Abstract
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.
Introduction and rationale
Unconventional shale plays have revolutionized the petroleum industry in the USA to the extent that the country has reverted to being self-sufficient for its energy needs and become a net gas exporter for the first time in over half a century. Founded on that exploration success, other plays have been investigated around the world, but with a varying degree of success. Selley (2005) suggested that there was shale gas potential in the UK, production from which could arrest the marked decline in gas supplied by UK offshore sources, and a number of basins were subsequently screened by Smith et al. (2010).
While the Jurassic of the Weald Basin and Lower Carboniferous (West Lothian Oil Shale Group) in the Midland Valley of Scotland provided some encouragement, Smith et al. (2010) suggested that the Carboniferous (Visean–Namurian) deep-water sediments ascribed to the Bowland and Hodder shales in the Craven Basin, Edale Basin, Widmerpool Gulf, Gainsborough Trough and Cleveland Basin of northern England (Fig. 1) were especially prospective. Subsequent studies have served to underline the potential (Roche 2012; Andrews 2013; Słowakiewicz et al. 2015; Yang et al. 2015; Könitzer et al. 2016; Hennissen et al. 2017; Harvey et al. 2018; Hughes et al. 2018; Newport et al. 2018) and extend the opportunity to include parts of North Wales too, where the corresponding units are known as the Holywell Shale Formation (Newport et al. 2016).
Early Carboniferous structural elements of north England and Wales highlighting the key basins and platform areas (modified after Fraser et al. 1990). The gross tectonic setting was that of a series of blocks and basins formed by contemporaneous rifting. Mudstone-dominated sequences (Bowland Shale and equivalents) filled the basins and extensive carbonate platforms developed on the blocks. The Ribblesdale Fold Belt and its offshore counterpart (the Môn–Deemster Fold Belt) are also highlighted (after Pharaoh et al. 2019).
The shale gas potential that was recognized by the aforementioned regional studies led to a renewed exploration interest in the Carboniferous basins, and a number of petroleum exploration and development licences (PEDLs) were bid for, and subsequently awarded, in the 13th and 14th Onshore Licensing Rounds in 2011 and 2015. Initial resource estimates for all of the main plays were published in 2013–14 and suggested that the UK's shale resources were large (Andrews 2013, 2014; Monaghan 2014). Collectively, the studies acted as a catalyst for renewed industry activity and promoted the notion that shale gas resources could be sufficient to help wean the country off its increasing dependency on petroleum imports (Harvey et al. 2018).
The aim of the paper is to present the results of an independent evaluation of the subsurface geology in a strategic part of the PEDL 165 licence in the Fylde area, NW Lancashire. The licence is situated in the southwesterly extension of the Craven Basin, where it is buried beneath a Permo-Triassic sedimentary cover (Fig. 2). The area was identified as a prime candidate for shale gas extraction and is the one licence where hydraulic fracturing has already taken place (Clarke et al. 2018).
Important new insights regarding the subsurface stratigraphy and structure of the area have emerged from our mapping including the role of Permo-Triassic extension, the severity of Late Carboniferous (Variscan) fold-and-fault deformation, and their combined effect on shale gas targets. The results have direct implications for unconventional shale gas exploration in the PEDL 165 licence, since the deformation compartmentalizes and limits the play with a consequent impact on resource estimates. The identification of fault networks also poses further challenges, as demonstrated by human-induced seismicity during hydraulic fracturing operations at two drill sites (Preese Hall-1 and Preston New Road (PNR): Clarke et al. 2014, 2019).
Simplified bedrock lithostratigraphy, geological structure and key well penetrations within the Craven Basin and surrounding areas. Also shown is the location of Figure 4, which illustrates the outline of the 3D seismic data volume used in this study. The figure draws upon British Geological Survey materials. © UKRI 2019. BABH, Becconsall–Ashnott buried high; BbF, Billsborrow Fault; Bc, Becconsall-1 well; Bk, Banks well; Bw, Boulsworth borehole; CA, Catlow Anticline; CFS, Clitheroe Fault System; CoA, Clitheroe Anticline; DF, Dent Fault; EF, Elswick Fault; GA, Gisburn Anticline; GF, Grimsargh Fault; HC, Holme Chapel well; Hk, Hesketh well; LbF, Larbeck Fault; NCF, North Craven Fault; OF, Oakenclough Fault; PF, Pendle Fault; PM, Pendle Monocline; Rw, Roddlesworth well; SA, Sykes Anticline; SbA, Slaidburn Anticline; SCF, South Craven Fault; SkA, Skipton Anticline; Sw, Swinden borehole; SwA, Swinden Anticline; TnA, Thornley Anticline; TtA, Thornton Anticline; TtF, Thistleton Fault; WF, Woodsfold Fault; Wm, Whitmoor well.
Regional setting
The 3D seismic data volume upon which this study is based was acquired in 2012 over an area where Permian and Triassic clastic red beds outcrop (Fig. 2), the occurrence of which represents the onshore expression of the East Irish Sea Basin (EISB). The basal sequences extend east and have an unconformable or faulted contact with truncated Carboniferous sediments belonging to the SW–NE-striking Craven Basin (Figs 1 and 2). The basin is one of a number of Late Devonian–Carboniferous (Mississippian) depocentres that formed in response to north–south- to NW–SE-orientated rifting (Fig. 1), the orientation of which appears to have been largely governed by the reactivation of Caledonian structures (Leeder 1982; Fraser and Gawthorpe 1990, 2003; Fraser et al. 1990; Corfield et al. 1996; Glennie and Underhill 1998; Worthington and Walsh 2011).
The Craven Basin underwent a phase of (post-rift) thermal subsidence during the Namurian and Westphalian (Pennsylvanian) before being affected by foreland deformation associated with the Variscan Orogeny to the south (Glennie and Underhill 1998). The main zone of deformation is known as the Ribblesdale Fold Belt, and consists of a suite of largely SW–NE-striking en echelon folds and reverse faults (Arthurton et al. 1988) that collectively plunge beneath the Permo-Triassic succession to the SW (Fig. 2). The SE margin of the fold belt is marked by the Pendle Monocline, which effectively separates the Craven Basin from the West Lancashire High and represents the surface expression of a reactivated (structurally inverted), NW-dipping planar normal fault (Chadwick and Evans 2005). Its northwestern margin counterpart separates the basin from the Lancaster High (Lawrence et al. 1987) and is marked by a SE-dipping (antithetic) fault against which a series of oblique folds occur (e.g. the Sykes and Catlow anticlines, etc.: Moseley 1962; Arthurton 1984; Brandon et al. 1998; Kirby et al. 2000) (Fig. 2). A recent study by Pharaoh et al. (2019) has also suggested that the fold belt extends across southern parts of the EISB to affect parts of NW Wales including Anglesey and called it the Môn–Deemster fold–thrust belt.
Uplift and peneplanation led to the erosional truncation of the Carboniferous sequences and formation of the Base Permian Unconformity (BPU: Fig. 3). Sedimentation was renewed during the Permian–Triassic when continental red beds onlapped and draped on to the BPU. The locus of basin subsidence lay to the west to create the EISB (Kirby et al. 2000); one of a series of major rifts that developed in response to east–west extension across the region. The development of the EISB was accommodated by large NNE–SSW-striking normal faults that transect the Carboniferous structure (Fig. 2). Some of the structures extend onshore to affect the Fylde area, with the local development of major normal faults to create local extensional depocentres, of which the Elswick Graben and Sollom Sub-basin that cut through PEDL 165 represent prime examples (Fig. 2) (Clarke et al. 2018).
Stratigraphic summary chart for the Carboniferous, Permian and Triassic sequences of NW England. The Lower Carboniferous sediments that accumulated in deep-water basins (e.g. Craven Basin) are differentiated from those that are extensively developed on adjacent platforms (e.g. West Lancashire High). Also included are key marine bands that define the Carboniferous chronostratigraphy in the region and seismic markers shown in the cross-sections presented in Figures 5–10.
The basin continued to subside through the Mesozoic before being uplifted during the Cenozoic, which brought the Carboniferous, Permian and Triassic sediments back to outcrop (Holford et al. 2008). The Ribblesdale area is also transected by numerous, late, NW–SE-striking extensional faults that include the Billsborrow and Grimsargh faults (Fig. 2). Since these structures offset the Permo-Triassic structures like the Woodsfold Fault (Fig. 2), they clearly formed later and may form part of a suite of Cenozoic structures that developed in response to pervasive Atlantic plate-margin deformation (Underhill 2009).
Stratigraphy
Lithostratigraphy
The Carboniferous stratigraphy of the Craven Basin is well displayed throughout the Ribblesdale Fold Belt and the Burnley Coalfield on its eastern margin. Distinct facies and thickness variations are seen between the basin depocentre and its margins during Dinantian times (Fig. 3), something that reflects the palaeo-bathymetric changes between upstanding blocks and intervening troughs resulting from (syn-rift) extension (Gawthorpe 1987).
Lower Dinantian (Tournaisian) and older rocks are absent from marginal blocks like the West Lancashire High, Askrigg Block and Lake District Massif, where deposition only began in the Visean (Arthurton et al. 1988). In these areas, condensed shallow- to marginal-marine platform carbonates (including those ascribed to the Great Scar Limestone Group) were established on highly deformed and weakly metamorphosed Upper Paleozoic (Caledonian) roots before passing up into cyclical clastics ascribed to the Yoredale Group (Fig. 3).
In contrast, deposition in the basinal areas started earlier with deposition of red-bed clastics like the Sedbergh Conglomerate Formation (Underhill et al. 1988) and sediments ascribed to the ‘Old Red Sandstone’ in the Boulsworth borehole (Ramsbottom 1974). The earliest sediments known at surface in the Craven Basin are mudstones belonging to the (Upper Courceyan (Ivorian)–Lower Chadian) Chatburn Limestone Formation (Fig. 3) (Fewtrell and Smith 1980), which were also seen in the Swinden-1 borehole near Skipton (Charsley 1984).
The Chatburn Limestone Formation is overlain by the (Chadian–Asbian) Worston Shale (or Craven) Group, which includes Waulsortian reef knolls (Miller and Grayson 1972) and boulder beds (Dunnington 1945) belonging to the Clitheroe Limestone Formation in its lower section (Fewtrell and Smith 1980). The remainder of the succession consists of the (Late Chadian–Holkerian) Hodder Mudstone, (Holkerian) Hodderense Limestone and (late Holkerian–Asbian) Pendleside Limestone Formations (Earp et al. 1961; Fewtrell and Smith 1980; Aitkenhead et al. 1992).
The Bowland Shale Group (BSG) sits conformably upon the Worston Shale Group and consists of two component parts: the (Late Asbian–Brigantian) Lower Bowland Shale and (Pendelian) Upper Bowland Shale Formations, the contact between which is marked by the Cravenoceras leion marine band (Aitkenhead et al. 1992) representing the boundary between the Dinantian and Silesian (Visean and Namurian). The upper boundary of the BSG is marked by the Cravenoceras malhamense marine band (Fig. 3). The BSG locally reaches thicknesses of over 1500 m and is organic rich with total organic carbon (TOC) values of up to 7% (Clarke et al. 2018). The unit is considered to be the main source rock for the oil and gas fields in the EISB (Lawrence et al. 1987; Armstrong et al. 1997) and East Midlands petroleum province (Fraser et al. 1990).
The Millstone Grit Group marks the onset of coarse clastic, southerly-prograding deltaic sedimentation across northern England (Collinson 1969; Collinson 1970; Steele 1988; Hampson 1997). The group can be subdivided into several component parts, the most notable of which are the (Pendleian E1–E2a) Pendleton (or Pendle Grit) Formation, Warley Grit, Sabden Shale, Kinderscout Grit, Middle Grit and Rough Rock. Their recognition across the region suggests that basin subsidence was more uniform by Namurian times. Upper Carboniferous deposition is completed by the fluvial-deltaic sediments of the Coal Measures Group, the lower boundary for which is defined by the Subcrenatum marine band (Fig. 3).
A major unconformity separates folded and truncated Carboniferous sediments from a Permo-Triassic continental red-bed succession, the base of which is marked by the Upper Permian (Rotliegend Group) Collyhurst Sandstone Formation and (Zechstein Group) Manchester Marl Formation (Fig. 3). Higher parts of the succession comprise the Sherwood Sandstone and Mercia Mudstone Group, which outcrop over western areas of Lancashire and extend offshore into the East Irish Sea. With the exception of the Preesall Halite, which is developed in and around Fleetwood, none of the evaporites that characterize offshore parts of the EISB is developed in the Fylde area (Wilson 1990). Apart from a thin Quaternary cover, the Triassic represents the youngest solid-rock stratigraphy in the area because of the effects of Cenozoic uplift and erosion.
Seismic stratigraphy
These variations in lithology lead to changes in density and/or seismic velocities that in turn create acoustic impedance contrasts which enable seismic interpretation. The most continuous stratigraphic surface that can be recognized and mapped is the BPU, which is marked by erosional truncation of Carboniferous sediments and structures, and by the onlap and drape of its Permo-Triassic cover. Additionally, a marked regional seismic facies contrast marks the top of the Lower Bowland Shale Formation, which enables the component parts of the Bowland Shale Group to be mapped. Our interpretations have focused on mapping these two prominent horizons and the base of the basinal Carboniferous, the results of which illustrate the main structural styles that characterize both the Ribblesdale Fold Belt and its Permo-Triassic (EISB) and younger tectonic overprint.
Data and methods
Seismic data
The Bowland-12 seismic survey upon which this study is based was acquired in 2012 by CGG for Cuadrilla Resources. The 3D seismic volume covers a c. 100 km2 area of the Fylde Peninsula (Fig. 4) and was released into the public domain in January 2018. We have gained access to the data as a result of public release through the UK Onshore Geophysical Library (UKOGL) and produced an interpretation thereof using Schlumberger Petrel software mounted on workstations housed at Heriot-Watt University. A processed zero-phase post-stack time migrated volume was used for this study, where the data has been interpolated into a cube consisting of 427 inlines and 433 crosslines; with a horizontal spacing of 25 m. Vertical resolution at the Bowland Shale level is c. 40 m, based on an average frequency of 30 Hz and a velocity of 4800 m s−1.
Location map illustrating the extent of the seismic coverage, locations of key wells and cross-sections illustrated from within the Bowland-12 3D seismic data volume. Line of section C–C′ intersects the PNR-1 well at the Base Permian Unconformity (BPU) and the line of section D–D′ intersects the PNR-1Z side-track at the Bowland Shale level.
The interpretation of the 3D volume has been combined with those derived from vintage regional 2D seismic data acquired in the 1980s and 1990s as part of an earlier campaign of conventional hydrocarbon exploration, including the UKOGL regional line published by Butler and Jamieson (2013). All of the seismic data are calibrated by five exploration wells (Table 1), leading to high confidence in the interpretations and mapping. All three shale gas exploration wells drilled by Cuadrilla Resources (Grange Hill-1Z, Preese Hall-1 and Preston New Road -1 (PNR-1)) have been integrated into the study to calibrate the seismic interpretation using their wireline logging suites, well reports and core studies. Electrical wireline logs and well reports for two legacy conventional British Gas wells (Thistleton-1 and Elswick-1) were also used. The shale gas well data were provided by the British Geological Survey (BGS) and the legacy wells obtained via CGG's Data Management Service.
Summary of the five key well sites (and two horizontal well side-tracks at Preston New Road (PNR)) that calibrate the interpretations described in this paper (see Fig. 4 for the locations). The wells are ordered by the date upon they reached TD (total depth)
Well ties and seismic interpretation
The wireline signature and seismic character at the Grange Hill-1Z, Preese Hall-1 and PNR-1 wells are illustrated in Figure 5. Synthetic seismograms were created for each of the well locations and correlated with the adjacent seismic data. Use was then made of stratigraphic horizons marked by acoustic impedance reflections and their terminations to interpret the whole 3D data volume.
Correlation of seismic to well ties for the Grange Hill-1Z, Preese Hall-1 and PNR-1 wells. Acoustic Impedance (AI) is plotted using wireline logs, transformed to reflection coefficients and convolved with a zero-phase 30 Hz Klauder wavelet to produce a synthetic trace equivalent to the seismic data volume to allow for a direct comparison. Interpreted structural and stratigraphic relationships between the wells are sketched with reference to an along-strike cross-section (Fig. 9). BG, Brennand Grit; Ch, Collyhurst Sandstone Formation; Ks, Kinderscout Grit Group; LB, Lower Bowland Shale Formation; Lc, Lower Coal Measures; MM, Manchester Marl Formation; PG, Pendle Grit; Rb, Roeburndale Formation; RR, Rough Rock; SS, Sabden Shale; UB, Upper Bowland Shale Formation; US, Upper Shales.
The most prominent horizons in the Permo-Triassic section belong to anhydrites of the Manchester Marl Formation, and can be correlated between Preese Hall-1 and PNR-1 wells. As the underlying Collyhurst Sandstone Formation is poorly developed over much of the study area, these provide a useful aid and are an effective proxy for mapping the Base Permian Unconformity (BPU).
Reflectivity in the Carboniferous section also permits the main stratigraphic units to be recognized and key horizons to be mapped. The underlying Millstone Grit is characterized by a series of strong reflectors indicative of thick sand units interbedded with shales. Preese Hall-1 and Grange Hill-1Z penetrated a full Millstone Grit sequence but it is absent in the PNR-1 well. The Upper Bowland Shale shows a contrasting seismic signature characterized by a lack of clear reflectivity. This formation consists mainly of silty shale interbedded with thin (5–10 m thick), low-impedance shales and very-thin (1–2 m thick) high-impedance carbonates/siltstones. The individual thicknesses of these contrasting units are, however, below seismic resolution at this depth (c. 40 m) and explain the lack of reflectivity response. Finally, the Lower Bowland Shale is characterized by the occurrence of strong reflectors. These are produced by thicker (20–30 m) low-impedance shales interbedded with high-impedance carbonates or sandstones (50–60 m thick) which show a more distinctive seismic character visible at the base of the Grange Hill-1Z and Preese Hall-1 wells. While this unit can still be identified at the base of the PNR-1 well, reflection amplitude is notably lower than that observed in the other wells (Fig. 5).
Three NW–SE-striking arbitrary dip sections (Figs 6–8), two (eastern and western) arbitrary strike lines (Figs 9 and 10) and selective seismic time slices (Figs 11 and 12) were initially used to tie the wells that lie within the seismic volume, constrain the horizon interpretation and identify the main faults. Good correlations were possible and no significant time shift was apparent at the three wells, lending confidence to the wider interpretation seeded by these data. The time–depth relationships obtained from check-shot surveys in the Thistleton-1 and Elswick-1 wells, and vertical seismic profiles (VSPs) for Grange Hill-1Z, Preese Hall-1 and PNR-1 were also used in depth conversion to determine stratigraphic horizon depths and estimate bedding dips.
NW–SE cross-section through the Bowland-12 3D seismic survey tie-ing the Grange Hill-1Z and Elswick-1 wells along the main direction of structural dip. The section has also been extended to pass through Roseacre Wood, the proposed location of a shale gas drilling pad. The section is shown without interpretation (left) and with interpretation (right) to enable a direct comparison between the data and our mapping. The main structural features and stratigraphic units are highlighted. Carb., Carboniferous (undifferentiated); GB, Greenhalgh Block; PB, Poulton Block; SB, Singleton Block; WB, Weeton Block. Please refer to Figure 5 for further acronyms used for the well tops.
NW–SE cross-section through the Bowland-12 3D seismic survey tie-ing the Preese Hall-1 and Thistleton-1 wells along the main direction of structural dip. The section is shown without interpretation (left) and with interpretation (right) for comparative purposes. The main structural features and stratigraphic units are highlighted. GB, Greenhalgh Block; PB, Poulton Block; SB, Singleton Block; TtF, Thistleton Fault; WB, Weeton Block. Please refer to Figure 5 for acronyms used for the well tops. The grey region in the NW of the line represents an area of the processed Bowland-12 3D cube where data was not acquired.
NW–SE cross-section through the Bowland-12 3D seismic survey tie-ing the PNR-1 well along the main direction of structural dip. The section is shown without interpretation (left) and with interpretation (right) for comparative purposes. The main structural features and stratigraphic units are highlighted. GB, Greenhalgh Block; WB, Weeton Block. Please refer to Figure 5 for acronyms used for the well tops. The grey region in the NW of the line represents an area of the processed Bowland-12 3D cube where data was not acquired.
NNE–SSW cross-section through the Bowland-12 3D seismic survey tie-ing the Grange Hill-1Z, Preese Hall-1 and PNR-1 wells along the main direction of structural strike. The section is shown without interpretation (left) and with interpretation (right) for comparative purposes. The main structural features and stratigraphic units are highlighted. GB, Greenhalgh Block; SB, Singleton Block; WB, Weeton Block. Please refer to Figure 5 for acronyms used for the well tops.
North–south strike section through the Bowland-12 seismic survey illustrating the syn-sedimentary thickening of the Permo-Triassic section in the Elswick Graben. The section is shown without interpretation (left) and with interpretation (right) for comparative purposes. The occurrence of a major erosional sequence boundary, seismic facies and direct calibration by the Elswick-1 well all point to there being an additional Permian section beneath the classic Collyhurst Sandstone unit in the graben as a result of syn-sedimentary extensional fault activity. Please refer to Figure 5 for acronyms used for the well tops. The undulating geometry of the Woodsfold Fault apparent on the diagram results from the fact that the seismic line runs sub-parallel to the structure.
A time slice extracted from the Bowland-12 survey at 600 ms, which corresponds to the Base Permian Unconformity (BPU) in the centre of the survey, and intra-Permo-Triassic in the west and east. The time slice is shown without (upper) and with (lower) annotated major faults. The intersection with the BPU surface map (Fig. 13) is also highlighted. El-1, Elswick-1; GH-1Z, Grange Hill-1Z; PH-1, Preese Hall-1; RW, Roseacre Wood; Th-1, Thistleton-1.
A time slice extracted from the Bowland-12 survey at 1200 ms, which is equivalent to the Lower Bowland Shale in the centre and the east of the survey. The time slice is shown without interpretation (upper) and with interpretation (lower). The lower panel includes interpreted fault planes and the four fault-bounded compartments identified within the footwall of the Thistleton Fault. The intersection with the Lower Bowland Shale surface map is also highlighted (Fig. 14). Please refer to Figure 11 for a list of well acronyms.
A number of key and consistent stratigraphic markers were apparent in the wells, and these provided a basis upon which to extend interpretations across the whole survey. Although all the prominent stratigraphic horizons have been mapped, we use the two that are most relevant to illustrate the critical stratigraphic and structural geometries for shale gas exploration in the area in this paper, namely:
the Base Permian (Variscan) Unconformity (BPU), which represents the boundary between the high-amplitude Permian Collyhurst Sandstone Formation and the lower-amplitude, tilted and truncated Carboniferous sands and shales;
the top of the Lower Bowland Shale, which represents the youngest hard calcareous sand unit within the shale.
The interpretation of the Bowland-12 survey using a combination of seismic sections and time slices has enabled us to define the principal structural features that affect the Bowland Shale and offset the BPU. Horizon and fault mapping was carried out across the study area to identify the main structural elements and produce time–structure maps. The maps were gridded using convergent interpolation and then displayed with overlays of the mapped fault planes (Figs 13 and 14). In some parts of the survey, however, interpretation was limited by data quality, and it has been necessary to infer fault traces or horizon picks honouring adjacent trends and regional knowledge.
Time–structure map for the Base Permian Unconformity (BPU) across the Bowland-12 dataset showing the distribution of the extensional fault network that largely affects the SE of the survey and defines the course of the Elswick Graben. Well-defined footwall highs lie to the west of the Thistleton and Elswick faults. In the case of the latter, the fault-bound three-way dip closure that hosts the Elswick gas field has been highlighted. The latter produced 1 Bcf of gas between 1996 and 2013; and while it is now depleted, it still constitutes a possible site for gas storage. Please refer to Figure 11 for a list of well acronyms.
Time–structure map for the Top of the Lower Bowland Shale. Comparison between the structures at the Top Lower Bowland Shale, at the Base Permian Unconformity (Fig. 13) and at outcrop (Fig. 2) serves to demonstrate the added structural complexity affecting the Lower Carboniferous shale gas target horizons, something that is not apparent from surface mapping. Mapping of three significant SW–NE reverse faults allows the area to the west of the (Permo-Triassic) Thistleton Fault to be subdivided into four main structural compartments. The identification of these structural subdomains places severe constraints on shale gas extraction. The Permo-Triassic normal faults that define the Elswick Graben overprint buried Carboniferous reverse faults and add structural complexity, meaning that shale gas exploration may be more challenged in this area. The line of section shown in Figure 8 intersects the PNR-1 well at the Base Permian Unconformity (BPU) and the line of section shown in Figure 9 intersects the PNR-1Z side-track at the Bowland Shale level. Please refer to Figure 11 for a list of well acronyms.
The western strike line (Fig. 9) was deliberately selected in order to compare our interpretations with those described by Clarke et al. (2018, fig. 3b). However, we found it impossible to replicate their line whilst honouring the check-shot-calibrated stratigraphy in the Grange Hill-1Z well since the latter lay on the downthrown side (footwall) of a major, large-displacement reverse fault, separating it from the corresponding upthrown side (hangingwall) in which the Preese Hall-1 well is situated. This fault is named here as the Summerer Fault after the farm and woods over which it is situated, and may correspond to the PH-1 Fault first identified as present to the north of the Preese Hall-1 well by the Oil and Gas Authority (2018). In order to duplicate Clarke et al. (2018)’s line requires the Grange Hill-1Z well to be projected across the aforementioned fault and time shifted up by c. 300 ms, neither of which can be justified. As a consequence, we present a new interpretation for the three-well tied strike line which demonstrates the role of the Summerer Fault that is evident between the Grange Hill-1Z and Preese Hall-1 wells (Fig. 9).
Permian and Triassic structure
Permo-Triassic strata outcrop or lie beneath a thin Quaternary cover across the whole of the Bowland-12 3D seismic survey. In eastern areas of the survey, the disposition of the Permo-Triassic is affected by the occurrence of several NNE–SSW-striking normal faults. The most significant structure is the WNW-dipping Woodsfold Fault, the surface trace of which lies outwith the 3D volume where it defines the NW limit of the West Lancashire Coalfield (Kirby et al. 2000).
The Woodsfold Fault defines the eastern edge of a major graben, the Elswick Graben of Clarke et al. (2018), and deeper parts of the fault are well imaged on the dip and strike lines (Figs 6–8 and 10) and time slices (Figs 11 and 12), allowing the course of the structure to be mapped over the SE part of the survey (Figs 13 and 14). The western part of the graben is defined by three east-dipping (antithetic) faults (from east to west: the Elswick, Thistleton and Larbeck faults: see Aitkenhead et al. 1992) (Fig. 6). While the Elswick Fault defines a terrace in the hangingwall to the large-displacement Thistleton Fault, the Larbeck Fault lies in the footwall to the Thistleton Fault and has a more modest offset. The faults are observed most clearly on dip sections in the north (Figs 6 and 7), and the Thistleton and Elswick faults have been mapped throughout the seismic volume (Figs 11 and 13). The southern part of the graben is separated by a large cross-fault which downthrows the Permo-Triassic and older sequences to the south (Fig. 10). This fault runs perpendicular to the Woodsfold and Thistleton faults, and is considered to be an example of a release fault (Destro 1995) formed in response to differential displacement along the strike, and in the hangingwall, of the former. It is named herein as the Wesham Fault after the village under which it lies.
The increase in thickness of the Manchester Marl Formation and the presence of the Collyhurst Sandstone Formation in the graben (Figs 6–8 and 10) demonstrate that the fault systems were active during Permo-Triassic deposition. Folding of upper parts of the Triassic sequence to create the Kirkham Syncline and Elswick Anticline at outcrop (Wilson 1990; Aitkenhead et al. 1992) may be a consequence of compactional drape over a buried western terrace that forms the fault-bound three-way dip closure that hosts the Elswick gas field itself (Fig. 13) or be a consequence of regional basin inversion.
One major effect of the Permo-Triassic extension is to add additional complexity to the Carboniferous section, with NNE–SSW striking faults transecting the earlier NE–SW striking Variscan structures and a consequent lower level of confidence in horizon mapping beneath the BPU. In contrast to eastern parts of the survey, its occurrence in the Grange Hill-1Z and Preese Hall-1 wells and its distinctive reflectivity demonstrate that the Permo-Triassic only forms a thin veneer to their Carboniferous subcrop in the west, thus allowing the Variscan structures that lie below to be mapped with confidence.
Although there is a complete absence of younger sediments in the area, porosity–depth plots, sonic velocity values and apatite fission-track analysis all point to there having been a substantive (>1 km) post-Triassic sedimentary cover in the area. Its removal is thought to have occurred in the Cenozoic in response to regional uplift and plate margin forces, the differential effects of which led to fault (re)activation. This mechanism may explain some of the normal fault offsets that affect the Permo-Triassic sequence in a similar way to that seen elsewhere in the UK (e.g. Argent et al. 2002; Guariguata-Rojas and Underhill 2017).
Carboniferous structure
Mapping of the Lower Bowland Shale has highlighted the severity of deformation in the area, which manifests itself as a series of high-angle reverse faults (that result from the contractual reactivation of normal fault precursors (tectonic inversion structures)) and by the folding and erosion of Carboniferous sequences. These features are best demonstrated through the use of three dip lines (Figs 6–8), two strike lines (Figs 9 and 10) and a time slice extracted at 1200 ms (Fig. 12).
A short dip line in the NW of the 3D survey tie-ing the Grange Hill-1Z and Elswick-1 wells with the site of the proposed Roseacre Wood well pad illustrates the presence of large reverse faults affecting the Carboniferous sequences (Fig. 6). To the west of the Grange Hill-1Z well, a SE-dipping reverse fault with a throw of c. 300 ms (c. 1 km) is present separating the block in which the well was drilled from an undrilled deeper block at 1800 ms (c. 3880 m). This fault is considered the northwards continuation of the Haves Ho Fault identified in the operator's hydraulic fracture plans for the PNR-1Z and PNR-2 wells (Cuadrilla Resources 2018a, b). To the east of the well, another SE-dipping reverse fault with a throw of c. 300 ms (c. 1 km) separates this block from another characterized by steeply-dipping continuous reflectors. This fault is the aforementioned Summerer Fault. Further SE, the Millstone Grit is truncated beneath the BPU in the footwall of another SE-dipping reverse fault (considered synonymous with the Moor Hey Fault of Cuadrilla Resources 2018a, b), resulting in the Upper Bowland Shale being brought directly beneath the unconformity. Within the hangingwall of the Thistleton Fault and within the Elswick Graben, the Upper and Lower Bowland Shale are buried beneath a thicker Permo-Triassic section. Elswick-1 was completed in a Carboniferous sequence, and the corresponding stratigraphic top lies directly beneath a strong reflector interpreted as the BPU.
A long dip line tie-ing the Preese Hall-1 and Thistleton-1 wells further illustrates the structural features (Fig. 7). The Haves Ho and Summerer faults appear to begin to converge and the block in which Grange Hill-1Z was drilled narrows. The seismic facies of the Lower Bowland Shale between the two tied wells is that of continuous but steeply-dipping (c. 30°) reflectors. Within the Elswick Graben, the Lower Bowland Shale is present at a shallower depth than observed in Figure 6 and a small west-dipping reverse fault is identified.
A dip line that ties the PNR wells illustrates a contrasting picture (Fig. 8). The Haves Ho and Summerer faults are absent on this line. The Lower Bowland Shale is present at c. 1200 ms (c. 2270 m) depth and is characterized as a series of gently undulating low-amplitude reflectors. The Moor Hey Fault is present east of the well, although reflectivity is very limited in the hangingwall. Within the hangingwall of the Thistleton Fault, a thick Triassic sequence is present, burying the Carboniferous to c. 1200 ms (c. 2270 m). The Lower Bowland Shale is interpreted as absent in this area.
Use of a composite section along the strike of the major structures (Fig. 9) and predominantly within the block between the Preese Hall-1 and Thistleton-1 wells illustrates a more continuous section but not one without structure complexity. The Summerer Fault is present between the Grange Hill-1Z and Preese Hall-1 wells. To the south, the Carboniferous sequences are brought up to beneath the BPU, with the Millstone Grit truncated beneath the unconformity to the north of the PNR wells. The latter entered the Upper Bowland Shale directly beneath the unconformity.
The features identified in these cross-sections were interpreted throughout the entire 3D volume, and have allowed us to map the Lower Bowland Shale surface and associated fault planes. This has concluded that the Bowland Shale is dissected into discrete fault-bound compartments, four of which can be mapped within the footwall of the Thistleton Fault (Fig. 12):
Poulton Block
Positioned in the far NW of the 3D survey, the Poulton Block (named after the business park situated above it) contains the Lower Bowland Shale buried to a depth of around 1800 ms two-way time (TWT: c. 3880 m) (Figs 6, 7, 12 and 14). The structural compartment ranges in width from 800 m to 2.5 km between the edge of the survey and the Haves Ho Fault, which defines the block to the SE. The Lower Bowland Shale forms a small syncline in the block which dips steeply (c. 50°) to the SE and terminates against the Haves Ho Fault. This block is undrilled to date and has been mapped based purely on seismic character.
Singleton Block
Named after the village beneath which the fault-bound compartment lies, the Singleton Block forms a narrow compartment separated from the Poulton Block by the Haves Ho Fault (Figs 6, 7, 9, 12 and 14). The block is 1.5 km wide in the north and narrows to the SW. It is defined in the east by the Summerer Fault. The Grange Hill-1Z well drilled in 2010 targeted the Bowland Shale within this specific fault block.
Weeton Block
The largest fault block is separated from the Singleton Block by the Summerer Fault. Its easternmost limit is defined by the Moor Hey Fault. We have termed this fault-bound compartment the Weeton Block after the most significant village lying above the feature (Figs 6, 7, 9, 12 and 14). It is synonymous with what Cuadrilla have termed the ‘Thistleton fairway’. The throw of the Moor Hey Fault is estimated at c. 300 m but this is uncertain due to lack of clear reflectivity in the area. The width of this block is the largest of those we can identify; varying between 1.5 and 3.2 km and also hosting the Thistleton-1, Preese Hall-1, and PNR-1, PNR-1Z and PNR-2 wells (Fig. 14). The block can be further subdivided into two contrasting portions. Towards the south and west, there is little deformation and beds are generally subhorizontal (Fig. 9: PNR well location). Towards the north and east, there is evidence of stronger deformation with steeply-dipping beds east of the Preese Hall-1 well (c. 30°), and further reverse faulting and folding present up to the Moor Hey Fault (Figs 6 and 7). The Millstone Grit is folded to the south and east, and truncated beneath the BPU in the footwall of the Moor Hey Fault, positioning the Upper Bowland Shale directly beneath the Permian sequences. Reflector strength begins to diminish in the east of this block, making for a more challenging interpretation and some uncertainty to the mapping in that subarea.
Greenhalgh Block
The final fault-bound compartment to be identified in the footwall of the Thistleton Fault is the Greenhalgh Block. The structural domain is defined in the west by the Moor Hey Fault and to the east by the Thistleton Fault. We have named it after the village of Greenhalgh that lies directly above the feature. The compartment forms a narrow block (maximum width of 1.5 km), where reflectivity is very limited, but the Upper Bowland Shale is interpreted as present at c. 600 ms (c. 950 m) directly beneath the BPU.
Other fault-bound compartments can be identified in the tract beneath the Elswick Graben in the SE part of the survey. However, the inherent structural complexity imposed by the subsequent Permo-Triassic extensional structures and their offsets has caused the Carboniferous sequences to be dissected, and mapping of the compartments containing the Bowland Shale is all the more challenging. As a consequence, we have not sought to define or name individual compartments in this area in the way that has proved possible to do in the area affected to the NW, where the effects of Permo-Triassic extension are absent.
We interpret the compartments defined by our mapping as the buried extension to the Ribblesdale Fold Belt since the structural signature observed is entirely consistent with the steep reverse faults and NE–SW-striking asymmetrical anticlines that characterize areas to the NE. The results of the seismic interpretation allow us to reconstruct the development and evolution of the area (Fig. 15). The structural geometries for the Variscan reverse faults are consistent with dip-slip contractional reactivation of normal faults (structural inversion), proposed by Gawthorpe (1987); however, we are mindful that our observations are at a seismic scale and so a component of strike- or oblique-slip (transpression), as proposed by Arthurton (1984), may also play a role.
Schematic cross-sections orientated approximately NW–SE through the Craven Basin illustrating the key tectonostratigraphic events that led to the development of the main structures affecting shale gas exploration in the area. (a) The Craven Basin is established during a phase of Lower Carboniferous extension and is bound by the West Lancashire High and the Pennine High (off the section in the NW). (b) Upper Carboniferous Variscan deformation results in the formation of reverse faults, the structural inversion of pre-existing normal faults and folding of Carboniferous strata, including the Bowland Shale. These lead to the creation of significant palaeotopography towards the end of the Carboniferous. (c) The Variscan topography is eroded to produce the Base Permian Unconformity (BPU) above which Permian–Triassic continental clastics are deposited. (d) Permian–Triassic extensional faulting leads to the creation of significant depocentres that overprint the Carboniferous structural configuration. While clearly important in its subsequent history of burial, the effects of Mesozoic subsidence and deposition, and Cenozoic uplift, exhumation and local fault reactivation are not shown in order to emphasize the Late Paleozoic (Variscan) and Permo-Triassic evolution that created the main structures seen on the 3D seismic dataset. BABH, Becconsall–Ashnott buried high; ElF, Elswick Fault; GB, Greenhalgh Block; HHF, Haves Ho Fault; LbF, Larbeck Fault; MHF, Moor Hey Fault; PB, Poulton Block; PF, Pendle Fault; SB, Singleton Block; SF,: Summerer Fault; TtF, Thistleton Fault; WB, Weeton Block; WF, Woodsfold Fault; WLH, West Lancashire High.
Implications for shale exploration
Reserve estimates and optimal drilling locations
An understanding of the subsurface structure is vital in any shale resource assessment and when seeking to evaluate the risk of intersecting critically stressed faults, upon which seismicity might be induced. Due to shale's low permeability, it is necessary to hydraulically fracture the rock at multiple stages along a horizontal well in order to initiate the flow of hydrocarbons. As such, long-reaching lateral wells are required in order to maximize contact with productive shale horizons and ensure the viability of the play. As an example, typical lateral sections within the Lower Carboniferous Barnett Shale play of the Appalachians in the USA usually have lengths of between 600 and 1800 m (Yuan et al. 2017). Furthermore, given the rapid decline in flow, it is necessary to drill wells at a close spacing during intense drilling programmes in order to produce hydrocarbons at commercial rates (Baranzelli et al. 2015; Clancy et al. 2018). Pad-based drilling allows this to be achieved efficiently, whereby multiple wells are drilled in varying directions out from a single surface location.
As our mapping has shown, the Bowland Shale is folded and transected by several large displacement faults, the combined effects of which lead to irregular bedding dips and create fault-bound compartments, the maximum width of which (3.2 km) occurs in the Weeton Block. Not only does this restrict the connected shale resource in the Fylde area, it also provides a constraint upon the drilling of long laterals in the optimal direction to maximize production. Similar structural challenges have been recognized in other areas of the UK, such as the Midland Valley of Scotland (Underhill et al. 2008), where shale gas extraction is likely to be equally challenged (Underhill 2018).
The productive shale plays in North America and Argentina are largely characterized by shallow dips, a lack of folding and absence of large faults, leading to lateral continuity of target horizons. In such relatively undeformed areas, wells are preferentially drilled such that fractures will propagate in the direction of the greatest principal stress (usually the maximum horizontal stress (SHmax) direction in deep reservoirs). Therefore, horizontal sections are typically drilled to align with the minimum horizontal stress orientation. There is limited borehole information available to determine the SHmax orientation in the Fylde region, however, Kingdon et al. (2016) reported a mean SHmax orientation of 154.5° from the Sellafield and Clarke et al. (2018) reported mean SHmax orientations of 173°, values that are consistent with regional data (Heidbach et al. 2016).
Based on these regional stress observations it would be necessary to drill horizontal wellbores at an azimuth of between 60° and 80° (WSW–ENE) in order to most effectively fracture the formation (Fig. 16a and b). However, as our mapping in the area has shown, the structural grain imposed by Upper Paleozoic (Carboniferous, Variscan and Permian) and younger activity comprises two main WSW–ENE and SSW–NNE fault networks, which both complicate the subsurface structure and place a major constraint on exploration. Using this wellbore orientation, even in the widest (Weeton) block, where the effects of the Permo-Triassic structure are absent, the spacing of the Variscan block-bounding steep, reverse, faults restrict the maximum achievable length to 4.2 km (Fig. 16d).
Schematic cartoon to illustrate the challenge and consequences of shale gas exploration drilling locations and wellbore pathways resulting from the fault patterns that affect the PEDL 165 licence. (a) A collection of maximum stress orientations, including the mean orientation from the Sellafield boreholes (Kingdon et al. 2016) and the distribution of orientations calculated at Grange Hill-1Z and Preese Hall-1 (Clarke et al. 2018). The average minimum horizontal stress of 80° shown was used in the production of map (d). (b) A schematic cartoon showing a plan view of a horizontal wellbore aligned with the minimum horizontal stress orientation showing zones of hydraulic fractures propagating in the direction of maximum horizontal stress. (c) Dip map for the Lower Bowland Shale with fault overlays added in the SE area of the map. (d) A common risk segment map classified using the angle of dip and continuity of reflectors. Some measurements of maximum compartment widths based on the maximum stress orientations shown in (a) are provided. These diagrams serve to illustrate how bedding continuity, dip and faulting can be used to define ranked common risk segments, and define, prioritize and rank the most preferential sites for unconventional drilling. When the orientation of the maximum horizontal compressive stress is introduced, it is possible use the optimal WSW–ENE drilling direction and the way in which the fault networks are orientated in the area to evaluate the position of well pads and the maximum length of a horizontal borehole pathway, which dictate the optimal drilling sites. Please refer to Figure 11 for a list of well acronyms.
A variation in bedding dip can also be observed within each major fault compartment, usually associated with smaller-scale faulting which will also provide a constraint upon where low-angle lateral wells can be drilled. In order to investigate this, a bedding dip angle map was calculated using a depth-converted Lower Bowland Shale surface, and the region was classified based on dip angle cut-offs and reflector continuity (Fig. 16c and d). This approach allows us to define four main common risk segments from which it is apparent that PNR, Grange Hill-1Z and the previously-proposed site at Roseacre Wood lie in the most prospective areas characterized by bedding dips of <30° and where there is good reflector continuity (Fig. 16d). While a 4.2 km lateral could potentially be achieved within the Weeton Block at a drilling azimuth of 60°–80°, the occurrence of high bedding dips, particularly in areas situated to the east of Preese Hall-1, adversely impacts prospectivity. The most prospective region is then limited to an area around the PNR site with a maximum width of 1.9 km (Fig. 16d).
One additional, significant consequence is that the optimal well pad locations lie along the eastern margin of a compartment, since that potentially allows the maximum lateral length of a wellbore to be drilled in continuous and largely undisturbed horizons outwith and away from zones of step dip (Fig. 16d). It is apparent that the Grange Hill-1Z well lies in a narrow compartment and Preese Hall-1 is situated on the western side of another, and so only PNR is positioned to the east of a compartment with the possibility to drill a longer lateral in the desired direction through gently-dipping shale target horizons (Fig. 16d).
Avoiding major structures could pose a significant challenge if drilling is along the azimuth of WSW–ENE. To counteract the effect of the spatial limits imposed by the fault-bound blocks and achieve maximum (commercial) flow rates might necessitate the hydraulic fracture of numerous, vertically-stacked, multi-lateral wells that target specific and numerous benches within the Bowland Shale. Naturally, adopting such an approach also presupposes several intervals with high TOC, suitable geomechanical properties (Anderson et al. 2019) and sufficient gas yields to be present; the latter of which has been recently been challenged by Whitelaw et al. (2019).
Induced seismicity on seismically resolvable faults
Fault reactivation and induced seismicity are well documented during hydraulic fracturing and are believed to be triggered by stress changes in the rock mass following injection, or by pore pressure changes along a permeable fault zone (Bao and Eaton 2016). Attempts have been made to quantify the distance at which a fault may be at risk of reactivation during hydraulic fracturing operations (Westwood et al. 2017) where the authors modelled ‘respect distances’ of between 63 and 433 m. However, this is clearly limited to those faults which are seismically resolvable and critically stressed. Our mapping has demonstrated the Lower Carboniferous to be heavily structured by seismically resolvable faults, which would limit the areas in which hydraulic fracturing could occur even using the maximum respect distance proposed by Westwood et al. (2017).
In 2011, prior to the acquisition of the Bowland-12 dataset, a hydraulic fracturing programme consisting of five fracture treatments across a 1000 m section of the Bowland Shale was carried out at Preese Hall-1 (Clarke et al. 2014). A total of 50 seismic events were recorded during and after operations; the largest two of which (ML = 1.5 and ML = 2.3) occurred 10 h after shut-in of stages 2 and 4, respectively (De Pater and Baisch 2011; Green et al. 2012). These events led to a review of the government regulations on induced seismicity around hydraulic fracturing operations and, ultimately, the development of a ‘traffic light system’. This scheme still remains in place, and requires the cessation of operations and the readjustment of fluid pressures/injection volumes if events of ML = 0.5 or larger occur (Green et al. 2012).
De Pater and Baisch (2011) reviewed a number of geomechanical and microseismic studies focusing on the Preese Hall-1 well, and concluded that reactivation of a critically stressed pre-existing fault was the likely mechanism to explain the patterns of induced seismicity. Following acquisition of the 3D seismic survey in 2012, Clarke et al. (2014) used a microseismic event recorded during the flowback phase with better spatial constraint to locate a previously unmapped fault directly to the east (Fig. 7) of the well. While their analysis is not disputed, the Summerer reverse fault mapped herein is interpreted to lie directly north and west of the well, and we speculate as to whether it may also have played a role in the induced seismicity recorded.
The role of subseismic faulting
The events at Preese Hall-1 stress the importance of the mapping of faults using high-quality 3D seismic data prior to any hydraulic fracturing operations. However, the limited vertical resolution of modern-day seismic data places a minimum constraint on the size of displacement, and therefore inferred fault, that an interpreter can confidently identify and map. As mentioned previously, the vertical resolution at the Bowland level is c. 40 m but it seems likely that in such a structurally complex area there are further faults with displacements below this cut-off that cannot be observed using this dataset. The nearby East Pennines Coalfield provides an analogue where fault patterns have been extensively studied within the Westphalian Coal Measures (Walsh and Watterson 1988; Watterson et al. 1996; Bailey et al. 2005), and those authors found a dense fault network of 7.58 faults per km2 (Bailey et al. 2005; Westwood et al. 2017) with throws as small as 0.91 m (Bailey et al. 2005).
In 2018, hydraulic fracturing operations were undertaken at the PNR-1Z lateral well. Fifteen hydraulic fracture treatments were made (of a planned 41) and resulted in the recording of 38 383 microseismic events, of which nine were reported to the Regulator under the aforementioned traffic light system. This led to four suspensions in drilling operations. Six of these large events occurred during pumping and three (including the largest ML = 1.5) were trailing events (Cuadrilla Resources 2019).
Clarke et al. (2019, their figure 5) produced maps showing the unusual pattern of microseismic events around the PNR-1Z well, notably the northward propagation away from each stage and an overall SW–NE trend. Their analysis of the microseismic focal mechanisms, events with ML > 0 and cumulative seismic moment led them to identify a subseismic plane striking at 237° and dipping at 70°, which was argued to be the cause of the microseismic pattern. Such a structure would be impossible to identify within the limits of modern seismic reflection data but a case can be made for real-time identification of these structures if operations can be adjusted accordingly.
Taken together, our results underline how important it is to characterize and understand the subsurface geology and regional stress field prior to the drilling of, and hydraulic fracturing along, long horizontal wells. The acquisition and accurate interpretation of high-quality 3D seismic datasets is crucial when seeking to identify and map all faults, including those that may be critically stressed prior to fluid injection. Our work seeks to directly address Green et al. (2012)'s recommendation that any potentially active faults be mapped prior to proceeding with hydraulic fracture operations. This recommendation was, in part, made following the conclusion that knowledge of the fault networks around the Preese Hall-1 well prior to fluid injection and associated induced seismicity was ‘poor’ (Green et al. 2012). However, exploration in such a structured area also highlights risks that cannot be mitigated through analyses of seismic data alone. The events at PNR-1Z confirms this and poses serious questions as to whether subseismic faulting is pervasive, induced seismicity is to be expected and how it should be planned for prior to hydraulic fracturing operations. Further induced seismicity has subsequently been recorded with magnitudes up to ML = 2.9 at the PNR-2 well (Oil and Gas Authority 2019).
Conclusions
Integration of regional geology, the analysis of exploration wells and the interpretation of a newly released 3D seismic data volume highlights the structural complexity of the Craven Basin in the Fylde area of NW Lancashire and its impact for shale gas exploration. The results demonstrate that deformation of the Lower Carboniferous is much more complex than at shallower levels or surface mapping would suggest. The (Dinantian–Early Namurian) Bowland Shale exploration targets are affected by a suite of folds and faults resulting from syn-sedimentary extension, Variscan contraction that led to large-offset reverse faulting and basin inversion, renewed extension in the Permo-Triassic, and renewed movement during subsequent Cenozoic uplift and exhumation. Structuration is most intense beneath the Base Permian Unconformity (BPU) and is consistent with it being part of the SW extension to the Ribblesdale Fold Belt that is exposed along strike. Variscan folds and steep reverse faults, together with other structures resulting from subsequent Permo-Triassic extension and post-depositional (Cenozoic) reactivation, define elongate fault-bound compartments containing steep and variable bedding dips. The occurrence, size and dimensions of the compartments place important constraints on the size of shale resources, the optimal location of well sites and on horizontal wellbore lengths. The accurate mapping of faults prior to the drilling and hydraulic fracturing of wells may also help to reduce the risk of fault reactivation and induced seismicity. However, in such a structurally complex area, subseismic faults should also be anticipated and plans drawn up to mitigate for their reactivation. Our structural interpretations suggest that the Bowland Shale gas play remains highly challenged, with significant uncertainty in its resource estimates, the planning of well site locations, horizontal wellbore pathways and risk of induced seismicity on faults that are seismically resolvable and those that are of subseismic scale.
Acknowledgements
The PhD project upon which this work is based forms part of the Natural Environment Research Council (NERC) Centre for Doctoral Training (CDT) in Oil and Gas, a UK-wide flagship research and training programme partnership of 17 universities and two NERC research centres led and managed by Heriot-Watt University (HWU). Jingsheng Ma, Xiaoyang Wu, Dorrik Stow and David Griffiths are all thanked for their input and discussion. Malcolm Butler, Mark Alldred and Neil Anderson are acknowledged for providing access to the data via the UK Onshore Geophysical Library (UKOGL). The British Geological Survey (BGS) and Compagnie Générale de Géophysique (CGG) are thanked for providing access to well data, and Schlumberger are thanked for the provision of Techlog and Petrel software under academic licence to Heriot-Watt University. We acknowledge use of the Ogilvie-Gordon 3D Audio-Visualisation Centre (OGAVC) in the Heriot-Watt Wing of the Lyell Centre as an aid in guiding and honing our subsurface interpretations of the structure contained within the Bowland-12 seismic survey. We appreciate Stavros Vrachliotis’ assistance when displaying the 3D data in the OGAVC facility. Jon Walsh and Richard Wrigley are thanked for their constructive and helpful reviews, Graham Yielding is acknowledged for his editorial support and Lucy Bell from the Geological Society's Publication House is thanked for excellent support through the publication process.
Funding
Iain Anderson acknowledges the support of a James Watt Scholarship from Heriot-Watt University (HWU) and the receipt of a British University Funding Initiative (BUFI) studentship award (grant No. GA/16S/024) from the British Geological Survey (BGS), which provides the funding for the PhD project upon which this work is based. The PhD forms part of the Natural Environment Research Council (NERC) Centre for Doctoral Training (CDT) in Oil and Gas (grant No. NE/M00578X/1).
Author contributions
IA: formal analysis (equal), methodology (equal), visualization (lead), writing – original draft (equal), writing – review & editing (equal); JRU: conceptualization (lead), data curation (lead), formal analysis (equal), investigation (equal), visualization (supporting), writing – original draft (equal), writing – review & editing (equal).
- © 2020 The Author(s). Published by The Geological Society of London for GSL and EAGE. All rights reserved
References
Structural constraints on Lower Carboniferous shale gas exploration in the Craven Basin, NW England
