof processes leading to their generation with processes responsiblefor their thermal breakdown into hydrocarbons (Pelet et al., 1986;Muscio et al., 1991; Michels et al., 1996). Similarly, recent artificialmaturation experiments have shown that kerogenmay decomposeinto heavy NSO compounds that rapidly undergo secondarycracking to generate hydrocarbons and new NSO fractions (Lewan,1985,1997; Behar et al., 2008a, 2008b, 2010). Asphaltenes and polarNSO compounds appear to not constitute simple intermediatesbetween kerogen and hydrocarbons during oil and gas generation:they actively participate in the chemical reactions involved inmaturation via second order reactions (e.g., Michels et al., 1996;Tiem et al., 2008). The chemical evolution of the extracted polarNSO fraction revealed by C-XANES spectroscopy in the presentstudy thus confirms progressive chemical evolution of compoundstypes differing in polarity.

Pyrobitumen compounds have been proved within the over-mature Haddessen samples. In natural systems, the aromaticcondensation and precipitation of pyrobitumen is seen asa required consequence of bitumen and oil cracking in reservoirs(Hill et al., 1996; Stasiuk, 1997; Mastalerz and Glikson, 2000). Thehigh oxygen content of the Haddessen pyrobitumen may resultfrom the incorporation of oxygen atoms within the high molecularweight aromatic units of the pyrobitumen where they form

thermally stable bonds (Michels et al., 1996). While this oxygenmay derive from organic oxygen, i.e. directly from asphaltenes andNSO compounds, molecular or mineral oxygen may constitutealternative sources. The origin of the oxygen richness of the polarNSO compounds and asphaltenes extracted from the investigatedHarderode and Haddessen samples is however not that clear. Inaccordance with the present results, Wilkes et al. (1998a, 1998b)have previously shown that ketone- and aldehyde-richcompounds quantitatively constitute an important fraction of thepolar NSO compounds of the Posidonia Shale bitumen and that theconcentrations of these oxygenated compounds strongly increasewith increasing maturity. Such oxygen enrichment is difficult toexplain by late oxidation events barring the assumption of a struc-ture highly prone to oxidation: the investigated Lower ToarcianPosidonia Shale were deposited under anoxic conditions and noextensive post-depositional oxidation event has ever been reported(Littke et al., 1991a, 1991b).

Alternatively, such oxygen enrichment may derive from theanoxic hydrothermal alteration of sedimentary organic matter, assuggested by the presence of authigenic albite within the investi-gated samples (cf discussion above). Previous surveys of the polarNSO fractions extracted from hydrothermally-altered sedimentarysequences has revealed the presence of oxygenated compoundscontaining phenols and ketones correlated with samples exhibitinga high degree of thermal maturity (Leif and Simoneit, 1995;Simoneit et al., 1996). Additionally, the presence of carboxylicacids in most oil-field brines at high concentrations has providedcompelling evidence that hydrocarbon oxidation in subsurfaceenvironments may represent a generally pervasive phenomenonthat accompanies the hydrothermal maturation of bitumen(Seewald, 2001a, 2001b). Furthermore, thermodynamic evalua-tions have suggested that the abundances of oxygenatedcompounds may be regulated by a state of redox-dependentmetastable thermodynamic equilibrium that involves hydro-thermal brines, carbon dioxide and carbonate minerals (Helgesonet al., 1993). The formation of oxygenated compounds within theinvestigated Posidonia samples may thus partly result fromhydrothermal processes, hydrothermal brines and iron-bearingminerals having provided available sources of hydrogen andoxygen (Seewald, 2001a, 2001b).

4.3.2. Spatially resolved insights into gaseous hydrocarbongeneration within gas shales

Considerable attention has been paid to the thermal stability ofcrude oils under geological conditions and the contribution of gasderived from oil and bitumen cracking to gas reservoirs (Mastalerzand Glikson, 2000; Waples, 2000; Hill et al., 2003; Tian et al.,2008; Tiem et al., 2008). The methodology used in this study,coupling TEM and synchrotron-based STXM observations, haselucidated the strong heterogeneous nature of the investigatedimmature (Wickensen), mature (Harderode) and overmature(Haddessen) samples at the nanometer scale (Figs. 6, 7 and 9). Allthe porosity of the investigated Wickensen samples appears to befilled with immature kerogen. In contrast, various organic macro-molecules are coexisting with kerogen particles within very smallareas of the investigated Harderode and Haddessen samples (Figs. 7and 9). Their systematic depletion in aromatics and enrichment inaliphatics compared to the associated kerogen (Figs. 7 and 9) haveallowed the identification of these compounds as different genera-tions of thermally generated asphaltene- and NSO-rich bitumens.

Within the investigated Harderode samples, the organicmacromolecules diplaying the C-XANES spectra 2, 3 and 4 (Fig. 7)are consistent with mature kerogens while the organic compoundsdisplaying the spectra 1 and 5 (Fig. 7) have been identified asasphaltene-rich bitumen, characterized by a variable amount of

S. Bernard et al. / Marine and Petroleum Geology 31 (2012) 70e8986

aromatic and oxygen-containing functional groups. Similarly to allthe gas shales worldwide, the investigated Harderode samplesexhibit a very low porosity (e.g., Curtis, 2002). Therefore, thethermally generated viscous bitumens might not have been able tomigrate over great distance and have filled the porosity madeavailable by kerogen degradation (Vandenbroucke and Largeau,2007; Loucks et al., 2009). The formation of gas might have thenbeen promoted by thermal cracking of first generations of bitumen,intimately connected to the thermally matured kerogen particles.Consistently, in addition to the overmature kerogen particles (Fig. 9- Spectra 2 and 3) and to the asphaltene- and NSO-rich bitumens(Fig. 9 - Spectra 4, 5 and 6), some pyrobitumen residues have beenevidenced within the investigated Haddessen samples (Fig. 9 -Spectrum 1). Such concomitant presence of aliphatic-rich bitumensand aromatic-rich pyrobitumens has been convincingly docu-mented within organic-rich source rocks at the stage of maturationcorresponding to the onset of gas generation using biomarkerresults and micro-FTIR techniques (e.g., Curiale, 1986; Mastalerzand Glikson, 2000). Under such a scheme, these pyrobitumenmacromolecules can be regarded as the solid residues of secondaryoil or bitumen cracking processes (Behar et al., 1992; Hill et al.,2003).

The fine-grained inorganic matrix porosity of all the investi-gated samples is almost totally filled with kerogen or bitumenparticles (Figs. 6e10). Some nano- to micropores locally appearassociated with fragmentary microfossil material as well as withpyrite framboids. In addition, irregular bubble-like nanopores ofabout 1e50 nm have been documented within the macromolecularstructure of some organic particles within the overmature Had-dessen samples. This intra-particle nanoporosity (pores of1e50 nm) constitutes the most obvious porosity of the investigatedovermature Posidonia Shale samples and most likely comes aboutby the exsolution of gaseous hydrocarbon. This porosity has thusbeen most likely hydrocarbon wet during the thermal maturationprocesses (Loucks et al., 2009). Similar intra-particle nanoporosityhas been previously documented within overmature samples ofBarnett Shale (Texas, USA) by Loucks et al. (2009), but has howevernot yet been attributed to any specific organic macromolecule. Thepresent study provides definitive proof that the organic macro-molecules displaying this intra-particle nanoporosity (pores of1e50 nm) are pyrobitumen residues.

This particular pore distribution has been progressively acquiredwith increasing maturity: neither intra-particle pore nor internalgrain heterogeneity can be seen within the Wickensen and Har-derode kerogen and bitumen particles. Although diageneticcompaction and burial might have influenced the rare inter-particleporosity to collapse (Katsube and Williamson, 1994; Kim et al.,1999), the diagenetic chemical and structural degradation of Pos-idonia organic matter, leading to the formation of pyrobitumenparticles, appears responsible for the creation and/or opening ofintra-pyrobitumen nanoporosity onto which gas may have sorbed(Loucks et al., 2009). Such pore distribution suggests that gaspermeability pathways have been greatly influenced, if notcontrolled, by the three dimensional arrangement of organicmacromolecules (Ross and Bustin, 2009; Nelson, 2009; Louckset al., 2009), and pyrobitumen particles in particular.

4.4. Concluding remarks: reservoir evaluation perspective

Many concepts regarding hydrocarbon generation and retentionprocesses occurring within gas shales have been carried overdirectly from conventional petroleum systems, thereby elucidatingthe influence of organic sedimentation, migration and retention onpetroleum quality and its concentration within the reservoir (e.g.,Curtis, 2002; Jarvie et al., 2007; Ross and Bustin, 2009). Based on

the TEM and synchrotron-based STXM characterization of organic-rich shale samples from a natural maturation series, the presentstudy documents geochemical and mineralogical heterogeneitiesdown to the nanometer scale. To the best of our knowledge, this isthe first attempt to constrain the fine-scale distribution of kerogenand newly formed organic macromolecules within organic-richshales over a maturation gradient. Different types of bitumen,very likely genetically derived from thermally degraded organicprecursors, have been detected in close associationwith authigenicminerals. The formation of pyrobitumen has been inferred fromSTXM-based C-XANES spectroscopy coupled to TEM observationswithin overmature samples. The concomittant appearance ofnanoscale porosity within the pyrobitumen structure has also beendocumented using TEM. By providing such in-situ evidence ofthe fate of bitumen as a response to the thermal evolution of themacromolecular structure of kerogen, the results reported in thecurrent contribution thus constitute a new step towards betterconstraining natural shale gas maturation processes.

This study has also major implications from a reservoir evalu-ation perspective. Improved understanding of the origin and fate ofsolid bitumen within gas shales is likely to have important impli-cations for geologic, engineering and economic evaluations of suchunconventional reservoirs as it can help to better evaluate pro-ducibility of bitumen-associated gaseous hydrocarbon. Solid bitu-mens have long been recognized as common occurrences in manypetroliferous basins and have been described in numerous reser-voir rocks (e.g., Curiale, 1986; Lomando, 1992; Hwang et al., 1998;Kelemen et al., 2010). However, the occurrence of solid bitumenin petroleum reservoirs presents technical challenges for fielddevelopment and production: it reduces the effective porosity andpermeability of reservoirs (Lomando, 1992; Hwang et al., 1998;Kelemen et al., 2010). By causing significant reservoir heteroge-neity as shown here, the presence of solid bitumen affects ultimaterecovery and reservoir response to stimulation. Although STXMand TEM images are micro-scale, and thus small compared to fieldsize, they provide essential insights into performance, producibilityand modeling of such strategic resources. The present studyemphasizes the fact that a better understanding of the relationshipsbetween the appearance of porosity and gaseous hydrocarboncontent is required for optimizing the processes for hydrocarbonrecovery in shale gas reservoirs.

Acknowledgments

We thank UlrichMann for providing the shale samples as part ofthe GASH project coordinated by the GFZ-Potsdam. STXM datawere acquired at beamline 5.3.2.2 at the ALS, which is supported bythe Director of the Office of Science, Department of Energy, underContract No. DE-AC02-05CH11231, and at beamline 10ID-1 at theCLS, which is supported by the NSERC, the CIHR, the NRC, and theUniversity of Saskatchewan. Special thanks go to David Kilcoyne forhis expert support of the STXM at the ALS and to Jian Wang and JayDynes for their expert support of the STXM at the CLS.We gratefullyacknowledge Karim Benzerara (IMPMC) and Estelle Couradeau(IMPMC) for their help with STXM data acquisition. Usefulcomments by two anonymous reviewers are greatly acknowledged.

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