The potential of shale reservoirs for gas extraction is largely determined by the permeability of the rock. Typical pore diameters in shales range from the μm down to the nm scale. The permeability of shale reservoirs is a function of the interconnectivity between the pore space and the natural fracture network present. We have measured the permeability of the Whitby Mudstone, the exposed counterpart of the Posidonia Shales buried in the Dutch subsurface and a possible target for unconventional gas, using different methods and established a correlation with the microstructures and pore networks present down to the nanometer scale. Whitby Mudstone is a clay rich rock with a low porosity. The permeability of the Whitby Mudstone is in the range of 10−18m2–10−21m2. 2D microstructures of the Whitby Mudstone show no connected pore networks, but isolated pore bodies mainly situated in the clay matrix, whereas 3D data shows that connected pore networks are present in less compacted parts of the rock. A closely spaced interconnected fracture network is often required to speed up transport of fluids from the matrix into a producing well. For fluids within the matrix the nearest natural fracture is on average at a distance of approximately 10cm in the Whitby Mudstone. The combination of the permeability data and the porosity data with natural fracture spacing of the fractures present in outcrops along the Yorkshire coast (UK) resulted in new insights into possible fluid pathways from reservoir to well.@en

The Toarcian (Early Jurassic) Posidonia Shale Formation is a possible unconventional gas source in Northern Europe and occurs within the Cleveland Basin (United Kingdom), the Anglo-Paris Basin (France), the Lower Saxony Basin and 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 samples from 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.@en

Finding generic trends in mechanical and physical rock properties will help to make predictions of the rock-mechanical behaviour of shales. Understanding the rock-mechanical behaviour of shales is important for the successful development of unconventional hydrocarbon reservoirs. This paper presents the effect of heterogeneities in mineralogy and petrophysical properties on the validity of generic trends on multiple scales in rock-mechanical and rock-physical properties of the Whitby Mudstone. Rock-mechanical laboratory experiments have been performed on Whitby Mudstone samples from multiple outcrops within five kilometres laterally in order to investigate the heterogeneity and possible trends on an outcrop scale. Unconfined compression tests and acoustic measurements have been conducted to obtain the rock-mechanical properties, including rock strength, Young's modulus, Poisson's ratio, and velocity anisotropy. The rock-physical properties, including mineralogy, porosity, and matrix density, were measured using X-ray fluorescence and helium pycnometry. Various methodologies have been applied to the resultant data in order to derive different brittleness indices. Significant heterogeneity in rock-mechanical and rock-physical properties is present on an outcrop scale. There is no obvious correlation between mineral content and rock-mechanical properties on an outcrop scale in the Whitby Mudstone. Comparison with shales from different basins show, however, correlations between composition and elastic properties. The presence of significant heterogeneities on an outcrop scale and between shales from different basins make it difficult to find generic trends in rock-physical and rock-mechanical properties.@en

Finding generic trends in mechanical and physical rock properties will help to make predictions of the rock-mechanical behaviour of shales. Understanding the rock-mechanical behaviour of shales is important for the successful development of unconventional hydrocarbon reservoirs. This paper presents the effect of heterogeneities in mineralogy and petrophysical properties on the validity of generic trends on multiple scales in rock-mechanical and rock-physical properties of the Whitby Mudstone. Rock-mechanical laboratory experiments have been performed on Whitby Mudstone samples from multiple outcrops within five kilometres laterally in order to investigate the heterogeneity and possible trends on an outcrop scale. Unconfined compression tests and acoustic measurements have been conducted to obtain the rock-mechanical properties, including rock strength, Young's modulus, Poisson's ratio, and velocity anisotropy. The rock-physical properties, including mineralogy, porosity, and matrix density, were measured using X-ray fluorescence and helium pycnometry. Various methodologies have been applied to the resultant data in order to derive different brittleness indices. Significant heterogeneity in rock-mechanical and rock-physical properties is present on an outcrop scale. There is no obvious correlation between mineral content and rock-mechanical properties on an outcrop scale in the Whitby Mudstone. Comparison with shales from different basins show, however, correlations between composition and elastic properties. The presence of significant heterogeneities on an outcrop scale and between shales from different basins make it difficult to find generic trends in rock-physical and rock-mechanical properties.@en

The presence of water in mudrocks has a largely negative impact on production of gas stored in these rocks, due to the fact that water causes swelling of the rock. Removing the water from the mudrock could potentially shrink the rock and increase the overall permeability of the rock. Investigation of the swelling/shrinkage behaviour of the rock during exposure to water vapour is of key importance in designing and optimizing unconventional production strategies. We have used outcrop samples of the Whitby Mudstone and the Posidonia shale, potential unconventional sources for gas in North-western Europe, to measure the swelling and shrinkage behaviour. Swelling and shrinkage of the rocks when exposed to water vapour was measured directly using 1 mm sample cubes in two different setups. The mm cubes were exposed to different levels of relative humidity either in an Environmental Scanning Electron Microscope (ESEM) or in a 3D dilatometer. Swelling of Whitby Mudstone and Posidonia shale is heterogeneous with 2–3 times more measured swelling strain perpendicular to the bedding. Volumetric swelling strains showed values between 0.6 and 2.2% for the Whitby mudstone and the Posidonia shale, respectively. The results suggest that it might be possible to increase permeability in the reservoir by decreasing the in-situ water activity due to shrinkage of the matrix.@en

As part of a study to investigate methods to enhance pore/crack connectivity between the shale matrix and the induced fractures, we have investigated the matrix microstructure of an exposed analogue of the Jurassic Posidonia shales in the Dutch sub-surface. A combination of Precision-Ion- Polishing and Scanning Electron Microscopy has been used to image the in-situ porosity and mineralogy in shale samples from Whitby (UK), which are an analogue for the Dutch Posidonia shale. First results show a fine-grained mudstone with cm-sub-mm scale stratification. The section at Whitby can be divided into a clay matrix dominated upper half and a coarser grained, calcite-rich, lower half of the section. Commonly occurring minerals are pyrite, calcite (fossils, grains and cement), quartz, mica and dolomite. Organic matter content varies from 0 -2 % in the calcite dominated layers to 5 -18% in the clay matrix dominated layers. The most porous phases are the clay matrix and calcite fossils. This microstructural study shows which intervals within Posidonia shale contain the largest porosity and organic matter contents for sweet spot analyses and forms a basis for future research on enhancing connectivity between the pores and induced fractures.@en

As part of a study to investigate methods to enhance pore/crack connectivity between the shale matrix and the induced fractures, we have investigated the matrix microstructure of an exposed analogue of the Jurassic Posidonia shales in the Dutch sub-surface. A combination of Precision-Ion- Polishing and Scanning Electron Microscopy has been used to image the in-situ porosity and mineralogy in shale samples from Whitby (UK), which are an analogue for the Dutch Posidonia shale. First results show a fine-grained mudstone with cm-sub-mm scale stratification. The section at Whitby can be divided into a clay matrix dominated upper half and a coarser grained, calcite-rich, lower half of the section. Commonly occurring minerals are pyrite, calcite (fossils, grains and cement), quartz, mica and dolomite. Organic matter content varies from 0 -2 % in the calcite dominated layers to 5 -18% in the clay matrix dominated layers. The most porous phases are the clay matrix and calcite fossils. This microstructural study shows which intervals within Posidonia shale contain the largest porosity and organic matter contents for sweet spot analyses and forms a basis for future research on enhancing connectivity between the pores and induced fractures.@en