• Samples show shale microstructures with silt-sized grains embedded in a fine grained matrix.
• Quartz content is homogeneous throughout the basin, whereas the carbonate content varies.
• Northern European Toarcian shales fall within the carbonate, mixed and argillaceous mudstones.
• No evidence was found that outcrop samples are microstructurally more altered than core samples.
M.E. Houbena, A.Barnhoornb, L. Waschc, J. Trabucho-Alexandrea, C.J.Peacha, M.R.Drurya
aFaculty of Geosciences, Utrecht University, PO-box 80.021, 3508TA Utrecht, The Netherlands. bFaculty of Civil Engineering & Geosciences, Delft University of Technology, PO-box 5048, 2600GA Delft, The Netherlands. cTNO, Princetonlaan 6, 3584 CB Utrecht, The Netherlands
Received 7 January 2016 – Accepted 4 August 2016
The Toarcian (Early Jurassic) Posidonia Shale Formation is a possible unconventional gas source in Northern Europe and occurs within the ClevelandBasin (United Kingdom), the Anglo-Paris Basin (France), the Lower Saxony Basinand the Southwest Germany Basin (Germany), and the Roer Valley Graben, the West Netherlands Basin, Broad Fourteens Basin, the Central Netherlands Basin and the Dutch Central Graben in The Netherlands. Outcrops can be found in the United Kingdom and Germany. Since the Posidonia Shale Formation does not outcrop in the Netherlands, sample material suitable for experimental studies is not easily available. Here we have investigated lateral equivalent shale samplesfrom six different locations across Northern Europe (Germany, The Netherlands,The North sea and United Kingdom) to compare the microstructure and composition of Toarcian shales.
The objective is to determine how homogeneous or heterogeneous the shale deposits are across the basins, using a combination of Ion Beam polishing, Scanning Electron Microscopy and X-ray diffraction.
The work presented here shows that the Toarcian shales of Northern Europe display considerable homogeneity in mineralogy and microstructure in the different investigated samples and formations, where the largest variability is the carbonate content ranging from almost zero up to 80%. We conclude that the outcrop locations in Germany and the United Kingdom are suitable analogues with respect to their mineralogy and microstructure for experimental studies on the Posidonia Shale in the Dutch subsurface.
The Toarcian (Early Jurassic) Posidonia Shale Formation (PSF) is one of the main hydrocarbon source rocks in the North Sea (Germany and The Netherlands) as well as in the Paris Basin (Tissot and Welte, 1978) with a thickness of circa 10 up to 70 m (30 m on average) and an average total organic carbon (TOC) content of 10% (Herber and de Jager, 2010). The lower Toarcian shales were deposited in the Cleveland Basin (Whitby Mudstone Formation), the Lower Saxony Basin, the Southwest Germany Basin (Posidonia Shale Formation) and the Anglo-Paris basin (‘Schistes Carton’; e.g. Rullköter et al., 1988, Littke et al., 1991; Fig. 1).
Fig. 1. Sedimentary logs of the Toarcian deposits of organic matter rich marls and shales deposited in the current North Sea basin and a map of the North Sea basin during the Sinemurian Aalenian modified after Röhl et al. (2001). Sample locations are indicated in the map either by the letters a.–d. corresponding to the logs or by the abbreviations used in the paper (F11, L5 and LOZ). a. Jet rock section of the Whitby Mudstone modified after Hesselbo et al., 2000. b. Posidonia Shale from Dotternhausen (DE) modified after Röhl et al., 2001. c. Schistes Carton from the Paris Basin modified after Emmanuel et al., 2006. d. Posidonia Shale from the Hils Area (DE) modified after Littke et al., 1991.
Lower Jurassic shales in the North Sea Basin are generally dark grey, thin bedded and bituminous, and they were deposited in an epicontinental shelf sea with variable energetic conditions and periodic benthic oxygen depletion (e.g.: Trabucho-Alexandre et al., 2012, French et al., 2014). The PSF in the Hils Area in Germany represents a large maturity range, from very early mature to over mature gas window (Littke et al., 1991). The formation only reaches gas maturity in small isolated synclines in the deepest parts of the West Netherlands Basin and Roer Valley Graben (de Jager et al., 1996).
The lower and upper boundaries of PSF are geochemically and petrographically abruptly bounded by less organic matter rich and less carbonate rich shales at the bottom and top that are displaying a lower hydrogen index than the Posidonia Shale Formation (Littke et al., 1991). Posidonia Shale units are fine-grained with porosities below 10 vol.% (Littke et al., 1991). On mechanically polished thin sections, no obvious visible primary connected pathways that might act as permeable conduits for the expulsion of fluids or gasses, are present in the PSF (Littke et al., 1988).
Interest in the PSF has increased since it was recognised as a possible source for unconventional oil/gas in Northern Europe (e.g.: Herber and de Jager, 2010). The work presented here summarizes the microstructures encountered in PSF samples sampled from cores and outcrops from the current North Sea basin and surrounding countries (Germany, The Netherlands and United Kingdom) with the aim to investigate how microstructurally and mineralogically similar the deposits are throughout the sampled areas.
Our aim is to determine the variability of Toarcian shales in the North Sea Basin and surrounding countries and to validate the use of outcrop samples for experimental studies as mechanical and petrophysical characterization, permeability and swelling/shrinking experiments. Characterization of the samples is a first step to better understanding ways to enhance the permeability of the rock and to increase gas flow from the rock to well.
Posidonia Shale Formation and Whitby Mudstone Formation samples
We investigated two samples from the Loon-op-Zand core (sample numbers S37 and S41; PSF-LOZ) which was cored in the West Netherlands Basin (the Netherlands) in the 1950s. The two samples originate from the lower half of the PSF section. Sample S37 was cored at a depth of circa 2495 m and sample S41 originated from a depth of circa 2507 m. Other PSF samples investigated, originated from the North Sea Basin (L5-4 core; PSF-L5) and were cored in 1985. The PSF interval was cored at a depth of 2824.65 to 2842.40 m. Four 1.2 cm diameter drill-cores and nine thin sections were used for microstructural research.
Furthermore, nine PSF thin sections from well F11-01 (North Sea Basin, Dutch Central Graben; PSF-F11) were investigated, where the samples originated from depths between 2657 and 2672 m. PSF samples were also collected at a quarry in Dotternhausen (Germany) in 2009 (PSF-D). The sample block used for microstructural research was taken in the quarry.
The Whitby Mudstone Formation (WMF) samples were sampled along the cliff coast north of Whitby in the United Kingdom near Runswick Bay and Port Mulgrave (see also Zhubayev et al., 2016). Location of the different samples can be found in Fig. 1. All samples were stored under atmospheric conditions and hence were air-dried during sample storage, either up to 60 years for some of the PSF samples or a couple of months for the WMF samples.
Scanning Electron Microscopy
Both mechanical polished thin sections (circa 2.5 × 4 cm) produced using conventional methods and argon ion beam polished samples using a Precision Ion Polishing System (PIPS; 8 mm in diameter; Houben et al., 2016) were investigated with Scanning Electron Microscopes (SEM: FEI XL30S FEG-SEM; FEI Nova 600 Nanolab; JEOL Neoscope II JCM-6000). All samples were polished perpendicular to the bedding.
Identification of pores and small minerals requires both good spatial and contrast resolution, which depends on the quality of the final sample polish, the SEM instrument, its detector capabilities and the imaging conditions used. The mechanically polished thin sections displayed good enough spatial resolution (300 nm pixel size, meaning minerals > 1 μm in diameter could be imaged) to get information about the mineralogy and the microstructure of the sample.
PIPS polished SEM samples were used to investigate both the mineralogy and the porosity (circa 25 nm pixel size, meaning only pores with diameters > 100 nm were imaged). With a Back Scattered Electron (BSE) detector density contrast in the micrographs was displayed to investigate the mineralogical variation in the samples in combination with an Energy-dispersive X-ray (EDX) detector. The grey scales in a BSE image from black to white correspond to: pores/cracks, organic matter, matrix/clay minerals/quartz/feldspar, calcite/dolomite, pyrite (see also Klaver et al., 2015a, Houben et al., 2016).
In addition, the SEM’s Secondary Electron (SE) detector has been used to image the pores in the PIPS-SEM samples. In order to make high resolution images of the PIPS-SEM samples in combination with imaging a large area, single SEM images were combined into one high resolution mosaic image using Microsoft Image Composite Editor.
For the PIPS-SEM samples a low magnification (200 ×) was used to make an overview image of the whole PIPS polished cross-section. Subsequently high resolution mosaics (magnification 5.000 ×) were made, aimed at mineralogically different layers, to image the 2D sample microstructures using the BSE detector. The high resolution BSE and SE mosaics were made in regions in between the largest cracks. The microstructures present in the mechanically polished thin sections were imaged with a slightly different approach; a single BSE image covering circa 300 × 400 μm2 was made (pixel size 300 nm).
The image was made in an area without any large cracks running through the entire polished section. In combination with EDX maps for the elements Si, Al, Ca, Mg, Fe, Na, K, and S the mineralogy in that single image was identified. Visible minerals (grains with diameters > 2 μm; silicates (quartz, feldspar), carbonates (calcite, dolomite), pyrite, organic matter) in both the thin sections and PIPS-SEM samples were segmented, either by using a combination of thresholding and edge detection in MATLAB 2010 (for pyrite and organic matter only) or manually using ArcMAP 10.1 (silicates, carbonates, mica’s and clay minerals) by making use of the BSE images and EDX maps.
Qmineral Analysis & Consulting (Heverlee, Belgium) identified and quantified both the bulk mineral composition and the mineralogy of the clay fraction. Samples were oven dried (40 °C) and ground before X-ray diffraction using CuKα radiation (Środoń et al., 2001). During the first stage of the experiment bulk mineralogy was investigated with Rietveld analysis (Rietveld, 1967) using the TOPAS software from Bruker. During the second stage of the experiment 5 g of the initial sample material was selectively disintegrated from aggregates that incorporate clay minerals (Jackson treatment; Jackson et al., 1976) and the fraction of the sample smaller than 2 μm was dissociated from the bulk by centrifugation. Oriented clay samples were prepared and used for the XRD analysis where quantification was done using the PONKCS-methods in the NEWMOD2 software (Moore and Reynolds, 1997, Scarlett and Madsen, 2006).
A selection of microstructures encountered in the investigated samples can be found in Fig. 2, Fig. 3. Comparing the sample overviews (Fig. 2) shows that some samples are more fractured than others. Whether these fractures are due to drilling, drying, sample preparation, or a combination of all is not clear, but the fractures are interpreted as not being part of the in-situ microstructure (see also Houben et al., 2013). Smaller cracks with widths up to about 1 μm account for up to 20% of the visible porosity in the PIPS-SEM mosaics, where some samples are more affected than others.
Microcrack shape, length and amount seems to be related to the microstructure/cementation of the sample and not to storage or drying time since no trend is visible that longer storage and hence longer drying or origination from lower depths generates more cracks in the microstructure of the different samples.
Fig. 2. Three PIPS polished SEM samples with sample diameters of 8 mm. a. WMF 4 with a large crack running through the sample along the bedding. b. PSF-L5-3 a mineralogically layered sampled on the sub-mm scale. c. PSF-D typical intact sample with larger fossil fragments visible in the sample overview.
Most cracks run through the matrix and surround the larger minerals. In addition, some sample overviews display mineralogically diverse layers easily differentiated based on the overall grey colour present in that certain layer in the overview (e.g. Fig. 2b) and the different microstructures in the high resolution mosaics (Fig. 3). Most samples show a dark grey fine grained matrix wherein organic matter (black) and pyrite (white) are easily distinguished from the background and single quartz, mica, calcite and dolomite grains are also present mostly floating within the matrix (Fig. 3).
Fig. 3. A selection of the microstructures encountered in the PSF and WMF samples from different locations. As a comparison for WMF15 (bottom row) high resolution mosaics made within one PIPS-SEM sample are displayed illustrating the difference in microstructure not only encountered in samples from different locations but also from within one sample. Sample locations within the basin can be found in Fig. 1.