Bowland Shale Research Articles

Baseline surface- and groundwater monitoring prior to an onshore shale gas operation in the Vale of Pickering, UK

A comprehensive programme of baseline groundwater hydrochemical monitoring has been carried out in connection with the proposed hydraulic fracturing of a 2–3 km deep Bowland Shale gas reservoir in borehole KM8 at Kirby Misperton, North Yorkshire, UK. The monitoring infrastructure encompassed five on-site boreholes with hydraulically open intervals ranging from shallow weathered cover to a c. 200 m deep Corallian limestone aquifer, six off-site wells (hydraulically open in superficial materials and/or Kimmeridge Clay) and four surface-water monitoring stations. Groundwater chemistry was high stratified with depth, ranging from slightly acidic, fresh, very hard Ca–HCO 3 –SO 4 waters in shallow weathered cover, to brackish, calcium-depleted, highly alkaline waters in the Corallian aquifer. Dissolved methane was detected in most boreholes, with 10 μ g l −1 being typical of shallow boreholes and around 50 mg l −1 in the Corallian. Low ethane concentrations and isotopic evidence suggest that the methane was predominantly microbial in origin (carboxylate fermentation at shallow depth, natural methanogenic CO 2 reduction at greater depth). Elevated dissolved ethane (20–30 μ g l −1 ) was found in one well of intermediate depth, suggesting admixture of a possible thermogenic component, although this could be derived directly from the Kimmeridge Clay penetrated by the well.

3D seismic interpretation and fault slip potential analysis from hydraulic fracturing in the Bowland Shale, UK

The Bowland Shale Formation is one of the most promising targets for unconventional exploration in the United Kingdom, with estimated resources large enough to supply the country's entire natural gas consumption for 50 years. However, development of the Bowland Shale has stalled due to concerns over hydraulic-fracturing-induced seismicity. Only three wells have been drilled and hydraulic-fractured to date in the Bowland Shale, and all three have produced levels of seismicity of sufficient magnitude to be felt at the surface. Susceptibility to induced seismicity will be determined by the presence of critically stressed faults. However, such faults can go undetected in conventional interpretation of 2D or 3D seismic surveys if they are shorter than the resolution retrievable from a seismic survey, or if they have low (and in some cases even zero) vertical displacement. In such cases, the faults that cause induced seismicity may only be visible via microseismic observations once they are reactivated. To better identify fault planes from 3D seismic images, and their reactivation potential due to hydraulic fracturing, a high-resolution fault-detection attribute was tested in a 3D seismic survey that was acquired over the Preston New Road site, where two shale-gas wells were hydraulic-fractured in the Bowland Shale in 2018 and 2019, obtaining fault planes with lengths between 400 and 1500 m. Fault slip potential was then estimated by integrating the obtained faults with the formation's stress and pore pressure conditions (with the Bowland shale also being significantly overpressured), and several critically stressed faults were identified near the previously hydraulic fractured wells. However, the faults that induced the largest seismic events in the Preston New Road site, of c. 200 m in length for seismic events of magnitudes below 3.0 (as imaged with a multicomponent, downhole microseismic monitoring array deployed during the hydraulic-fracturing stimulations), could not be identified in the 3D seismic survey, which only mapped fault planes larger than 400 m in length.

Nanoscale geochemical heterogeneity of organic matter in thermally-mature shales: An AFM-IR study

The composition and characteristics of organic matter within shales can reveal important information on hydrocarbon generation, migration and accumulation processes during catagenesis. It may also impact upon the chemical properties and mechanical behavior of shales acting as hydrocarbon reservoirs or caprocks/seals for carbon and subsurface energy storage. Here we apply advanced atomic force microscopy-based infrared spectroscopy (AFM-IR) to capture the in-situ chemical structure of nanoscale molecular composition of individual organic particles and map the distribution of typical functional groups nondestructively; something that cannot be achieved via conventional analysis since some methods damage the sample, some only collect bulk measurements and some do not achieve nano-scale resolution. The nanoscale geochemical characteristics of organic matter in thermally mature samples of a Baltic Basin shale, from Lithuania and the Bowland Shale, from the UK, obtained from AFM-IR, are compared in this study. Significant chemical heterogeneity can be observed in migrated solid bitumen. In contrast, graptolite, alginite and inertinite generally display homogeneous molecular composition. The potential role of laminae as barriers to the vertical migration of hydrocarbons is also apparent. This research provides insights into nanoscale geochemical information on different organic matter types in thermally-mature shales to enhance the understanding of molecular structure transformation of organic matter during hydrocarbon expulsion and migration processes, and also sheds light on implications of nanogeochemistry for enhanced gas recovery, carbon sequestration and underground hydrogen storage-related energy applications. We also characterised specific materials which have not been reported in other geological studies using AFM-IR. Some of them display similar characteristics to other organic matter particles in the samples, but the possibilities of unknown organic contaminants introduced to the sample during sample storage or preparation stages cannot be ruled out. A comprehensive analysis on this undefined organic material is provided. Standardized approaches during sample preparation and storage are required for future research to dislodge potential surface-attached contaminants.

Determining reservoir intervals in the Bowland Shale using petrophysics and rock physics models

SUMMARY An evaluation of prospective shale gas reservoir intervals in the Bowland Shale is presented using a wireline log data set from the UK's first shale gas exploration well. Accurate identification of such intervals is crucial in determining ideal landing zones for drilling horizontal production wells, but the task is challenging due to the heterogeneous nature of mudrocks. This heterogeneity leads to stratigraphic variations in reservoir quality and mechanical properties, and leads to complex geophysical behaviour, including seismic anisotropy. We generate petrophysical logs such as mineralogy, porosity, and organic content and calibrate these to the results of core studies. If ‘reservoir quality’ is defined by combined cut-offs relating to these parameters, we find that over 100 m of reservoir quality shale is present in the well, located primarily within the upper section. To examine the link between geophysical signature and rock properties, an isotropic rock physics model is developed, using effective medium theories, to recreate the elastic properties of the shale and produce forward-looking templates for subsequent seismic inversion studies. We find that the mineralogical heterogeneity in the shale has a profound impact on modelled elastic properties, obscuring more discrete changes due to porosity, organic content and water saturation and that the best reservoir quality intervals of the shale bear a distinctive response on rock physics cross-plots. Finally, we consider the density of natural fractures in the shale by developing an anisotropic rock physics model to reflect high-angle fractures observed on micro-imagery logs. We invert crack density using shear wave splitting well log data and find a crack density of up to 4 per cent which correlates well with micro-imagery observations. Our work further supports previous authors’ core-based studies in concluding that the Bowland Shale holds good reservoir characteristics, and we propose that there are multiple intervals within the shale that could be targeted with stacked horizontal wells, should those intervals’ mechanical properties also be suitable and there be adequate stress barriers between to restrict vertical hydraulic fracture growth. Finally, our rock physics templates may provide useful tools in interpreting pre-stack seismic data sets in prospective areas of the Bowland Shale and picking the best locations for drilling wells.

Bowland Shale well placement strategy – Part 1: Determining landing intervals using geomechanical properties

The production performance of a shale reservoir is directly affected by the geomechanical characteristics of the formation. A target shale interval will ideally develop hydraulic fractures upon stimulation that stay open with the aid of injected proppant. However, shales are geomechanically complex due to heterogeneities in their rock properties such as mineralogy and porosity and the extent to which they may be naturally fractured. These characteristics can complicate the task of identifying the ideal target interval for placing horizontal wells. Whilst the Bowland Shale is the UK's most prospective shale gas target, long horizontal wells are generally not feasible or practical in the Craven Basin, due to the existence of many, large-offset reverse faults and high bedding dips. An alternative to this approach could include drilling shorter, stacked horizontal wells targeting different stratigraphic intervals. However, it is unclear if there are enough intervals within the stratigraphic section with the desired geomechanical properties to target with stacked horizontal wells, nor if there are adequate intervals that can limit vertical hydraulic fracture growth between those wells. The absence of the latter may ultimately lead to well interference and reduced production. These issues were addressed by the creation of a series of wireline log-based geomechanical logs at well Preese Hall-1, calibrated to pressure test data. Aided by the results of a cluster analysis model, the upper section of the Bowland Shale was classified into geomechanical zones to identify the optimal intervals for hydraulic fracturing and barriers to vertical hydraulic fracture growth. Three intervals are highlighted with low effective stress, low fracture toughness and high brittleness which may form excellent landing zones. Importantly, these landing zones are also separated by intervals of high effective stress that may limit vertical hydraulic fracture growth and mitigate the risk of well interference.

Linking multi-scale 3D microstructure to potential enhanced natural gas recovery and subsurface CO2storage for Bowland shale, UK

This paper quantitatively characterises the microstructure in shales across five scales in 3D, builds a multi-scale model of CH4and CO2flow pathways and storage, and assesses the potential of enhanced gas recovery and CO2 storage simultaneously.

Interplay of southern, western, and northern sources during deposition of North Wales Carboniferous sandstones, determined from heavy minerals, mineral chemistry, and detrital zircon ages

A study of heavy mineral assemblages, rutile compositions, and detrital zircon age populations in Namurian to Westphalian sandstones in North Wales, deposited in the southwestern part of the Pennine Basin of the United Kingdom, has revealed considerable complexity in their provenance history. The Pendleian to Marsdenian Cefn‐y‐fedw Sandstone Formation, which is known to have been derived from the Wales–Brabant Massif (WBM) to the south on the basis of palaeocurrent and petrographic information, has a heavy mineral assemblage characteristic of this southerly provenance, and includes Late Neoproterozoic zircons apparently of local WBM origin. However, the formation is dominated by zircons that were most probably recycled from sandstones of northern origin from the Caledonian Belt that had been previously deposited over the massif during Late Devonian times, and, as such, has a comparable history to the sedimentologically‐equivalent Morridge Formation of the English Midlands. Heavy mineral analysis shows that sandstones in the Brigantian Minera Formation and Yeadonian part of the Bowland Shale Formation are of similar southern provenance. The Yeadonian Gwespyr Sandstone, previously interpreted as having a northern origin, similar to the time‐equivalent Rough Rock, has variations in heavy mineral analysis and zircon spectrum that suggest a complex provenance history and includes sand bodies with northern, western, and southern origins.

Geochemical element mobilisation by interaction of Bowland shale with acidic fluids

Hydraulic fracturing is widely used to exploit unconventional hydrocarbon sources, to enhance exploitation of geothermal energy and to aid in carbon sequestration through underground storage of captured CO2. The hydraulic fracturing fluids, which are commonly acidic, cause dissolution of minerals and desorption of elements which can lead to groundwater contamination. Batch reactor experiments were conducted to explore the interaction of simulated fracturing fluids with two end member compositions of basinal shales of the Bowland-Hodder unit (Carboniferous, UK) whereby the impact of temperature, fluid acidity, and rock/fluid ratio conditions were investigated. The results demonstrate that the fluid acidity is mainly controlled by the oxidative dissolution of pyrite and the dissolution of calcite, impacting mobilisation and fate of major and trace elements. The dissolution of calcite and pyrite significantly dominates the leaching of Sr and As, respectively. Generally, increased fluid acidity and temperature facilitate element mobilisation due to enhanced mineral dissolution and ion desorption, whereas higher rock/fluid ratio (higher mass of carbonate minerals) raises the buffering capacity and may promote the immobilisation of some metal ions by adsorption and precipitation (e.g. Ba, Pb, Fe, Al, and Mn). Moreover, the surface topography of different minerals in polished shale sample sections after fluid-rock interaction indicates that mineralogical compositions may play an important role in determining the pore structure. This research identifies chemical reaction pathways of geochemical elements (including contaminants) in fracturing fluids over a range of fluid chemistries and environmental conditions, and helps to evaluate element mobilisation from shale reservoirs with differing mineralogies.

Micro- and Nano-Scale Pore Structure in Gas Shale Using Xμ-CT and FIB-SEM Techniques

Shale is a complex rock composed of a complex mixture of matrix minerals and kerogen and having a complex pore microstructure. The pore microstructure is highly dependent upon the scale at which it is considered. Such a microstructure is important for the assessment of the potential of gas shales based on the connectivity and pores at each scale and the ability of the rock to be hydraulically fractured. In this work, the three-dimensional (3D) structure of Bowland shale has been investigated at both microscopic and nanoscopic scales on the same sample for the first time using (i) a combination of serial sectioning, using focused ion beam (FIB) milling and scanning electron microscopy (SEM), and (ii) X-ray micro-computed tomography (Xμ-CT). The reconstructed matrix, kerogen, and pore space volumes from each approach showed significant scale-dependent differences in the microstructure. The shale samples displayed a high kerogen content with high connectivity. Porosity in the shale rock sample was observed to be prevalent in either the inorganic matrix, the kerogen, or both. Furthermore, the porosity from the reconstructed shale volumes was found to vary with locations, as sampled by FIB-SEM, within the shale samples taken for Xμ-CT. Pore volume, scale invariant surface area to volume ratios, and two orthogonal pore aspect ratio distributions were extracted from the reconstructed image data by 3D image analysis. These data show that voids within the rock are oblate at all scales. However, the smaller pores visible by FIB-SEM present higher scale invariant surface area to volume ratios, indicating that they are more likely to interlink the larger pores visible by Xμ-CT and form a small scale but highly connected pore network for fluid flow. Permeabilities have been calculated from both the FIB-SEM and Xμ-CT images and fall in the range 2.98 to 150 nD, broadly agreeing with experimental determinations from another author.

Origin and implications of early diagenetic quartz in the Mississippian Bowland Shale Formation, Craven Basin, UK

Silica cementation exerts a key control on the compaction and geotechnical properties of mudstones, and by extension, the style of hydrocarbon and/or mineral systems present in a given sedimentary basin. Integrated microscopic and bulk geochemical observations demonstrate that siliceous mudstones in the Bowland Shale Formation, a target for UK shale gas extraction, exhibit abundant dispersed, discrete, μm-scale quartz cements, and exhibit silica enrichment (‘excess’) above a local detrital Si/Al threshold of 2.5. Dissolution of siliceous radiolarian tests during early diagenesis is identified as the main source of silica (opal A) required for quartz precipitation, either via opal CT or directly to quartz, and potentially generated as a product of anoxic marine ‘weathering’ (dissolution) of reactive silicates during early diagenesis. Excess silica correlates with free hydrocarbons (S1) normalised to total organic carbon (oil saturation index; OSI); we propose early diagenetic quartz precipitation suppressed pore collapse (‘buttress effect’), retaining the pore space capacity to host oil. Quartz precipitation was likely catalysed, for example via low porewater pH, elevated Al and/or Fe oxide content, and/or abundant labile organic matter. Juxtaposition of siliceous mudstones and mudstones lacking quartz cement indicates silica was immobile beyond the bed scale. Thus metre-scale siliceous packages likely represent more prospective units within the Bowland Shale (in terms of unconventional hydrocarbons), on the basis of early diagenetic biogenic-derived quartz cementation leading to improved hydrocarbon storage capacity coupled to enhanced brittleness. These findings are relevant for shale oil and shale gas systems, specifically where oil retained in pores subsequently cracks to generate gas. These findings also suggest the Bowland Shale is a sub-class of black shale, defined by the potential to host a relatively large volume of early diagenetic fluids, derived from anoxic bottom waters, which were potentially S- and/or metal-bearing. This is potentially relevant for understanding the genesis of adjacent and related Pb-Zn mineral deposits.

Structural constraints on Lower Carboniferous shale gas exploration in the Craven Basin, NW England

Detailed interpretation of a 3D seismic data volume reveals the detrimental effect that post-depositional tectonic deformation has had on buried Lower Carboniferous (Dinantian–Namurian) shales and its consequences for shale gas exploration in the SW part (Fylde area) of the Craven Basin in NW England. The structural styles primarily result from Devono-Carboniferous (syn-sedimentary) extension, post-rift subsidence and Variscan inversion, a renewed phase of Permo-Triassic extension, and Cenozoic uplift and basin exhumation. In contrast to the shallow dips and bedding continuity that characterizes productive shale gas plays in other basins (e.g. in the USA and Argentina), our mapping shows that the area is affected by deformation that results in the Bowland Shale Formation targets being folded and dissected into fault-bound compartments defined by SW–NE striking (Lower Carboniferous and Variscan) reverse faults and SSW–NNE to N–S striking (Permo-Triassic) normal faults. The fault networks and the misalignment between the elongate compartments they contain and the present-day minimum horizontal stress orientation limit the length over which long lateral boreholes can remain in a productive horizon, placing an important constraint on optimal well positioning, reducing the size of the shale gas resource and affecting well productivity. Our subsurface mapping using this high-fidelity dataset provides an accurate picture of the Upper Palaeozoic structure and demonstrates that faulting is denser and more complex than apparent from geological mapping of the surface outcrop. That structural complexity has direct and significant consequences for: the location of well pads; the lateral continuity of target shale gas horizons; the evaluation of the risk of inducing seismicity on seismically resolvable (large displacement) fault planes prior to drilling; and the likelihood of faults with small throws (below seismic resolution) being present.

Shale oil and gas resource evaluation through 3D basin and petroleum systems modelling: a case study from the East Midlands, onshore UK

Technological advances in horizontal drilling and hydraulic fracturing have led to a re-evaluation of the UK Carboniferous sequences for shale oil and gas potential. In the Gainsborough Trough, hemipelagic mudstones known collectively as the Bowland Shale were deposited during the Pendleian Substage (Late Mississippian). In this study the interpretation of heritage 2D and recent 3D seismic data allowed the reconstruction of the tectonic evolution of the basin, which was simulated in a 3D basin and petroleum systems model. The model enabled the first prediction of generated, adsorbed, retained and expelled hydrocarbon volumes. Between 8 and 26 Bbbl of STOIIP, and between 11 and 38 tcf of GIIP have been estimated to lie within the Bowland Shale in the Gainsborough Trough. However, at the present time, there is considerable uncertainty concerning these in-place volumes, and no tests have proven the recoverability of oil and gas from the Bowland Shale in this area. Importantly, the Bowland Shale has been modelled as a single homogeneous layer, and the in situ volume numbers need to be corrected for a net to gross factor, once the criteria required for the definition for net reservoir in this formation are better understood.

A Mississippian black shale record of redox oscillation in the Craven Basin, UK

Early diagenetic redox oscillation processes have been rarely recognised in the ancient rock record but potentially exert an important control on mineral authigenesis, hydrocarbon prospectivity and supply of metals and/or reduced S as part of associated mineral systems. The upper unit of the Mississippian Bowland Shale Formation is a candidate record of diagenetic redox oscillation processes because it was deposited under a relatively high sediment accumulation rate linked to a large delta system, and under dominantly anoxic and intermittently sulphidic bottom-water conditions. In order to characterise the syngenetic and early diagenetic processes, sedimentological and geochemical data were integrated through the Upper Bowland Shale at three sites in the Craven Basin (Lancashire, UK). Organic matter (OM) comprises a mixture of Type II, II-S, II/III and III OM. ‘Redox zones’ are defined by patterns of Fe-speciation and redox-sensitive trace element enrichment and split into two groups. ‘Sulphidic’ zones (EUX, AN-III, AN-I and AN-IT) represent sediments deposited under conditions of at least intermittently active sulphate-reduction in bottom-waters. ‘Non-sulphidic’ zones (OX-RX, OX-F and OX) represent sediments deposited under non-sulphidic (oxic to ferruginous anoxic) bottom-waters. Operation of a shelf-to-basin ‘reactive Fe’ (FeHR) shuttle, moderated by sea level fluctuation and delta proximity, controlled the position and stability of redoxclines between zones of Fe and sulphate reduction, and methanogenesis. Early diagenetic redoxclines were capable of migration through the shallow sediment column relatively quickly, in response to sea level fluctuation. Preservation of syngenetic and early diagenetic geochemical signals shows redoxclines between Fe and sulphate reduction, and the upper boundary of sulphate-methane transition zone, were positioned within decimetres (i.e., 10 s cm) of seabed. Falling sea level and increasing FeHR supply is recognised as a switch from zones EUX (high sea level), AN-III and ultimately AN-I and AN-IT (low sea level). Zone AN-I defines the operation of ‘redox oscillation’, between zones of Fe and sulphate reduction in shallow porewaters, associated with enhanced degradation of OM and complete dissolution of primary carbonate. Preservation of OM and carbonate, in this system, was a function of changing bottom and pore water redox processes. Redox oscillation operated in a siliciclastic, prodeltaic environment associated with a relatively high sediment accumulation rate and high loadings of labile organic matter and metal oxides. These findings are important for understanding Late Palaeozoic black shales in the context of hydrocarbon and mineral systems.

From marine bands to hybrid flows: Sedimentology of a Mississippian black shale

AbstractOrganic‐rich mudstones have long been of interest as conventional and unconventional source rocks and are an important organic carbon sink. Yet the processes that deposited organic‐rich muds in epicontinental seaways are poorly understood, partly because few modern analogues exist. This study investigates the processes that transported and deposited sediment and organic matter through part of the Bowland Shale Formation, from the Mississippian Rheic–Tethys seaway. Field to micron‐scale sedimentological analysis reveals a heterogeneous succession of carbonate‐rich, siliceous, and siliciclastic, argillaceous muds. Deposition of these facies at basinal and slope locations was moderated by progradation of the nearby Pendle delta system, fourth‐order eustatic sea‐level fluctuation and localized block and basin tectonism. Marine transgressions deposited bioclastic ‘marine band’ (hemi)pelagic packages. These include abundant euhaline macrofaunal tests, and phosphatic concretions of organic matter and radiolarian tests interpreted as faecal pellets sourced from a productive water column. Lens‐rich (lenticular) mudstones, hybrid, debrite and turbidite beds successively overlie marine band packages and suggest reducing basin accommodation promoted sediment deposition via laminar and hybrid flows sourced from the basin margins. Mud lenses in lenticular mudstones lack organic linings and bioclasts and are equant in early‐cemented lenses and in plan‐view, and are largest and most abundant in mudstones overlying marine band packages. Thus, lenses likely represent partially consolidated mud clasts that were scoured and transported in bedload from the shelf or proximal slope, as a ‘shelf to basin’ conveyor, during periods of reduced basin accommodation. Candidate in situ microbial mats in strongly lenticular mudstones, and as rip‐up fragments in the down‐dip hybrid beds, suggest that these were potentially key biostabilizers of mud. Deltaic mud export was fast, despite the intrabasinal complexity, likely an order of magnitude higher than similar successions deposited in North America. Epicontinental basins remotely linked to delta systems were therefore capable of rapidly accumulating both sediment and organic matter.

Can One-Run-Fixed-Arrhenius Kerogen Analysis Provide Comparable Organofacies Results to Detailed Palynological Analysis? A Case Study from a Prospective Mississippian Source Rock Reservoir (Bowland Shale, UK)

Organofacies analysis, a fundamental component within source rock appraisal based on the study of kerogen within a source rock, is typically produced from microscopy (palynological) and geochemical (kerogen kinetic) data, both of which are costly to acquire. One-Run-Fixed-Arrhenius (ORFA) kerogen kinetic analysis based on Rock–Eval pyrolysis offers a substantially cheaper kinetic dataset. Here, ORFA and palynological analyses are compared in organofacies characterization of a prospective Mississippian source rock reservoir (Bowland Shale, UK). Two-end-member organofacies were determined based on the abundance of the 56 kcal/mol activation energy peak derived from ORFA data: absence (< 5%) indicating ‘organofacies A’ containing the highest proportion of algal material (Type I kerogen); and presence (> 15%) indicating ‘organofacies B’ containing the highest proportion of sporomorphs (Type II kerogen). A mud-dominated slope setting for the rock reservoir was also used to test the accuracy of organofacies analysis in determining depositional environment. Organofacies A found within lithofacies deposited from dilute waning density flows and hemipelagic suspension settling occurred between shelf edge, slope and basin. Organofacies B found within lithofacies deposited from dilute waning density flows, and low-strength cohesive debrites occurred only within the lower slope. This study demonstrates that ORFA kerogen kinetic analysis provides comparable net results to palynological analysis, enabling cheaper and faster organic characterization during initial source rock appraisal. However, caution must be exercised in drawing interpretations as to biological source(s), organic matter mixing and preservation state(s) without additional investigation using data from detailed palynological analysis.

Deformation Experiments on Bowland and Posidonia Shale\u2014Part II: Creep Behavior at In Situ pc\u2013T Conditions

To unravel their long-term creep properties at simulated reservoir conditions, we conducted constant stress deformation experiments at elevated confining pressures, pc = 50–115 MPa, and temperatures, T = 75–150 °C, on Posidonia (GER) and Bowland (UK) shale, which exhibit varying petrophysical and mechanical properties. Depending on applied pc–T conditions and sample composition, recorded creep curves exhibit either only a primary (decelerating) or additionally a secondary (quasi-steady state) and a tertiary (accelerating) creep phase during deformation. At high temperature and axial differential stress and low confining pressure, creep strain is enhanced and a transition from primary towards secondary and tertiary creep behavior is observable. Creep strain of Posidonia shale, which is rich in weak constituents (clay, mica, and organic content), is enhanced when compared to creep strain recorded during deformation of either carbonate- or quartz-rich Bowland shale. Electron microscopy observations revealed that creep strain is mainly accommodated by the deformation of weak minerals and local pore space reduction. In addition, microcrack growth occurred during secondary creep. An empirical correlation between creep strain and time based on a power law was used to describe the decelerating creep phase, also accounting for the influence of confining pressure, temperature, and axial differential stress. The results suggest that the primary creep strain can be correlated with mechanical properties determined from short-term constant strain rate experiments such as static Young’s modulus and triaxial compressive strength.

Shale gas reserve evaluation by laboratory pyrolysis and gas holding capacity consistent with field data

Exploration for shale gas occurs in onshore basins, with two approaches used to predict the maximum gas in place (GIP) in the absence of production data. The first estimates adsorbed plus free gas held within pore space, and the second measures gas yields from laboratory pyrolysis experiments on core samples. Here we show the use of sequential high-pressure water pyrolysis (HPWP) to replicate petroleum generation and expulsion in uplifted onshore basins. Compared to anhydrous pyrolysis where oil expulsion is limited, gas yields are much lower, and the gas at high maturity is dry, consistent with actual shales. Gas yields from HPWP of UK Bowland Shales are comparable with those from degassed cores, with the ca. 1% porosity sufficient to accommodate the gas generated. Extrapolating our findings to the whole Bowland Shale, the maximum GIP equate to potentially economically recoverable reserves of less than 10 years of current UK gas consumption.

Lithological and chemostratigraphic discrimination of facies within the Bowland Shale Formation within the Craven and Edale basins, UK

The Carboniferous Bowland Shale Formation of the UK is a proven hydrocarbon source rock and currently a target for shale gas exploration. Most existing analysis details lithofacies and geochemical assessment of a small number of boreholes. Given a paucity of relevant borehole cores, surface samples provide a valuable contribution to the assessment of this unconventional gas source. This study reviews existing literature on the formation's hydrocarbon geochemistry and provides new lithological descriptions of seven lithofacies, XRD mineralogy and hydrocarbon-specific geochemical data for 32 outcrop localities within the Craven and Edale basins, respectively in the northern and southern parts of the resource area. Low oxygen indices suggest that the majority of samples are relatively unaltered (in terms of hydrocarbon geochemistry), and therefore suitable for the characterization of the shale organic character. Total organic carbon (TOC) ranges from 0.7 to 6.5 wt%, with highest values associated with maximum flooding surfaces. Mean T max values of 447 and 441°C for the Edale and Craven basins, respectively, suggest that nearly all the samples were too immature to have generated appreciable amounts of dry gas. The oil saturation index is consistently below the &gt;100 mg g −1 TOC benchmark, suggesting that they are not prospective for shale oil. Supplementary material: A table summarizing the location, geological description and age of all of the samples in this paper is available at https://doi.org/10.6084/m9.figshare.c.4444589

Controls on amorphous organic matter type and sulphurization in a Mississippian black shale

Paleoredox proxies (Fe speciation, trace element and δ34Spy) integrated with sedimentological and palynological observations link the distribution and type of particulate organic matter (OM) preserved to hydrocarbon source rock potential. In the Mississippian Bowland Shale Formation (Lancashire, UK), particulate OM is dominated by “heterogeneous” amorphous OM (AOM), primarily “sharp-edged, pellet-like” (AOMpel) and “heterogeneous, granular” (AOMgr) types. AOMpel is abundant in muds deposited under anoxic and moderately to highly sulphidic conditions and most likely represents the fecal minipellets of zooplankton and/or pellets of macro-zooplankters. We recognize two intervals, “A” and “B,” which exhibit Sorg/TOC > 0.04, suggesting a bulk Type II-S kerogen composition. The Interval A palynofacies is typified by pyritized AOMpel (AOMpyr) particles that contain high-relief organic spheres surrounding individual pyrite framboids, within each AOMpyr particle. These textures are interpreted as sulphurized OM local to pyrite framboids (Sorg-PF). Sorg-PF is rarely observed in Interval B, and absent in all other samples. Redox oscillation between ferruginous and euxinic conditions during early diagenesis of Interval A likely promoted S cycling in microenvironments surrounding pyrite framboids, which generated reactive S species and reactive OM required for sulphurization. Early diagenetic redox oscillation processes were apparently triggered by relative sea level fall, associated with an increased supply of FeHR from adjacent shelves into the basin. Interval B represents deposition during the late stages of basin infill and transition from anoxic to (sub)oxic bottom waters, where AOMpel is replaced by AOMgr as the dominant type of AOM. A large particle diameter at the limit of the mesh size (500 μm), sheet-like, fragmented character, and presence of candidate organic sheaths suggests AOMgr at least partially represent fragments of benthic microbial mats, probably as sulphide-oxidizers. A ternary plot of AOMpel + AOMpyr versus AOMgr versus spores + phytoclasts links the observed palynofacies to bottom and pore water redox conditions, water column productivity and proximity to fluvial (deltaic) supply of spores and phytoclasts. These variables were moderated by changing basin accommodation, driven primarily by eustatic sea level fluctuation. A sequence-stratigraphic control on AOM type and sulphurization is important for understanding the link between source rock heterogeneity and the timing of hydrocarbon generation and expulsion from this source rock.

Enhanced Shale Gas Recovery: Gas Sorption Controls on Recoverable Gas and CO2 Storage Capacity

The natural gas industry has seen a revolution in recent times due to the boom in shale gas extraction. The process can be made more efficient by using CO2 injection as part of enhanced recovery. This would serve two key advantages: (1) Preferential shale adsorption of CO2 leading to CO2 storage and (2) Displacement of CH4 leading to higher recovery of natural gas. Executing this process on a field-scale level would need an understanding of the controls on gas adsorption in shale and the overall Gas-in-Place (GIP). In this study, CO2 and CH4 adsorption have been gravimetrically measured at high pressure on a sample of the Bowland shale at 80°C using a Rubotherm Magnetic Suspension Balance. To quantify the compositional effect on adsorption, mesoporous carbon, a synthetic material, has also been used in similar experiments, as an analogue of the organic matter in shale. Both sets of adsorption isotherms have been used to calculate the GIP at reservoir conditions. Good agreement between the two independent sets of data suggests that shales are largely mesoporous and that the organic matter is the main contributor to gas adsorption in shale.