**2. Geological structure and stratigraphy**

The study area is in southeast England, near the boundary between the counties of Surrey and West Sussex, 40 km WSW of central London, on the northern flank of the Weald Basin (**Figures 1** and **6**). The outcrop geology and shallow subsurface structure of this area are documented by Dines and Edmunds [14] and Gallois and Worssam [15]; Trueman [16], DECC [17], and others have discussed the history of petroleum exploration. Many authors have discussed the origin and structure of the Weald Basin, or Weald sub-basin of the wider Wessex Basin (e.g., [12, 13, 18–25]). As these and many other works demonstrate, this basin has developed near the northern margin of the Variscan orogenic belt, Variscan reverse faults having been reactivated as normal faults during the Mesozoic. Chadwick [19] resolved two phases of Mesozoic extension, during the Early Jurassic (Hettangian to Toarcian; extension factor, β, 1.12) and Late Jurassic and earliest Cretaceous (late Oxfordian to Valangian; β = 1.10). The Jurassic and Cretaceous sedimentary formations that accompanied and followed this extension are documented in many works and summarised in the British Geological Survey (BGS) stratigraphic lexicon (https:// www.bgs.ac.uk/lexicon/); **Table 1** summarises the local stratigraphy, based on the HH1 well record. This basin experienced Cenozoic inversion, when some of the Mesozoic normal faults were reactivated as reverse faults (e.g., [23]; **Figure 6**). As a result of this history, some faults have normal offsets within the syn-rift succession but show reverse slip in younger sediments, as illustrated in **Figure 2**.

Most oil reservoirs in the study area are in the Upper Portland Sandstone, a shallow marine sandstone of Upper Jurassic age, sealed above by the overlying impermeable Purbeck Anhydrite, deposition of which reflects isolation of the Weald Basin interior from the sea (e.g., [21]; **Table 1**). The oil–water contact for the Horse Hill reservoir has been inferred as 580 m TVDSS [26], thus roughly at the mid-point of the Upper Portland Sandstone. The modelled extent of this 3 km square reservoir is illustrated in **Figure 7**; to the north and south it is sealed by

*The labelled horizons were not explained by Hicks et al. [1], but appear to be the top Portland group, top Kimmeridge clay formation, and top coralline Oolite formation (cf. Table 4). Faults designated by Hicks et al. [1] are identified thus: COF, Collendean fault; LHF, Leigh fault; and NGF, Newdigate fault. CF denotes the Crawley fault. The depth scale from Hicks et al. [1], which appears to be based on their velocity model for earthquake location (Table 2), is 'greyed out', being now considered inaccurate. The new depth scale, using the seismic interval velocities from well HH1 (Table 1) and now considered more accurate, is emphasised. Additional interpretation has also been added, including the interpreted top Penarth group / base Lias group reflector and its offset by the main strand of the Newdigate fault, and some of the additional lesser fault strands forming the multi-stranded Newdigate fault zones, other strands being evident in Figure 3 and in the uninterpreted version of this seismic section provided by Hicks et al. [1] in their online supplement.*

reservoir in the Kimmeridge Limestone (e.g., [26]) (the 'Kimmeridge micrites' in **Table 1**). However, the production from this reservoir shows no correlation with seismicity (**Figure 4**), indicating that this deeper reservoir is not hydraulically

*Seismicity at Newdigate, Surrey, during 2018–2019: A Candidate Mechanism…*

The smaller (2 km 1 km) Brockham reservoir (**Figure 8**) has a permeability of up to 200 mD (e.g., [27]), its base at 582 m TVDSS [27]. As **Figure 8** shows, it is sealed to the southeast by the Brockham Fault and in other directions by the dip of the Purbeck Anhydrite. However, although the Brockham Fault has 40 m of throw and acts as an impermeable reservoir seal, 1 km farther SW, where the top Portland Sandstone is deeper than the reservoir base, this fault has low throw and the permeable sandstone is juxtaposed on both sides. This aspect of the fault geometry (discussed in more detail in the supplement) is key to the proposed conceptual model (**Figure 5**), as it makes possible a permeable hydraulic connection between the Brockham reservoir and the seismogenic Newdigate Fault to the southeast. In contrast, Hicks et al. [1] depicted the Brockham Fault as sealing the reservoir to the SE for 3 km distance (**Figure 1**). Together with the different geometry of other faults in the area, this portrayal would preclude the possibility of any permeable hydraulic connection between this reservoir and the Newdigate Fault. The Angus [27] fault reconstruction is favoured in the present study, being assessed as more soundly based than either the Hicks et al. [1] version or the earlier interpretation by Europa [28] (see supplement). As detailed in the supplement, during prolonged production from Brockham well BRX2Y from 1998 to 2016, amounting to 36,900 m3 [11], the reservoir pressure had decreased from 900 to 500 psi. Angus [29] reported that at the end of production in 2016 the reservoir pressure was 490 psi or 3.4 MPa, indicating a decline by 2.7 MPa below the expected

The geometry of the Newdigate Fault also differs radically between the Xodus [26] and Hicks et al. [1] interpretations (**Figures 1** and **7**). Hicks et al. [1] depict it as

Newdigate area to the vicinity of Horley, its eastern part thus forming the southern seal to the Horse Hill reservoir. In contrast, Xodus [26] depicted it as dying out eastwards 3 km SW of the HH1 well, at the eastern end of the Newdigate seismicity (**Figure 7**). In this interpretation the fault that forms the southern seal of the Horse Hill reservoir (here designated the Hookwood Fault, HKF; **Figure 7**) is separated from the Newdigate Fault (NGF) by a 1 km 'gap' around Charlwood. To investigate this difference, the records from seismic lines C79–36 and TWLD90– 21, which run N-S through this area, were obtained from the UK Onshore Geophysical Library (OGL; https://ukogl.org.uk/) archive. Neither seismic line shows any stratigraphic offset transecting this area, favouring the Xodus [26] interpretation, thus also casting doubt on other aspects of the Hicks et al. [1] analysis where they differ from other interpretations (such as for the aforementioned Brockham Fault), and initiating thorough checking of their article, which identified other

The base of the Jurassic sequence lies at 2100–2200 m depth in the study area (e.g., [12, 13]; **Figure 6**). This sequence is locally underlain by thin Triassic deposits overlying pre-Variscan (Palaeozoic) 'basement' at depths of > 2200 m [30]. The uppermost 'basement' in much of this area is known from borehole records to be Dinantian (Early Carboniferous) limestone [13, 30]. Thus, the HH1 well log (**Table 1**) indicates that the Jurassic Lias Group is underlain by 60 m of latest Triassic Penarth Group ('Rhaetic') rocks, then 50 m of the Triassic Mercia Mudstone, then 10 m of dolomitic conglomerate of uncertain age, then 70 m of Dinantian limestone, above a mudstone-dominated Upper Devonian succession.

a continuous structure with total E-W length 13 km, extending from the

connected to the seismogenic Newdigate Fault.

*DOI: http://dx.doi.org/10.5772/intechopen.94923*

hydrostatic pressure of 6.1 MPa at its 622 m depth.

problematic aspects (see **Figure 1** caption, also below).

**65**

#### **Figure 3.**

*Excerpt from the record section for seismic line TWLD-90-15 between shot points 1400 and 2070, as provided by UKOGL, illustrating the Newdigate fault. As geo-located using documentation provided by UKOGL, this excerpt extends between BNG references TQ 21618 38952 and TQ 20381 46852, a distance of 8 km. The location of this seismic line adjoining the Newdigate fault is illustrated in Figures 1 and 7.*

normal faults (the Horse Hill Fault and Hookwood Fault) across which the Purbeck Anhydrite is downthrown, whereas in other directions it is sealed by the dip of the impermeable cap rock. This reservoir, hydraulically connected to the HH1 and HH2Z wells, from which production began in mid 2018 and late 2019, respectively (see supplement), is thus much larger than the hydraulic 'radius of influence' depicted by Hicks et al. [1] in **Figure 1**. The neighbouring Collendean Farm reservoir is to the north, separated by the Collendean Farm Fault, was reached by the Collendean Farm-1 (CF1) borehole (drilled in 1964; BGS ID TQ24SW1; at TQ 2480 4429; **Figure 7**). Oil has also been produced by the HH1 well from a deeper

*Seismicity at Newdigate, Surrey, during 2018–2019: A Candidate Mechanism… DOI: http://dx.doi.org/10.5772/intechopen.94923*

reservoir in the Kimmeridge Limestone (e.g., [26]) (the 'Kimmeridge micrites' in **Table 1**). However, the production from this reservoir shows no correlation with seismicity (**Figure 4**), indicating that this deeper reservoir is not hydraulically connected to the seismogenic Newdigate Fault.

The smaller (2 km 1 km) Brockham reservoir (**Figure 8**) has a permeability of up to 200 mD (e.g., [27]), its base at 582 m TVDSS [27]. As **Figure 8** shows, it is sealed to the southeast by the Brockham Fault and in other directions by the dip of the Purbeck Anhydrite. However, although the Brockham Fault has 40 m of throw and acts as an impermeable reservoir seal, 1 km farther SW, where the top Portland Sandstone is deeper than the reservoir base, this fault has low throw and the permeable sandstone is juxtaposed on both sides. This aspect of the fault geometry (discussed in more detail in the supplement) is key to the proposed conceptual model (**Figure 5**), as it makes possible a permeable hydraulic connection between the Brockham reservoir and the seismogenic Newdigate Fault to the southeast. In contrast, Hicks et al. [1] depicted the Brockham Fault as sealing the reservoir to the SE for 3 km distance (**Figure 1**). Together with the different geometry of other faults in the area, this portrayal would preclude the possibility of any permeable hydraulic connection between this reservoir and the Newdigate Fault. The Angus [27] fault reconstruction is favoured in the present study, being assessed as more soundly based than either the Hicks et al. [1] version or the earlier interpretation by Europa [28] (see supplement). As detailed in the supplement, during prolonged production from Brockham well BRX2Y from 1998 to 2016, amounting to 36,900 m3 [11], the reservoir pressure had decreased from 900 to 500 psi. Angus [29] reported that at the end of production in 2016 the reservoir pressure was 490 psi or 3.4 MPa, indicating a decline by 2.7 MPa below the expected hydrostatic pressure of 6.1 MPa at its 622 m depth.

The geometry of the Newdigate Fault also differs radically between the Xodus [26] and Hicks et al. [1] interpretations (**Figures 1** and **7**). Hicks et al. [1] depict it as a continuous structure with total E-W length 13 km, extending from the Newdigate area to the vicinity of Horley, its eastern part thus forming the southern seal to the Horse Hill reservoir. In contrast, Xodus [26] depicted it as dying out eastwards 3 km SW of the HH1 well, at the eastern end of the Newdigate seismicity (**Figure 7**). In this interpretation the fault that forms the southern seal of the Horse Hill reservoir (here designated the Hookwood Fault, HKF; **Figure 7**) is separated from the Newdigate Fault (NGF) by a 1 km 'gap' around Charlwood. To investigate this difference, the records from seismic lines C79–36 and TWLD90– 21, which run N-S through this area, were obtained from the UK Onshore Geophysical Library (OGL; https://ukogl.org.uk/) archive. Neither seismic line shows any stratigraphic offset transecting this area, favouring the Xodus [26] interpretation, thus also casting doubt on other aspects of the Hicks et al. [1] analysis where they differ from other interpretations (such as for the aforementioned Brockham Fault), and initiating thorough checking of their article, which identified other problematic aspects (see **Figure 1** caption, also below).

The base of the Jurassic sequence lies at 2100–2200 m depth in the study area (e.g., [12, 13]; **Figure 6**). This sequence is locally underlain by thin Triassic deposits overlying pre-Variscan (Palaeozoic) 'basement' at depths of > 2200 m [30]. The uppermost 'basement' in much of this area is known from borehole records to be Dinantian (Early Carboniferous) limestone [13, 30]. Thus, the HH1 well log (**Table 1**) indicates that the Jurassic Lias Group is underlain by 60 m of latest Triassic Penarth Group ('Rhaetic') rocks, then 50 m of the Triassic Mercia Mudstone, then 10 m of dolomitic conglomerate of uncertain age, then 70 m of Dinantian limestone, above a mudstone-dominated Upper Devonian succession.

Busby and Smith [30] estimated using gravity modelling that these Devonian rocks are typically 1–2 km thick, their base at a typical depth of 3.5–4 km, being underlain in the central Weald Basin by many kilometres of Lower Palaeozoic metamorphic basement. Around the northern margin of the basin and the southern margin of the adjoining London Platform the Dinantian limestone and underlying Devonian rocks are well imaged seismically at <1 s two way time (TWT), indicating depth < 1500 m (e.g., [18]). Both these subdivisions are locally several hundred metres thick, the limestone being relatively unreflective and the Devonian succession highly reflective. Moving southward, as the overlying Mesozoic succession thickens, the Dinantian limestone gradually becomes thinner and its boundaries become more difficult to interpret seismically (e.g., [18]). Busby and Smith [30] noted reports of this limestone in many boreholes beneath the Weald Basin; in their view it persists southward beneath most of the basin, almost to the English

*Seismicity at Newdigate, Surrey, during 2018–2019: A Candidate Mechanism…*

*DOI: http://dx.doi.org/10.5772/intechopen.94923*

More recently, Pullan and Butler [13] have presented a new map (their Fig. 21) showing the pre-Variscan subcrop beneath much of the Weald Basin (including most of the area of **Figure 1**) as Devonian, the Dinantian limestone being inferred to be absent. As interpreted by these workers, this limestone dies out 2.5 km SW of the HH1 well, indicating that it is absent in the vicinity of the Newdigate fault and the associated seismicity. However, seismic lines in this area (e.g., that in **Figure 2**, which is part of line TWLD90–15), as well as lines C79–36 and TWLD90–21 that have also been scrutinised as part of this study, do not clearly resolve whether this limestone is present or not; as Pullan and Butler [13] showed, there is no well control for 20 km distance SW of the HH1 well, so no direct evidence either way. Pullan and Butler [13] noted dipmeter evidence that in the HH1 well this limestone dips northward at 20–30°; their inference that it dies out not far away seems based on structural projection given its thickness (**Table 1**) and assuming continued northward dip. However, it is clear from other seismic sections (e.g., [18]) that in other parts of the basin the Dinantian limestone is folded. At this stage it is unclear whether this lithology is present across the study area or not. The proposed conceptual model (**Figure 5**) requires a highly permeable lithology, such as this,

The two key issues already noted will now be addressed. The distribution and

properties of calcite 'beef' will first be discussed. Second, the geolocation of

*timeline for production at Brockham up to 2016: The correct timeline is shown in Fig. 11 of Angus [27]. (a) Installation dates of stations forming the local temporary seismic monitoring network. From Fig. 3(a) of Hicks et al. [1]. (b) Detected earthquakes, cumulative number of events, and inferred variations in the completeness threshold magnitude MC. From Fig. 3(b) of Hicks et al. [1]. Note that some of the magnitudes ML depicted here differ slightly from those listed in supplementary Fig. S2 of Hicks et al. [1], which feature in discussion in the text. (c) Summary timeline for activities at the Horse Hill 1 well, indicating (based on the information sources discussed in the text and details in part (d)) the phases of production from each reservoir. Notes refer to details discussed in the text, thus, regarding the HH1 well: 1, first known intervention affecting the well, 5 April 2018; 2, removal of bridge plug that had isolated the Portland reservoir from the surface, 4 July 2018; 3, production from KL3; 4, production from KL4; and 5, 'co-mingled' production from both KL3 and KL4. Regarding the BRX2Y well, based on the timeline reported by Hicks et al. [1]: 6 denotes the restart of production on 22 march 2018; 7 denotes a later resumption, with injection of water starting on 25 June and (net) production restarting on 28 June (but with both injection and production occurring on 27 June); and 8 denotes the end of production on 15 October 2018. (d) More detailed operations timeline for activities at the Horse Hill 1 well, with flow-period averaged production and cumulative production over time. From Fig. 3(c) of Hicks et al. [1], with further details, including dates, provided in their supplementary Table S4. The information provided by Hicks et al. [1] is much more detailed than that which has been otherwise released into the public domain, and must have been obtained from the developer. However, their reporting of the information does not identify the hydrocarbon reservoirs to which the activities relate (see part (c)), which is essential to reveal the pattern of correlation between seismicity and activities affecting the Portland reservoir*

Channel coastline.

beneath the Mesozoic sediment in this area.

*(see also the online supplement).*

**67**

features, mislocated by Hicks et al. [1], will be considered.

#### **Figure 4.**

*Time series of Newdigate earthquakes and other activities affecting the Horse Hill 1 and Brockham X2 wells, based on Figure 3 of Hicks et al. [1]. Note that their original Fig. 6(d) has been omitted as it depicts an incorrect*

*Seismicity at Newdigate, Surrey, during 2018–2019: A Candidate Mechanism… DOI: http://dx.doi.org/10.5772/intechopen.94923*

Busby and Smith [30] estimated using gravity modelling that these Devonian rocks are typically 1–2 km thick, their base at a typical depth of 3.5–4 km, being underlain in the central Weald Basin by many kilometres of Lower Palaeozoic metamorphic basement. Around the northern margin of the basin and the southern margin of the adjoining London Platform the Dinantian limestone and underlying Devonian rocks are well imaged seismically at <1 s two way time (TWT), indicating depth < 1500 m (e.g., [18]). Both these subdivisions are locally several hundred metres thick, the limestone being relatively unreflective and the Devonian succession highly reflective. Moving southward, as the overlying Mesozoic succession thickens, the Dinantian limestone gradually becomes thinner and its boundaries become more difficult to interpret seismically (e.g., [18]). Busby and Smith [30] noted reports of this limestone in many boreholes beneath the Weald Basin; in their view it persists southward beneath most of the basin, almost to the English Channel coastline.

More recently, Pullan and Butler [13] have presented a new map (their Fig. 21) showing the pre-Variscan subcrop beneath much of the Weald Basin (including most of the area of **Figure 1**) as Devonian, the Dinantian limestone being inferred to be absent. As interpreted by these workers, this limestone dies out 2.5 km SW of the HH1 well, indicating that it is absent in the vicinity of the Newdigate fault and the associated seismicity. However, seismic lines in this area (e.g., that in **Figure 2**, which is part of line TWLD90–15), as well as lines C79–36 and TWLD90–21 that have also been scrutinised as part of this study, do not clearly resolve whether this limestone is present or not; as Pullan and Butler [13] showed, there is no well control for 20 km distance SW of the HH1 well, so no direct evidence either way. Pullan and Butler [13] noted dipmeter evidence that in the HH1 well this limestone dips northward at 20–30°; their inference that it dies out not far away seems based on structural projection given its thickness (**Table 1**) and assuming continued northward dip. However, it is clear from other seismic sections (e.g., [18]) that in other parts of the basin the Dinantian limestone is folded. At this stage it is unclear whether this lithology is present across the study area or not. The proposed conceptual model (**Figure 5**) requires a highly permeable lithology, such as this, beneath the Mesozoic sediment in this area.

The two key issues already noted will now be addressed. The distribution and properties of calcite 'beef' will first be discussed. Second, the geolocation of features, mislocated by Hicks et al. [1], will be considered.

*timeline for production at Brockham up to 2016: The correct timeline is shown in Fig. 11 of Angus [27]. (a) Installation dates of stations forming the local temporary seismic monitoring network. From Fig. 3(a) of Hicks et al. [1]. (b) Detected earthquakes, cumulative number of events, and inferred variations in the completeness threshold magnitude MC. From Fig. 3(b) of Hicks et al. [1]. Note that some of the magnitudes ML depicted here differ slightly from those listed in supplementary Fig. S2 of Hicks et al. [1], which feature in discussion in the text. (c) Summary timeline for activities at the Horse Hill 1 well, indicating (based on the information sources discussed in the text and details in part (d)) the phases of production from each reservoir. Notes refer to details discussed in the text, thus, regarding the HH1 well: 1, first known intervention affecting the well, 5 April 2018; 2, removal of bridge plug that had isolated the Portland reservoir from the surface, 4 July 2018; 3, production from KL3; 4, production from KL4; and 5, 'co-mingled' production from both KL3 and KL4. Regarding the BRX2Y well, based on the timeline reported by Hicks et al. [1]: 6 denotes the restart of production on 22 march 2018; 7 denotes a later resumption, with injection of water starting on 25 June and (net) production restarting on 28 June (but with both injection and production occurring on 27 June); and 8 denotes the end of production on 15 October 2018. (d) More detailed operations timeline for activities at the Horse Hill 1 well, with flow-period averaged production and cumulative production over time. From Fig. 3(c) of Hicks et al. [1], with further details, including dates, provided in their supplementary Table S4. The information provided by Hicks et al. [1] is much more detailed than that which has been otherwise released into the public domain, and must have been obtained from the developer. However, their reporting of the information does not identify the hydrocarbon reservoirs to which the activities relate (see part (c)), which is essential to reveal the pattern of correlation between seismicity and activities affecting the Portland reservoir (see also the online supplement).*

#### **Figure 5.**

*Cartoons summarising the proposed conceptual model linking the Newdigate seismicity to reductions in fluid pressure in oil wells. (a) Cross-section showing large-scale processes. Production of oil (1) will reduce the pressure within the Portland reservoir near the production well. This will cause flow of oil towards the well from more distal parts of the reservoir (2). This will be accompanied by flow of groundwater into the volume vacated by the oil, which is inferred to be hydraulically connected (presumably by fractures) to the 'hyper-permeable' calcite 'beef' fabric in the underlying lower Portland sandstone a short distance below (3), which will cause flow within this fabric (4). This 'beef' fabric is inferred to be continuous as far as the Newdigate fault zone. The flow within it will therefore draw groundwater from greater depths (5), up one or more strands of this fault zone, which is assumed permeable, reducing the pressure in the section where this fault transects the Dinantian limestone. This pressure reduction will act to draw groundwater from the permeable Dinantian limestone into the fault (6). The associated compaction of the Dinantian limestone will cause separation of its two surfaces across the seismogenic fault (7). Surface interventions affecting the wells, such as bleeding pressure following shut-in, will reduce the pressure inside the well and have a similar overall effect. In contrast, the Kimmeridge reservoir is below the 'beef' layer, so no corresponding downward hydraulic connection from this reservoir exists, therefore production from this reservoir causes no equivalent effect on the hydrology and state of stress within the Dinantian limestone. (b) Plan view of a patch of the seismogenic fault showing processes on a micro-structural scale on the seismogenic fault, where separation of the fault surfaces by a small distance Δx, from configuration (i) to configuration (ii), affects three model asperities (1, 2 and 3). After this change, at asperity 1 the fault surfaces are no longer in contact, at asperity 2 what was an interference fit between the fault surfaces has become a clearance fit, and the rocks forming at asperity 3 have decompressed elastically, so they remain in contact but with a reduced normal stress and thus a reduced limiting shear stress that can maintain fault stability.*

been used by quarry workers on account of similarity to the fibrous structure of meat. This fabric (illustrated by many authors, including [35–37]), is now recognised in mudstone formations worldwide (e.g., [35]). Following the abovementioned early reporting its mode of origin was widely debated; the view has become accepted relatively recently that 'beef' develops by natural hydraulic fracturing associated with overpressure during hydrocarbon maturation and migration (e.g., [32, 33, 35, 36, 38–41]; cf. [36, 42]). This fabric is indeed sometimes designated as 'hydrocarbon-expulsion fractures' (e.g., [43]). The conditions for calcite 'beef' development include palaeo-temperature 70–120 °C [35]. In the central Weald Basin, such conditions are expected throughout the Jurassic succession,

*although the latter is misplaced. Modified from part of Fig. 4(a) of Butler and Pullan [12].*

*Seismicity at Newdigate, Surrey, during 2018–2019: A Candidate Mechanism…*

*DOI: http://dx.doi.org/10.5772/intechopen.94923*

*Map of the structure of British National Grid 100 km 100 km quadrangle TQ, showing the depth of base Jurassic (in feet below O.D., with contours at 200 ft. or 60 m intervals) and locations where the base Jurassic is offset by faults. Faults in the vicinity of the present study area are named: C and M correspond to the Crawley and Maplehurst faults [15]; B and H appear to denote the Box Hill and Holmwood faults (cf. Figure 1),*

**Figure 6.**

**69**
