Multi-scale detection of organic and inorganic signatures provides insights into gas shale properties and evolution

Chemie der Erde 70 (2010) S3, 119–133

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Chemie der Erde journal homepage: www.elsevier.de/chemer

Multi-scale detection of organic and inorganic signatures provides insights into gas shale properties and evolution Sylvain Bernard a,n, Brian Horsfield a, Hans-Martin Schulz a, Anja Schreiber b, Richard Wirth b, a ¨ , Herbert Volk c, Tiem Thi Anh Vu a, Ferdinand Perssen a, Sven Konitzer Neil Sherwood c, David Fuentes c a

Deutsches GeoForschungsZentrum GFZ, Section 4.3. Organic Geochemistry, Telegrafenberg 14473 Potsdam, Germany Deutsches GeoForschungsZentrum GFZ, Section 3.3. Chemistry and Physics of Earth Materials, Telegrafenberg 14473 Potsdam, Germany c CSIRO Petroleum, P.O. Box 136, North Ryde, NSW 1670, Australia b

a r t i c l e in fo

abstract

Article history: Received 18 January 2009 Accepted 5 May 2010

Organic geochemical analyses, including solvent extraction or pyrolysis, followed by gas chromatography and mass spectrometry, are generally conducted on bulk gas shale samples to evaluate their source and reservoir properties. While organic petrology has been directed at unravelling the matrix composition and textures of these economically important unconventional resources, their spatial variability in chemistry and structure is still poorly documented at the sub-micrometre scale. Here, a combination of techniques including transmission electron microscopy and a synchrotron-based microscopy tool, scanning transmission X-ray microscopy, have been used to characterize at a multiple length scale an overmature organic-rich calcareous mudstone from northern Germany. We document multi-scale chemical and mineralogical heterogeneities within the sample, from the millimetre down to the nanometre-scale. From the detection of different types of bitumen and authigenic minerals associated with the organic matter, we show that the multi-scale approach used in this study may provide new insights into gaseous hydrocarbon generation/retention processes occurring within gas shales and may shed new light on their thermal history. & 2010 Elsevier GmbH. All rights reserved.

Keywords: Gas shales Bitumen Nanoscale chemical and mineralogical characterisation STXM XANES spectroscopy TEM Raman microspectroscopy Laser ablation pyrolysis GC–MS

1. Introduction Gas shales are of great economic significance because they constitute self-contained source–reservoir systems of large continuous (unconventional) dimensions. They can contain thermogenic, biogenic or combined biogenic–thermogenic gases (Jenkins and Boyer, 2008). These formations are typically rich in organic material (OM), tens to hundreds of metres thick, and sufficiently brittle and rigid to allow fracturing for production. They are characterised by widespread gas saturation, subtle trapping mechanisms, seals of variable lithology and relatively short hydrocarbon migration distances (e.g., Curtis, 2002; Jarvie et al., 2007; Jenkins and Boyer, 2008). Generated gas can be stored as free gas in intergranular porosity and natural fractures, adsorbed onto kerogen and clay particles surfaces or dissolved in kerogen and bitumen (Curtis, 2002). Understanding the geological and geochemical nature of gas shale formations and improving their gas producibility have thus been at the heart of millions of dollars worth of research since the 1970s (e.g., Curtis, 2002).

n

Corresponding author. E-mail address: [email protected] (S. Bernard).

0009-2819/$ - see front matter & 2010 Elsevier GmbH. All rights reserved. doi:10.1016/j.chemer.2010.05.005

Historically, optical microscopy has been a cornerstone of characterising OM in sediments because of the detailed insights it provides into maceral habitat and distribution, as well as levels of thermal maturation. Additionally, a wide range of bulk analyses have been routinely used for decades in order to estimate the content and type of kerogen and bitumen present, the level of maturity of the rock, as well as how much and what kind of hydrocarbons has been generated (Tissot and Welte, 1984). Such information has been used to evaluate the potential and producibility of gas shale systems (Jarvie et al., 2007). The most frequently used analytical methods for this purpose are total organic carbon (TOC) content estimation, Rock–Eval pyrolysis, open-system pyrolysis gas chromatography and vitrinite reflectance measurements (e.g., Tissot and Welte, 1984; Horsfield, 1989; Dembicki, 2009). However, it goes beyond just source rock richness, quality and maturity when evaluating gas shale potential: the challenge in unconventional gas shale systems is to find areas, where high gas-in-place is accompanied by high production efficiency (e.g., Curtis, 2002; Jenkins and Boyer, 2008). Therefore, the thickness, areal extent, porosity and mechanical properties of the gas shale system need to be also taken into account to determine the volume of gas that may be commercially produced (Jarvie et al., 2007). Additionally, elucidating the pore structure and sorption characteristics of such organic-rich shales

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appear crucial to predict potential gas capacities (Pollastro et al., 2007; Ross and Bustin, 2009; Loucks et al., 2009). Geochemical, sedimentological, petrophysical and geomechanical methods have thus been used in parallel towards this end (see Jenkins and Boyer, 2008). Within such complex self-contained source–reservoir systems, three distinct processes may result in the formation of thermogenic gas: (1) the decomposition of kerogen to gas and bitumen; (2) the decomposition of bitumen to oil and gas; and (3) the secondary cracking of oil to gas and a carbon-rich coke or pyrobitumen residue (Tissot, 1969; Lewan et al., 1979; Braun and Rothman, 1975; Jarvie et al., 2007). Primary kerogen cracking and bitumen cracking occur between 80 and 180 1C, whereas secondary oil to gas cracking has been suggested to begin in shales at about 150 1C and in sandstone reservoirs at 200 1C (e.g., Larter and Horsfield, 1993; Dieckmann et al., 1998; Horsfield et al., 1992; Waples, 2000; Jarvie et al., 2007). The volume of gas generated by secondary cracking directly depends on oil retention in the system, i.e. on the nature of organic and inorganic phases and their specific adsorption capabilities as well as on organic/ inorganic structural relationships, pores and fracture network. Cooles et al. (1986) have shown that oil expulsion efficiency is directly correlated with generative yield (kg/tonne) and inversely correlated with secondary gas yield. High gas yields, signalling inefficiency of petroleum expulsion, have been described as ‘‘dysfunctional’’ in the conventional petroleum system sense (Hamblin, 2006). Due to these successive cracking reactions, gas shale systems may be expected to display various remnants of organic precursors and products both vertically and laterally. Heterogeneity can also be expected with associated inorganic phases which may influence gaseous hydrocarbon generation and retention processes. Organic petrography allows insights into the distribution of kerogen, bitumen and oil in organic-rich rocks (e.g., Littke et al., 1997). Scanning electron microscopy has been used less widely, but has revealed additional textures and genetic relationships at the micrometre-scale (e.g., Mann et al., 1997). Nevertheless, despite the very important vertical and spatial variability of organic-rich shales and the sub-micrometric size of most of their organic and inorganic constituents, chemical and structural characterisation at the nanometre-scale has never been performed on such samples, and gas shales in particular. Transmission electron microscopy (TEM) and synchrotronbased scanning transmission X-ray microscopy (STXM) are intrinsically well suited for nanoscale characterisation of such heterogeneous and organic-rich samples. In particular, STXM is a spectromicroscopy technique which has already proved valuable for the nanoscale characterisation of organic specimens in polymer science (Ade et al., 1992; Hitchcock et al., 2002, 2005), biology (Jacobsen et al., 1995; Jacobsen, 1999), medicine (Benzerara et al., 2006a), geobiology (Benzerara et al., 2006b; Miot et al., 2009a, 2009b), paleontology (Boyce et al., 2002, 2004; Bernard et al., 2007, 2009; Lepot et al., 2008, 2009), river and marine chemistry (Brandes et al., 2004; Haberstroh et al., 2006), soil science (Lehmann et al., 2005, 2008), carbon physics (Felten et al., 2006; Brandes et al., 2008), cosmochemistry (Cody et al., 2008) and environmental sciences (Bluhm et al., 2006). Apart from the pioneer STXM studies of organic and inorganic microstructures of bituminous and sub-bituminous coals performed by Botto et al. (1994) and Cody et al. (1995a, 1995b), only a few STXM applications in organic geochemistry can be found in the literature (e.g., Cody et al., 1996, 1998). The present paper aims to introduce and illustrate the capabilities of a multi-scale and multi-technique methodology using the example of a potential gas shale of Europe and supplemented by reference source rock samples from around

the world. First, based on the combination of compositional organic geochemistry data as well as Raman, TEM and STXM observations, we document microscale and nanoscale chemical and mineralogical heterogeneities within an overmature calcareous gas shale from northern Germany (the precise location of this sample will remain confidential). We then discuss the complementarity of the different techniques used in this study for the multiscale characterisation of such complex organic-rich samples. Finally, we discuss these microscale and nanoscale observations in the light of thermal history constraints and hydrocarbon generation/retention processes.

2. Materials and methods 2.1. Material description 2.1.1. Calcareous shale sample from northern Germany The sample investigated here is from the Mesozoic of northern Germany and, for the purposes of this investigation, fulfils the basic criteria demanded of potential gas shales, namely: it is organic-rich; it is from a formation that is 15–80 m thick and was deposited in a reducing marine environment; it contains organic matter that was originally Type II, but is now at an advanced level of maturity; quartz and especially carbonate exceed clay minerals (Littke et al., 1997). Bulk organic geochemical data (TOC content and Rock–Eval parameters) of the investigated sample are given in Fig. 1. This shale sample has a TOC content of 5.65%. Its remaining hydrocarbon generating potential value (S2) is 4.9 mg/g rock, meanwhile its free hydrocarbon content (S1) is relatively small and in range of 0.72 mg/g rock. The kerogen type and petroleum potential of the investigated shale sample can be evaluated from the HI vs. OI and HI vs. Tmax diagrams (Fig. 1) following Espitalie´ et al. (1977, 1984). The investigated sample appears in Fig. 1 at the very end of the mean evolution pathways of kerogen Types I and II, at the onset of gaseous hydrocarbon generation, which is consistent with its Tmax value of 466 1C. According to Jarvie et al. (2007), all these parameters are consistent with a high shale gas potential. 2.1.2. Reference materials Five kerogens from well-known source-rocks, coal macerals and organic-rich sediments have been used as references in this study to assist in interpreting the STXM data for the potential gas shale under study. Australian Torbanite (Permian), which consists of algal remains resembling Botryococcus braunii, and Alaskan Tasmanite (Jurassic) are both classified as Type I yielding primarily paraffinic oils in nature (Horsfield, 1989). The so-called Cannel Coal (Cretaceous) from Utah, which in this case is rich in bituminite and has strong algal affinities, and a sporinite concentrate from the High Hazles Seam (Whitwell colliery, Westphalian, U.K.) are Type II kerogens (Horsfield, 1989). Vitrinite handpicked from High Volatile Bituminous coals of the Talang Akar Formation (Oligocene) of Java fall on the Type II/III kerogen evolution curve (Horsfield et al., 1988; Horsfield, 1989). 2.2. Sample preparation procedures and analytical methods The multi-scale characterisation methodology presented in this study is based on a combination of organic geochemistry, microscopy and spectroscopy techniques. The nature and the maturity level of the investigated macerals are first determined using organic petrography and vitrinite reflectance measurements. Then, molecular-level information on individual macerals

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Fig. 1. Top: TOC content and Rock–Eval parameters of the investigated sample. Bottom: hydrogen index vs. oxygen index diagram (left) and hydrogen index vs. Tmax diagram (right). The investigated sample appears at the very end of the mean evolution pathways of kerogen Types I and II (red dot). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

are obtained from laser-ablation pyrolysis gas chromatography mass spectrometry (LA-Py) measurements, while open-system pyrolysis, gas chromatography provide the maceral mixture composition. The organic and inorganic constituents of the investigated sample are then characterised at the micrometrescale using Raman microspectroscopy and Raman mapping. Finally, ultrathin sections are extracted from the investigated sample using focused ion beam (FIB) milling for chemical and structural characterisation at the nanometre-scale using TEM and STXM. 2.2.1. Organic petrography and vitrinite reflectance measurements Standard methods of petrographic sample preparation were used to prepare a polished block of sample for reflectance measurements and maceral analyses. Reflectance values were taken on randomly oriented phytoclasts in non-polarised light (Rmo%) in oil immersion having a refractive index of 1.518 at 2371 1C. Zeiss equipment was employed, using a  40 objective with a measuring diaphragm giving a 3 mm diameter spot size and an interference filter having a passband peak of 546 nm. Where possible, a minimum of 25 reflectance measurements are made on vitrinite/vitrinite-like matter in order to achieve a stable mean. Fewer readings were taken on other macerals for comparative purposes. The photometer was calibrated against a synthetic Yttrium–Aluminium garnet standard of 0.92% reflectance. Maceral analysis was done using a stepping stage and point-counter, with the stepping interval set to enable complete coverage of the sample in 10 traverse lines and a minimum of 500 points counted. 2.2.2. Open-system pyrolysis, gas chromatography About 30 mg of the sample was analysed by an open-system pyrolysis using a Quantum MSSV-2 Thermal Analysis System& interfaced with an Agilent GC-6890A (Horsfield, 1989). The sample was placed into the central part of a glass tube (26 mm long—inner sleeve diameter 3 mm). The remaining volume was filled with purified quartz wool that had been cleaned by heating at 630 1C in air for 30 min. The sample was then heated in a flow of helium, all products released up to 300 1C being vented. The

sample was then pyrolysed at 50 1C/min from 300 to 600 1C (3 min, isothermal) and pyrolysis products collected in a cryogenic trap (liquid nitrogen cooling at  190 1C, glass beads substrate) for condensation, from which they were later liberated by ballistic heating (held at 320 1C for 10 min). A HP-Ultra 1 dimethylpolysiloxane capillary column (50 m length, inner diameter of 0.32 mm, film thickness of 0.52 mm) connected to a FID was used with helium as carrier gas. The temperature of the gas chromatograph was programmed from 30 to 320 1C at a rate of 5 1C/min, followed by an isothermal phase of 35 min. The gas chromatogram was interpreted and analysed with the ChemStation software. An external butane standard was used for quantification. Response factors for all compounds were assumed the same, except for methane whose response factor is 1.1.

2.2.3. Laser-ablation pyrolysis gas chromatography mass spectrometry (LA-Py) The laser extraction GC–MS instrument used in this study was a modified version of the set-up described by Greenwood et al. (1998). It comprises a Hewlett Packard 6890 gas chromatograph (GC) interfaced to a Hewlett Packard 5973 mass selective detector (electron energy 70 eV, electron multiplier 1200 V, source temperature 250 1C, 0.1 amu resolution) and an Olympus BX60M system microscope with a custom-built laser chamber and inlet system. The microscope was used with long working distance objectives (50  , 0.25, N/  f¼180). The laser is a continuous wave (CW) Nd:YAG laser (Laser Applications Inc.) capable of delivering 50 W maximum power. Laser power was attenuated with a beam splitter window to 10% transmittance. The full scan analysis (50–550 amu) of the maceral pyrolysates were performed on a DB-5MS column (J&W, 60 m, 0.25 mm I.D, 0.25 mm film thickness) with helium as a carrier gas. The sample chamber was held at approximately 100 1C. Helium used as the sweep gas with a flow of 100 mL/min was maintained throughout the chamber. Products liberated by the laser ablation process were cryogenically trapped in a coiled nickel loop using a liquid nitrogen bath. After 2 min of trapping, a 6-port Valco valve was rotated to transfer the 1 mL/min column helium flow through the

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nickel trap. The contents of the nickel trap were then desorbed by heating to 320 1C and the products cryo-focused in a loop of GC column immersed in a liquid nitrogen bath. The GC oven was programmed from an initial temperature of 40 1C (2 min hold), followed by heating at 4 1C/min to 310 1C (30 min hold).

2.2.4. Raman microspectroscopy and Raman mapping Raman data were obtained with a Renishaw INVIA spectrometer operating at IMPMC, Paris. Spectra were measured at a constant temperature (20–22 1C) with the 514.5 nm wavelength of a 20 mW spectra physics Argon laser focused through a Leica microscope with a 50  objective (NA¼0.75). This configuration yields a planar resolution of  1 mm and a laser power delivered at the sample surface of around 1 mW (an instrumental configuration which limits radiation damage). Light was dispersed by a holographic grating with 1800 grooves/mm and the signal analysed with a RENCAM CCD detector. Spectra were collected using the software Wire 3.2 provided by Renishaw. Raman microspectroscopy of carbonaceous materials has been performed on polished section following the recommendations of Beyssac et al. (2003). Dynamic line-scanning Raman mapping (Renishaw Streamline) was performed using an XYZ motorised stage following the procedure described in Bernard et al. (2008).

2.2.5. Reference material preparation Reference samples were finely ground in an agate mortar and suspended in water. A droplet was then deposited on a Si3N4 window and air dried. Particles of about 200 nm of diameter have been measured using scanning transmission X-ray microscopy.

2.2.6. Sample preparation by focused ion beam (FIB) milling TEM and STXM are transmission techniques, and thus require the samples to be electron and X-ray transparent, i.e. thinner than 250 nm. Focused ion beam (FIB) milling was used to prepare such ultrathin samples. This sample preparation technique preserves microtextural information, while avoiding harsh chemical extraction or staining. Ultrathin foils, typically 15  5  0.10 mm3, were prepared using the FIB single beam device (FEI FIB 200 TEM) operating at GFZ Potsdam. The FIB extraction procedure, which has become a standard sample preparation technique in various domains of Earth sciences (e.g., Kempe et al., 2005; Benzerara et al., 2007; Schiffbauer and Xiao, 2009), is described in detail in Heaney et al. (2001), Benzerara et al. (2005) and Wirth (2004, 2009). A thin layer of platinum is deposited on the sample surface in order to protect it during the milling process. A 30 kV Ga + beam emitted from a Ga liquid metal ion source operating at  20 nA allows excavating the sample from both sides of the Pt layer to a depth of  5 mm. Before removal of the thin slide, the sample is further thinned to 100 nm with a glancing angle beam at much lower beam currents of  100 pA. A line pattern is then drawn with the ion beam along the side and bottom edges of the thin foil allowing its removal. Finally, the foil is transferred at room pressure with a micromanipulator onto the membrane of a carbon-coated 200 mesh copper grid for subsequent TEM and STXM analyses. Advantages and limitations of this sample preparation technique are discussed in details in Obst et al. (2005), Langford (2006) and Drobne et al. (2007). Bernard et al. (2009) have recently shown that FIB milling does not induce significant changes in the speciation of carbon in model polymers as measured by STXM at the C K-edge.

2.2.7. Scanning transmission X-ray microscopy (STXM) STXM is a synchrotron-based transmission spectromicroscopy technique using a monochromated X-ray beam produced by synchrotron radiation. This technique allows both microscopic observations – i.e. imaging at the 25-nm scale with speciation sensitivity, and spectroscopic measurements – i.e. recording X-ray absorption near edge structure (XANES) spectra, which provide information on the bonding environment of carbon in organic compounds at the same spatial scale. When X-rays pass through matter, some photons are absorbed (their extent depending on the sample nature, thickness and density) and cause excitation of inner shell (core) electrons of sample atoms. These excited core electrons can then be promoted to unoccupied energy levels (molecular orbitals) to form a short lived excited state, or completely removed to form an ionised state. For STXM imaging, the X-ray beam is focused on the sample using a zone plate, and a 2-D image is collected by scanning the sample at a fixed photon energy at a spatial resolution of about 30 nm. The image contrast results from the differential absorption of X-rays, which depends on the sample speciation. By subtracting an image taken below the carbon edge from another taken at the energy corresponding to the absorption of a specific C-containing functional group, it is possible to obtain a chemical map showing the spatial distribution of this specific functional group. STXM can also be used to perform high-spatial and energy resolution X-ray absorption near edge structure (XANES or NEXAFS for near edge X-ray absorption fine structure) spectroscopy (in the energy range 280–300 eV at the carbon K-edge). XANES spectra can be obtained by collecting image stacks, which are taken by scanning the sample in x–y directions of selected sample areas with energy increments of 0.1 eV over the energy range of interest (280–300 eV at the C K-edge). Here, the x–y plane refers to the plane perpendicular to the X-ray beam direction. The procedure for collecting a stack image thus consists of measuring the XANES spectrum on each pixel (one pixel can be as small as 25-nm  25-nm) of the image. Counting times are of the order of a few milliseconds or less per pixel. Normalization and background correction of the C K-edge XANES spectra were performed by dividing each spectrum by a second spectrum taken at a C-free location on the same sample. These near edge absorption bands are sensitive indicators of the local chemical bonding environment surrounding the carbon atoms in question ¨ (Stohr, 1992) and correspond to transitions from inner shell 1s electrons to both unoccupied pn (antibonding) and low lying sn orbitals. The different peaks observed in C K-edge XANES spectra can thus be tentatively assigned to different types of C-containing functional groups based on numerous previous XANES studies on coal and natural carbon particles. As a result, the STXM technique allows in situ organic geochemical characterisation at the 25-nm scale. Extensive databases of reference XANES spectra measured on hundreds of C-containing compounds at the C K-edge, sometimes supported by theoretical calculations using multiple scattering approaches are available (e.g., http://unicorn.mcmas ¨ ter.ca/corex/cedb-title.html; Stohr, 1992; Myneni, 2002). More details about the physical basis of XANES spectroscopy can be ¨ found in Stohr (1992). The methods used for STXM data acquisition and analysis as well as examples of STXM applications in various fields can be found, for example, in Jacobsen et al. (2000), Hitchcock (2001), Benzerara et al. (2004) and Bluhm et al. (2006). Measurements of the present study were done using the STXM located on the X1–A1 beamline (outboard branch) of the National Synchrotron Light Source, Brookhaven National Laboratory (e.g., Winn et al., 2000). The X-ray source is an undulator on the 2.5 GeV electron storage ring. Energy calibration was accomplished using the wellresolved 3p Rydberg peak at 294.96 eV of gaseous CO2 for the

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C K-edge. The numerous images of each stack were aligned using an automated image alignment program, which optimised the location of each image relative to the others using features of strong contrast (Jacobsen et al., 2000). Spectral peak positions, intensities and widths were determined using the Athena software package (Ravel and Newville, 2005). 2.2.8. Transmission electron microscopy (TEM) Transmission electron microscopy was performed using a TECNAI F20 XTWIN TEM operated at 200 kV with a field emission gun (FEG) as the electron source. This TEM, operating at GFZPotsdam, is equipped with a Gatan TridiemTM energy filter, an EDAX GenesisTM X-ray analyzer and a Fishione high angle annular dark filed detector (HAADF) allowing an image acquisition in the scanning transmission electron microscopy (STEM) mode. Energy dispersive X-ray spectroscopy (EDXS) element maps are always displayed as background subtracted intensity maps.

3. Results 3.1. Characterisation at the micrometre-scale 3.1.1. Organic petrological findings Optical microscopy observations show that organic particles are widespread in the shale sample, either as bituminite/micrinite finely disseminated within the mineral matrix or concentrated as structured particles. The kerogen of this overmature sample is mainly composed of meta-alginite (Littke et al., 1997), inertinite and a few vitrinite-like particles (Table 1 and Fig. 2). The metaalginite likely originated from prasinophyte cysts (tasmanitids), while the inertinite includes both faunal and floral relics (Fig. 2). Abundant bitumen particles have also been documented within the investigated sample (Fig. 2). Reflectance measurements have been carried out on the different macerals. Reflectance values of

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vitrinite-like compounds range from 1.3 to 1.7% Ro, for a mean value of 1.45% Ro (Fig. 3). Bitumen reflectance values are relatively high and range from 1.10 to 1.65% Ro. Two populations of bitumen can be identified from these reflectance measurements (Fig. 3). The sample was also examined using an incident fluorescence excitation (450–490 nm) with a 100 W Hg lamp as a source, but no fluorescence was observed, even for liptinites, because of the high thermal maturity of the investigated sample.

3.1.2. Laser-ablation pyrolysis gas chromatography mass spectrometry (LA-Py) LA-Py was utilised to gain an insight into the composition of individual macerals. Before doing so, a whole rock sample was pyrolysed using an open-system pyrolysis gas chromatography to establish the average chain length distribution and aromaticity of the combined maceral assemblage described above. The gas chromatogram (Fig. 4) shows a mixture of paraffins and aromatic hydrocarbons. Normal hydrocarbon doublets appear as an abundant compound group and show a decrease in relative abundance with increasing carbon numbers from n-C6 to n-C27, which is typical for many Type II kerogens (Horsfield, 1989; Muscio et al., 1994; Clegg et al., 1997). Alkylaromatics were also in abundance (benzene, toluene, m-p-o-xylenes, naphthalene and methylnapthalenes), but neither alkylphenols, typical of land plant lignocellulosic organic matter, nor sulphur-compounds, usually associated with low maturity anoxic sediments, were detected. Due to the high maturity of the sample, only low abundances of LA-Py products were yielded per shot. 70, 60 and 50 shots producing laser craters of ca. 40–50 mm size were thus amalgamated for the bituminite, prasinophytes and bitumen macerals, respectively. LA-Py products of all these macerals appear very similar as regards to major pyrolysate component distributions (Fig. 5), being dominated by light aromatic compounds and

Table 1 Abundance, mean, maximum and minimum reflectance values of the different macerals and bitumen observed within the investigated sample (N: number of analyses, SD: standard deviation).

Vitrinite-like Inertinite Liptinite Bitumen Bituminite/micrinite

Abundance % (whole rock)

Abundance % (TOC)

Mean Ro %

Minimum %

Maximum %

N

SD

o 0.2 0.4 0.7 0.9 6.9

o 2.0 4.5 7.9 10.1 77.5

1.47 1.91 1.75 1.36 –

1.29 1.8 1.51 1.07 –

1.71 2.11 1.85 1.64 –

26 5 19 13 –

0.12 0.13 0.09 0.2 –

Fig. 2. Photomicrographs of the investigated sample in reflected white light (oil immersion).

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Fig. 3. Reflectance histogram of liptinite, inertinite, vitrinite-like and bitumen particles of the investigated sample.

Fig. 4. Pyrolysis gas chromatogram of the investigated sample. Numbers refer to chain length of n-alkene/-ane doublets. B ¼ benzene, T ¼toluene, E ¼ethylbenzene, M ¼meta- and para-xylenes, O¼ ortho-xylene, N ¼naphthalene.

n-alkanes. No alk-1-enes, which are typical open-system pyrolysis products from aliphatic precursor structures were detected. The latter feature may not reveal insights into macromolecular organic matter compositions but rather, because of the mild thermal extraction treatment employed prior to laser pyrolysis, represent free hydrocarbons in the rock. Subtle compositional differences between macerals can be recognised despite the high maturity level of the sample (Fig. 5).

3.1.3. Raman microspectroscopy and Raman mapping Raman spectra of the various macerals observed within the sample are identical (Fig. 6). They display a broad band at 1350 cm  1 (defect band D1) and a second major feature at 1600 cm  1 (including the so-called graphite band G and the defect band D2). In addition, a D3 band, a D4 band and even a D5 band are observed at 1500, 1175 and 1250 cm  1, respectively (Fig. 6). These additional defect bands are typical of very disordered OM (Beyssac, personal communication). Their presence prevents the use of the geothermometer calibrated by Beyssac et al. (2002) to precisely estimate the maximum temperature experienced by the sample. However, a temperature of about 180 1C can be qualitatively estimated based on the characteristics of these Raman spectra (Beyssac, personal communication), this

being broadly similar to the 170 1C that would be anticipated based on kinetic analysis (Dieckmann et al., 1998). The mineral matrix of the investigated sample is predominantly composed of clay particles. Unfortunately, their very small size prevents Raman spectra to be collected. Within the matrix, some disseminated pyrite, calcite and anatase crystals can be observed. Noteworthy, some sparsely distributed quartz and albite crystals have also been seen. All these minerals have been unambiguously identified from their Raman spectra (Fig. 6). Indeed, the Raman spectrum of pyrite displays vibrational bands at 343, 379 and 430 cm  1, while the Raman spectrum of calcite exhibits vibrational bands at 155, 280, 710 and 1085 cm  1, the Raman spectrum of anatase shows vibrational bands at 140, 395, 510 and 640 cm  1, the Raman spectra of quartz displays vibrational bands at 204, 355 and 460 cm  1 and the main vibrational bands of the Raman spectrum of albite can be seen at 160, 185, 290, 475 and 505 cm  1 (Fig. 6). Raman maps collected on the investigated Posidonia sample evidence its strong heterogeneity at the micrometre-scale, as well as the close association of OM with these different inorganic phases (Fig. 6). However, due to the fluorescence in the Raman signal induced by the very small size of clays and of some other minerals, some regions of the sample could not be imaged using Raman mapping.

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Fig. 5. Total ion chromatograms (TIC) for the laser pyrolysis products of bituminite, prasinophyte and bitumen from the investigated sample. C ¼ CS2, B ¼benzene, M ¼methylcyclohexane, T ¼ toluene, S¼siloxane. Numbers on peaks refer to the n-alkane carbon chain length.

3.2. Characterisation at the nanometer-scale 3.2.1. XANES spectroscopy of reference materials As a prelude to the detailed analysis of the high maturity sample, high-spatial and energy resolution XANES spectra were first acquired at the C K-edge on five reference organic compounds which display unique spectral signatures (Fig. 7), indicating that they contain several C-bearing functional groups in different concentration ratios. The presence of several peaks in a XANES spectrum at the C K-edge reflects different types of carbon speciation, which can be related to different types of bonding of carbon in the sample (Cody et al., 1995a, 1995b, 1996, 1998; Boyce et al., 2002). The first step of a XANES spectroscopy study thus consists in identifying these different peaks. The following peak assignments for the C K-edge XANES spectra of kerogens have been done based on previous peak assignments made by Cody et al. (1995a, 1995b, 1996, 1998), Boyce et al. (2002) and Urquhart and Ade (2002). The five reference macerals contain various amounts of (1) aromatic carbon groups (peak at 285.1 eV), (2) aliphatic carbon groups (shoulders at 287.7 and 288.1 eV) and (3) oxygencontaining carbon functional groups (peaks at 286.6, 287.1 288.6 and 289.5 eV) as major constituents (Fig. 7). The signal at 285.1 eV is interpreted as 1s-pn transitions in aromatic or olefinic carbon (CQC) (Cody et al., 1995a, 1995b, 1996, 1998; Boyce et al., 2002,

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2004). The 286.6 eV peak is assigned to ketonic (Cody et al., 1996; Urquhart and Ade, 2002) or phenolic (Lehmann et al., 2005) functional groups (C¼O) (1s-2pn). The 287.1 eV peak is indicative of the presence of alcoholic (C–OH) or phenolic (Ar–OH) groups (1s-pn) (Boyce et al., 2002). The shoulder at 287.7 eV has been attributed to the presence of unsaturated bonds between carbon and heteroatoms such as oxygen (Braun et al., 2005), nitrogen (Shi et al., 2005), or sulphur (Liu et al., 2005), but is most commonly attributed to 1s-3p/sn transition of aliphatic carbon bonded to one, two, or three hydrogens (C–H1–3) (Cody et al., 1998; Boyce et al., 2002; Braun et al., 2005; Hitchcock et al., 2007). The shoulder at 288 eV is due, in a large part, to the presence of aliphatic carbon (C–H1–3) (1s-3p/sn) (Cody et al., 1998). The 288.6 eV peak is indicative of carboxylic (COOH) functional groups (1s-pn) (Cody et al., 1996), while the peak at 289.5 eV likely corresponds to the presence of hydroxylated (alcoholic) functional groups (C–OH) or ether linked sp3 hybridised carbon (1s-pn) (Cody et al., 1998; Boyce et al., 2002). Similarly to all electric–dipole transition spectroscopies, as XANES spectroscopy adheres to Beer’s law, variations in peak intensity directly result from variations in concentration of the functional groups absorbing at these specific energies. However, the intensity of a given peak is a function of the specific oscillator strength of the transition (Ishii and Hitchcook, 1988; Francis and Hitchcock, 1992). Without a precise estimation of these oscillator strengths, the respective concentrations of the different functional groups cannot be quantitatively estimated. Additional experimental and theoretical studies, as well as a reliable calibration based on reference materials similar to the studied samples, are needed in order to better understand the physics beyond such XANES data and allow quantitative estimations of the contribution of the different moieties within the organic compounds. Nevertheless, given that the XANES spectra presented here have been normalized to the total carbon amount (corresponding to the absorption at 300 eV) and assuming that the oscillator strength of a given functional group is essentially the same in organic compounds of similar chemistry, the relative concentrations of the different functional groups can be discussed qualitatively. Indeed, based on the previous assignments, it can be inferred that the reference vitrinite (Type III) is mostly composed of aromatic groups and contains a few amount of phenolic and carboxylic groups, while the kerogen from the Cannel Coal and the reference sporinite (both Type II) contain a variable concentration of carboxylics (probably fatty acids) and alcohols (C–OH) and/or alkylphenols (Ar–OH) as well as a substantial amount of aromatic and aliphatic polymers. Carboxylic groups appear as a major constituent of the Alaskan tasmanite and the Australian torbanite (both Type I), which contain a lower amount of aromatic groups and phenolic or alcoholic groups than the other reference macerals measured here. XANES spectroscopy may thus not only allow identifying the bulk kerogen type, but may also allow discriminating different kerogens of the same type.

3.2.2. TEM and XANES spectroscopy of the investigated gas shale sample Powder analysis: XANES analyses performed at the C K-edge at the sub-micrometre-scale reveal that the OM of this sample is not spectroscopically homogeneous. Three chemically distinct organic materials have indeed been identified within the sample (Fig. 8). The XANES spectrum of the main fraction, identified as kerogen in Fig. 8, displays absorption features at 284.5, 285.1, 286.1 and 288.6 eV, interpreted as electronic transitions of benzoquinone (C6H4O2) (1s-pn), aromatic or olefinic groups (CQC) (1s-pn), ketonic groups (CQO) (1s-2pn) and carboxylic

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Fig. 6. Raman analysis of the investigated sample. (A) Raman spectrum of OM from the overmature sample of Posidonia Shales, showing two dominant vibrational bands at 1350 cm  1 (D1—main defect band) and at 1600 cm  1 (G—graphite band including the D2—defect band), and additional defect bands (D3, D4 and D5 observed at  1500, 1175 and 1250 cm  1, respectively). This spectrum is typical of very disordered carbonaceous material. (B) Raman spectra of minerals associated with OM in the investigated sample: albite in yellow (main vibrational bands at 160, 185, 290, 475 and 505 cm  1), quartz in pink (vibrational bands at 204, 355 and 460 cm  1), calcite in red (vibrational bands at 155, 280, 710 and 1085 cm  1), pyrite in grey (vibrational bands at 343, 379 and 430 cm  1) and anatase in green (vibrational bands at 140, 395, 510 and 640 cm  1). (C) Photomicrographs in reflected light of different zones of the investigated sample and corresponding Raman maps showing the spatial distribution of OM in blue and associated mineral phases with an imaging resolution of 1 mm. Albite appears in yellow, quartz in pink, calcite in red, pyrite in grey and anatase in green. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 7. XANES spectra of reference organic compounds: Vitrinite, Cannel Coal maceral, Sporinite, Tasmanite and Torbanite. Indexation of electronic transitions corresponding to the different peaks is reported in the text.

groups (COOH) (1s-pn), respectively (Cody et al., 1996; Boyce et al., 2002). Authentic standards are currently being utilised to confirm or refute the presence of acid groups in macromolecules at high thermal maturity levels. The important surface of the aromatic peak (  285 eV) relatively to the absorption feature at 288.6 eV ostensibly related to the presence of carboxylic groups is consistent with an overmature organic material (e.g., Cody et al., 1996, 1998). Additionally, more gentle absorption features are

observed at 290.3, 297.3 and 300 eV and likely result from the close association of carbonates (290.3 eV) and K-bearing phases (297.3 and 300 eV) with the kerogen. Indeed, carbonate functional groups specifically absorb at 290.3 eV (Benzerara et al., 2004), making their identification unambiguous, and K-bearing compounds have a specific spectral signature in the C K-edge energy range as they show two specific peaks at 297.2 and 300 eV related to the K L2,3-edges (de Groot et al., 1990). This spectrum

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Fig. 8. STXM characterisation of the investigated sample at the carbon K-edge. (A) STXM image of the FIB foil extracted from the investigated sample collected below the carbon absorption edge (280 eV). Carbon particles do not absorb at this energy, and thus appear in bright, non-containing carbon minerals appear darker and the platinum layer appears in black. The bottom region of the FIB appears darker as the result of a thickness variation. (B) STXM map of aromatics groups (bright areas) in the FIB foil obtained by subtraction of a STXM image collected at 280 eV to an image of the same area collected at 285.1 eV. (C) C-XANES spectra of the different constituents of the sample: kerogen in green, bitumen I in red, bitumen II in pink, carbonates in dark and light blue and K-bearing clays in yellow. The XANES spectrum of bitumen from a fracture is shown as reference (black spectrum). Indexation of electronic transitions corresponding to the different peaks is reported in the text. (D) Compositional map showing the spatial distribution of the different constituents over the whole FIB foil (no bitumen II can be observed). Color code is the same than XANES spectra in C. The phases of constant absorption over the C K-edge energy range appears in grey (likely silicates), while the platinum layer on top of the FIB section appears in white. The red square indicates the location of the close-up shown below. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

also exhibits a broad region of absorption around 292 eV, which principally corresponds to the ionization edge of carbon. The intensity of this absorption is, however, modulated by a superposition of broad 1s-sn transitions. Surprisingly, some organic compounds from the same sample display very different XANES spectra which exhibit more intense absorption peaks at the specific absorption energies of aliphatic and oxygen-containing functional groups (Fig. 8). The presence of such aliphatic-rich organic compounds is consistent with the pyrolysis results presented above and others on overmature kerogens from

around the world (Horsfield, 1989), which argue for aliphatic structures being preserved throughout and beyond the stage of catagenesis. These organic materials are also closely associated with carbonates and K-bearing phases as evidenced by the presence of gentle absorption features at 290.3, 297.3 and 300 eV. Two different types of aliphatic-rich organic macromolecules have been identified using the XANES spectroscopy. The XANES spectrum of the first type of aliphatic-rich organic compounds display small absorption peaks at 285.1 and 285.4 eV, corresponding to electronic transitions of aromatic or olefinic carbon (CQC)

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Fig. 9. STEM image (HAADF mode) of the FIB foil (top left), in which OM 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 foil shown in this figure (carbon, oxygen, sulphur, potassium, silicium, aluminium, sodium, chlorin, calcium, magnesium and iron), while the red dashed square indicates the location of the STXM compositional map shown in Fig. 8D. See text for details.

(1s-pn), as well as clearly individualised peaks at 286.6, 288 and 289.5 eV (Fig. 8). These peaks respectively indicate the presence of phenolic groups (Ar–OH) (1s-pn), aliphatic groups C–H1–3 (1s3p/sn) and alcoholic or ether groups (C–OH or C–O–C) (1s-3p/sn) (Cody et al., 1996, 1998; Boyce et al., 2002). The XANES spectrum of the second type of aliphatic-rich organic compounds is slightly different: the peak at 285.4 eV only appears as a shoulder on the aromatic peak at 285.1 eV, the intensities of the peaks at 286.6, 288 and 289.5 eV are lower relatively to the aromatic peak and an additional peak at 288.6 eV indicative of carboxylic functional groups (1s-pn) (Cody et al., 1996) is observed. These two types of aliphatic-rich organic compounds are both thought likely to correspond to bitumen, because absorption peaks at the same energies have been observed in the XANES spectrum of bitumen-filled fractures within the investigated sample, thus indicating a similar chemistry and the presence of the same C-bearing functional groups. Therefore, the aliphaticrich organic compounds of the first and the second types have been labelled as bitumens I and II, respectively (Fig. 8). FIB section analysis: an ultrathin FIB foil has been extracted from a polished section of the investigated sample for nanoscale characterisation using STXM and TEM (Figs. 8 and 9). Energyfiltered STXM imaging at the C K-edge allowed locating organic materials specifically within the FIB foil (as illustrated by the enhanced aromatic distribution map of the FIB foil shown in Fig. 8). XANES spectroscopy performed at the C K-edge on the FIB foil reveals the presence of kerogen particles as well as of aliphatic-rich particles corresponding to the components labelled above as bitumen I, while no particles of bitumen II have been found within the FIB foil. As carbonate functional groups specifically absorb at 290.3 eV (Benzerara et al., 2004) and K-bearing compounds display specific absorption features at 297.2 and 300 eV (de Groot et al., 1990), carbonate crystals and K-bearing

particles have been easily identified from their XANES spectra (Fig. 8). The carbonates of this FIB foil display two XANES spectra, which are different in terms of intensity of the peak at 290.3 eV (1s-pn transitions of carbonate functional groups) relatively to the peak at 300 eV (1s-sn transitions of carbonate functional groups) (Fig. 8). These differences can be related to the relative orientation of the carbonate crystals with respect to the X-ray beam (Zhou et al., 2008; Benzerara et al., in press). Cluster analyses performed on STXM data collected at the C K-edge (see Lerotic et al., 2004, 2005 for details) were used to assess the spatial distribution of the kerogen and bitumen I particles, as well as of carbonate phases and K-bearing compounds present within the FIB section (Fig. 8). The compositional STXM map obtained from these data reveals the strong heterogeneity of the sample at the 30-nm scale as well as the close relationships existing between these different phases (Fig. 8). In particular, the kerogen and the bitumen I appear to be spatially connected (Fig. 8). This spatial relationship between kerogen and bitumen within the FIB foil can be clearly observed on the TEM-EDXS elemental maps reported in Fig. 9. The results also confirm the presence of oxygen within the bitumen phase, while no oxygen is detected within the kerogen. In contrast, traces of sulphur are detected within both kerogen and bitumen. Additionally, the EDXS mapping results allow identifying the nature of the carbonates as well as the different carbon-free minerals present within the FIB foil (Fig. 9). Hence, pure calcite (CaCO3), dolomite (Ca,Mg)(CO3) and ankerite ((Fe,Mg)Ca(CO3)2) can been observed directly associated with OM. The K-bearing phase evidenced by STXM likely corresponds to clays. Quartz (SiO2) and pure albite (NaAlSi3O8) have also been identified in close association with both the kerogen and the bitumen. Noteworthy, the albite phase appears to contain sub-micrometric NaCl-rich inclusions, which can be identified as

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halite crystals. All these phases are present within a region of the sample as small as 2  4 mm2 (Fig. 9). These elemental maps of the FIB foil thus clearly evidence the strong chemical and mineralogical heterogeneity of the investigated sample at the nanometre scale (Fig. 9).

4. Discussion 4.1. Multi-scale chemical and mineralogical heterogeneities: complementarity of the different techniques used in this study The novel multi-scale and multi-technique characterisation of the gas shale performed in this study has documented strong chemical and mineralogical heterogeneities at all scales of observation. All the techniques used here appear as highly complementary. While open-system pyrolysis GC-MS analyses provide bulk molecular-level information, organic petrography and vitrinite reflectance measurements allow identifying the nature and estimating the maturity of the different macerals present within the sample. LA-Py measurements offer molecularlevel information on petrologically distinct constituents. Raman microspectroscopy together with Raman mapping appear to be sensitive to the degree of OM organization and provide spatially resolved information related to the nature of the different inorganic constituents associated with OM at the micrometrescale. STXM is the only currently available method that offers the possibility of both imaging organic and inorganic components of gas shales at a sub-micrometre scale. Furthermore, it allows distinguishing organic components with different compositions as XANES spectroscopy at the C K-edge provides fine chemical signatures of organic compounds at the 25-nm scale. Finally, TEM allows performing elemental mapping, and thus identifying the various inorganic components associated with OM at the nanometer-scale. Some apparent discrepancies between Raman, LA-Py and XANES results are addressed here and can be reconciled. The similarities in Raman spectra collected from different macerals and bitumen indicate that their aromatic skeletons reached the same organization degree during maturation, independent of the nature of the chemical precursor. In contrast, LA-Py measurements show slight chemical variations between the different macerals, as well as compared to bitumen. Furthermore, XANES spectroscopy indicates that different bitumens and macerals contain different functional groups and/or different relative amounts of the same functional groups. LA-Py, Raman and XANES analyses thus clearly provide complementary information for determining the chemical and structural signature of the OM constituting gas shales. It has to be noted that a precise estimation of the relative proportions of different OM types as measured by XANES would require numerous additional measurements. Indeed, the XANES data presented in this study only provide qualitative information regarding the chemical nature of the different types of organic compounds which can be observed within the studied sample, as well as regarding their structural and textural relationships with inorganic phases. Micro-Fourier transform infrared (FTIR) spectroscopy may have been an alternative to LA-Py measurements. FTIR spectroscopy indeed allows recognizing common types of atomic bonds within organic compounds, and thus identifying the nature of their functional groups at the micrometre-scale. However, the LA-Py technique allows molecularly probing petrologically distinct constituents, thus offering the possibility to directly compare the results with open pyrolysis GC measurements performed on bulk samples, this latter technique being classically used in organic geochemistry (e.g., Horsfield, 1989 and references

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therein). In addition, TEM-based electron energy loss spectroscopy (EELS) may have been an alternative technique to XANES spectroscopy, but its poor spectral resolution on conventional TEM ( 0.8 eV), curtailing the detection of carbon functional groups (only a few tenths of eV separate peaks attributed to various C-functional groups), and the difficulty of getting energy-filtered images on tens of micrometres wide areas make it far less attractive than STXM. In addition, radiation damage per unit of analytical information has been shown to be typically 100–1000 times lower in STXM-based XANES spectrocopy than in TEM-based EELS (Rightor et al., 1997; Braun et al., 2005; Hitchcock et al., 2008), thus conferring a clear advantage to STXM and XANES spectroscopy for the study of organic-rich samples. Except for STXM and TEM, all the measurements performed in this study have been done on polished sections; harsh chemical extraction and related artefacts have thus been avoided. As STXM and TEM are transmission techniques, the maximum thickness of a sample cannot exceed about 250 nm so that it does not completely absorb the X-ray or electron incident beam. As illustrated here, FIB milling appears to be an appropriate sample preparation technique for such organic-rich rock samples. Indeed, in contrast to the harsh chemical procedures widely used to extract OM from rock samples, FIB extraction maintains textural integrity even in the case of loosely consolidated materials and preserves the speciation of carbon in complex organic compounds (Bernard et al., 2009). The results obtained from the application of the STXM technique as a probe of the chemistry of macerals and bitumen within gas shales appear extremely promising. Especially, the occurrence of carboxyl functionalities at high maturity is fascinating and, if supported by an ongoing calibration using authentic standards, is possibly indicative of organic–inorganic interactions during maturation. Therefore, although it has to be noted that STXM requires access to a specialised synchrotron radiation beamline (only six STXM beamlines worldwide allowing access to the C K-edge are in operation in 2010), it clearly appears that this technique might provide new insights into gaseous hydrocarbon generation and retention processes within gas shales. In the near future, the various organic compounds constituting such gas shale samples shall also be measured at the O K-edge. Indeed, data acquired in this spectral range will allow the unambiguous identification of the oxygen functionality within different macerals and/or bitumen at the nanometer-scale, and thus facilitate the assignment of spectral features across the C K-edge to specific oxygen-containing carbon functional groups.

4.2. Mineralogical heterogeneities: insights on the thermal history of the investigated sample The occurrence of sparsely disseminated albite crystals has been documented within the investigated sample. These crystals contain very little K and Ca and display a very restricted compositional and structural variability, their composition being consistent with a 99 mol% stoichiometric end-member Ab component. Although rarely abundant volumetrically, these albite crystals are widespread within the investigated sample and no K- or Ca-feldspars have been observed. Based on these character¨ et al. (1999), istics, following Kastner and Siever (1979) and Spotl the albite crystals described here might be identified as authigenic feldspars. Not strictly synonymous with diagenetic, authigenic is commonly used in the literature when referring to minerals that formed in situ during either high-grade diagenesis ¨ or low-grade metamorphism (Kastner and Siever, 1979; Spotl et al., 1999).

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These authigenic albite crystals display nanometric halite inclusions, which may result from the participation of evaporite¨ derived brines in the crystallisation of albite. Indeed, Spotl et al. (1999) have previously identified authigenic albite as a potentially diagnostic tracer of palaeobrine–carbonate-(shale) interactions. In fact, NaCl-type brines have been shown to thermodynamically favour albite over K-feldspar and K-bearing clay minerals (Aagaard et al., 1990; Bazin et al., 1997). Such saline fluid may have constituted the source of Na and Si needed for Albite crystallisation, as the solubility of Si increases with the ionic strength, while Al may have come from the alteration and/or breakdown of clay minerals from the matrix. Once initiated, such feldspar crystallisation might have been maintained by the pH-buffering capacity of carbonates as long as ¨ et al., 1999 and references the ionic supply lasted (e.g., Spotl therein). As the investigated shale formation displays a very low porosity, and thus a very low permeability, fluid access might have been limited. The crystallisation of authigenic albite from brine–carbonate interactions might have thus needed high temperature, and thus occurred during deep burial or incipient metamorphism, at least at higher temperatures than those prevailing in shallowly buried settings. However, the scarcity and size of halite inclusions observed within the authigenic albite described here precludes a precise determination of their temperature of crystallisation. Nevertheless, previous studies have suggested that albite authigenesis is retarded in low porosity carbonate-rich shales relative to feldspar-producing reactions in sandstones which usually display higher porosity (Crampon, ¨ et al., 1999). Such crystallisation of authigenic albite 1973; Spotl in carbonate-rich shales has only been reported in deep burial ¨ et al., 1999), at temperatures settings (e.g., Crampon, 1973; Spotl ranging from high-grade diagenesis (150–200 1C) to incipient metamorphism (200–250 1C). This temperature range is consistent with the temperature qualitatively estimated from the Raman characteristics of the investigated sample OM and kinetics considerations.

4.3. Nanoscale chemical heterogeneities: insights on hydrocarbon generation/retention processes The multiscale methodology used in this study allowed different types of organic materials to be characterised within the investigated sample. The presence of bitumen has been evidenced from organic petrography observations, LA-Py results having confirmed the aliphatic-rich nature of these compounds. In addition, reflectance measurements have indicated that two different types of bitumen might be present. This observation has been confirmed by STXM-based XANES analyses: two different types of bitumen have been observed within the sample, the first one appearing as enriched in aliphatics and oxygencontaining functional groups compared to the second. Various origins have been assigned to solid bitumens, ranging from identification as thermally immature to thermally postmature crude oil, including several stages of possible alteration (e.g., devolatilization, biodegradation) between these two extremes (e.g., Curiale, 1986, and references therein). Understanding the fate of in situ source rock bitumen during maturation is thus very important as far as hydrocarbon generation processes are concerned. Indeed, both the kerogen and bitumen of source rocks contain macromolecular OM, and hence may contribute to petroleum and gas formation (Vu et al., 2008; Mastalerz and Glikson, 2000 and references therein). It has been recently shown that in situ bitumen has a profound influence on the mechanism of petroleum formation, its presence or absence dictating the degree

of condensation, recombination and cross linking in the solid structures via bimolecular reactions (Mansuy et al., 1995; Vu et al., 2008). Here, we briefly consider the origin of the bitumens. The two types of bitumen described here may be derived from two different in situ sources. Being more aliphatic, the bitumen I may be derived from a more aliphatic starting material such as liptinite, while the more aromatic bitumen II could be derived from a more aromatic source such as vitrinite. The chain length distribution of kerogens and genetically related asphaltenes are indeed similar (Muscio et al., 1991). In this case, we would expect bitumen I to be more abundant, because the bulk composition of the organic matter as a whole is Type II, and liptinite strongly predominates. A direct genetic relationship between kerogen and bitumen is entirely plausible from the standpoint of carbon skeletons, but does not take into account the high content of oxygen in the more aliphatic bitumen I. On the other hand, both types of bitumen may come from a unique source, either a liptinite or vitrinite or a mixed liptinite/ vitrinite source. In such a case, the higher aromaticity of the bitumen II might result from the loss of some aliphatic compounds due to generation of gas from the bitumen I, the resultant residue becoming more aromatic. In such a scenario, the bitumen I would represent an early phase, containing only a few aromatic groups and from which no light gaseous compounds have been formed. The bitumen I might thus be seen as a pre-oil bitumen, while the bitumen II would correspond to a post-oil bitumen. Such pre- and post-oil bitumen have been convincingly documented within organic-rich source rocks at the stage of maturation corresponding to the onset of gas generation using biomarker results and micro-FTIR techniques (e.g., Curiale, 1986; Mastalerz and Glikson, 2000). Under such a scheme, pre-oil solid bitumens are early-generation products of rich source rocks, probably extruded from their sources as a very viscous fluid, and migrated the minimum distance necessary to reach fractures and voids in the rock (Curiale, 1986). In contrast, post-oil solid bitumens, usually pyrobitumen, are products of the alteration of a once-liquid crude oil, generated and migrated from pre-oil bitumen and/or mature kerogen, and subsequently degraded (e.g., Curiale, 1986). Indeed, early thermally generated OM are usually rich in heteroatoms and aliphatic groups, viscous, and not susceptible to long flow distances (e.g., Curiale, 1986 and references therein). Highly mature bitumens formed from this NSO-rich precursor are isotropic and exhibit a very fine mosaic (Stasiuk, 1997). Here, the connection observed at the nanometer-scale between the kerogen and the bitumen I supports the pre-oil nature of the bitumen I. Indeed, since the investigated sample exhibits a very low porosity, as it is the case for gas shales worldwide (e.g., Curtis 2002), this pre-oil bitumen might not have been able to migrate over great distance. In contrast, the liquid-crude oil, of which the bitumen II potentially constitutes the altered solid residue, may have migrated further. This would be consistent with the fact that no bitumen II has been observed in close association with macerals within the FIB foil. The possibility of detecting pre- and post-generation bitumen can be very valuable in understanding the role of solid bitumen in oil and gas generation. Indeed, in contrast to conventional petroleum system, pre-oil bitumen as well as newly formed liquid crude oil remain entrapped within gas shales due to the very low porosity of such unconventional source rock. A subsequent increase of temperature may then promote the formation of gas by thermal cracking of these pre-oil bitumen and crude oils. It thus clearly appears here that the characterisation of solid

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bitumen in gas shales may provide new insights on gaseous hydrocarbon generation and retention processes. Additionally, from a reservoir evaluation perspective, although these poorly understood organic deposits constitute a cement, neutron logs record solid bitumen as porosity and resistivity logs may record them as conventional oil (e.g., Hwang et al., 1998). This lack of recognition in logs may cause tremendous difficulties in estimating gas recoverable reserves in gas shales. Furthermore, as the presence of solid bitumen reduces porosity and permeability of reservoirs by filling pore space (Lomando, 1992; Hwang et al., 1998), it can thus cause significant reservoir heterogeneity as shown here and affect ultimate recovery and reservoir response to stimulation. Better understanding the origin and behaviour of solid bitumen within gas shales would thus have important implications in the future for geologic, engineering and economic evaluations of such unconventional reservoirs as it might help better evaluating producibility of bitumen-associated gaseous hydrocarbon.

5. Concluding remarks Significant advances have already been achieved in unravelling the role played by organic matter in shale gas generation and retention (Curtis, 2002; Hamblin, 2006; Jarvie et al., 2007). Many concepts were simply carried over directly from conventional petroleum systems, thereby elucidating the influence of organic sedimentation, maturation, migration and retention on petroleum quality and concentration within the reservoir. However, process understanding in time and space is only fragmentary, especially concerning phase behaviour and adsorptive capacity during polyphasic geological history. The results reported in the current contribution constitute the first step towards unravelling the relationships between kerogen structure, bitumen occurrence, kinetics of transformation into gas and gas retention, all as a function of maturation level. Integration down to the nanometerscale had never been attempted before. Having reached this objective, we can now assert that a multiscale and multitechnique approach is highly likely to open important avenues in the elucidation of how gas shales are formed and how they are likely to behave on production. In particular, as illustrated here, STXM provides a novel and unique way of mapping organic chemical heterogeneities at the 25-nm scale and organic/inorganic phases structural relationships in such complex samples. In parallel, XANES spectroscopy at the carbon K-edge provides information on organic constituent speciation, thereby allowing organic geochemistry to be performed at the same submicrometric scale. In addition, TEM allows performing elemental mapping, and thus identifying the various inorganic components associated with OM at the nanometer-scale In subsequent publications, we shall apply these technologies to known and potential gas shales of the United States and Europe in a regional context, as part of the GeoEn and GASH projects.

Acknowledgements We thank Ulrich Mann for providing the potential gas shale sample as part of the GASH project. We gratefully acknowledge Sue Wirick (NSLS), Karim Benzerara (IMPMC) and Olivier Beyssac (IMPMC) for their help with STXM and Raman experiments. Useful comments by two anonymous reviewers are greatly acknowledged. Funding by BMBF (Grant 03G0671A/B/C) is gratefully acknowledged.

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