Abstract
Variability in the Lower Bowland shale microstructure is investigated here, for the first time, from the centimetre to the micrometre scale using optical and scanning electron microscopy (OM, SEM), X-Ray Diffraction (XRD) and Total Organic Carbon content (TOC) measurements. A significant range of microtextures, organic-matter particles and fracture styles was observed in rocks of the Lower Bowland shale, together with the underlying Pendleside Limestone and Worston Shale formations encountered the Preese Hall-1 Borehole, Lancashire, UK. Four micro-texture types were identified: unlaminated quartz-rich mudstone; interlaminated quartz- and pyrite-rich mudstone; laminated quartz and pyrite-rich mudstone; and weakly-interlaminated calcite-rich mudstone. Organic matter particles are classified into four types depending on their size, shape and location: multi-micrometre particles with and without macropores: micrometre-size particles in cement and between clay minerals; multi-micrometre layers; and organic matter in large pores. Fractures are categorized into carbonate-sealed fractures; bitumen-bearing fractures; resin-filled fractures; and empty fractures. We propose that during thermal maturation, horizontal bitumen-fractures were formed by overpressuring, stress relaxation, compaction and erosional offloading, whereas vertical bitumen-bearing, resin-filled and empty fractures may have been influenced by weak vertical joints generated during the previous period of veining. For the majority of samples, the high TOC (>2 wt%), low clay content (<20 wt%), high proportion of quartz (>50 wt%) and the presence of a multi-scale fracture network support the increasing interest in the Bowland Shale as a potentially exploitable oil and gas source. The microtextural observations made in this study highlight preliminary evidence of fluid passage or circulation in the Bowland Shale sequence during burial.
Highlights
Characterisation of the properties of shales is an essential process in the estimation of their gas and oil resource potential
The major goal of this study is to present the first multi-scale and petrologic characterisation of the different microtextures of Bowland shale encountered in the Preese Hall-1 borehole, a significant and recent exploration borehole in Lancashire, NW England, using optical and scanning electron microscopy in combination with X-Ray Diffraction (XRD) and Total Organic Carbon content (TOC) analysis
Microtexture types were defined according to the thin section observations, on the basis of (a) the absence or presence of laminae, (b) lamina thickness, (c) grain type, (d) grain properties and (e) the structure, content and nature of organic matter particles
Summary
Characterisation of the properties of shales is an essential process in the estimation of their gas and oil resource potential. Important shale gas and oil resources are believed to exist in Western Europe (EIA, 2013) and in particular in the UK (EIA, 2015a). From the millimetre to the nanometre scale, shale gas is stored as free gas in natural fractures, within both intergranular and intra-granular porosity and adsorbed onto kerogen and clay-particle surfaces. It may be dissolved in kerogen and bitumen (Curtis, 2002). Data are sparse for the Bowland and Hodder Shales, despite forming an important potential oil and gas resource. Detailed petrographic studies of these shales are required to allow a better comparison with other shale formations, especially the relatively well understood shale reservoirs from northern America
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