**1. Introduction**

#### **Highlights**.

Earthquakes at Newdigate in 2018–2019 correlate with oil production from Portland sst. A cause-and-effect relation via a high-permeability hydraulic connection is proposed. The model seismogenic fault incorporates internal structure with asperities. Testing by Mohr-Coulomb failure analysis incorporates effects of poroelasticity.

A 'swarm' of earthquakes with magnitudes up to 3, starting on 1 April 2018, has affected the Newdigate area of Surrey, in the Weald Basin of southeast England (**Figures 1**–**3**). As is detailed in the online supplement, a potential connection with

local oilfield activities, in the nearby Brockham-X2Y (BRX2Y) and Horse Hill-1 (HH1) wells, was immediately suspected, but dismissed by petroleum developers (e.g., [2, 3]). Concerns about the possibility that activities in these oilfields were indeed causing these earthquakes were raised through correspondence in The Times newspaper in August 2018 [4]. A workshop, convened by the Oil & Gas Authority (OGA), followed on 3 October 2018, the OGA being a UK government body with responsibilities that include the licencing of exploration and development of onshore oil and gas resources in England, including managing the risk of seismicity from such operations. A summary of the proceedings of this workshop was reported [5], including the statement that 'the workshop participants concluded that, based on the evidence presented, there was no causal link between the seismic events and oil and gas activity although one participant was less certain and felt that this could only be concluded on "the balance of probabilities" and would have liked to see more detailed data on recent oil and gas surface and subsurface activity' ([5], p. 1). It has subsequently been argued that there is indeed no such cause and effect connection (e.g., [1, 6]), developers having repeatedly issued strong public statements to this effect (e.g., [3, 7, 8]). However, a major issue, not noted in any of the above-mentioned works, is the clear temporal pattern of earthquake occurrence (**Figure 4**), with earthquakes strongly concentrated at times when oil is being produced from the Upper Portland Sandstone via the HH1 and/or BRX2Y wells. Production will reduce the fluid pressure in the reservoir being pumped. Fluid pressure changes within faults are well known as a cause of anthropogenic seismicity (e.g., [9, 10]); however, rather than a decrease, the causative change is usually an increase in fluid pressure as, for example, for the Preese Hall earthquake sequence in 2011, caused by injection of water under pressure during 'fracking' for

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

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

Given the geology of the Weald Basin [12, 13], a conceptual model can be envisaged whereby pressure reduction the Portland reservoir might bring nearby faults to the Mohr-Coulomb condition for slip, as illustrated in **Figure 5**. As

summarised in the figure caption, this model also provides a natural explanation for why production from the deeper Kimmeridge reservoir does not have an equivalent effect. Nonetheless, testing this model is difficult, for several reasons. The map and cross-section reported by Hicks et al. [1] provide the most detailed documentation of the Newdigate seismicity available (**Figures 1** and **2**), and thus serve as a basis for further discussion. However, a first reason why model testing is difficult is that use of these outputs is problematic because of mistakes in their preparation; before they can be used their geolocation has to be improved (as discussed in the present online supplement, also below). A second reason is uncertainty in the hydraulic properties of elements of the proposed model; this includes the distribution of the permeable 'calcite "beef"' fabric within clay-dominated lithologies that are otherwise impermeable. Each of these aspects will be investigated in this study. A third reason why

*BHF, Box Hill fault; CF, Crawley fault; COF, 'Collendean fault'; FGF, 'Faygate fault'; HF, Holmbush fault; HHF, Horse Hill fault; HKF, Hookwood fault; HWF, Holmwood fault; KFF, Kingsfold fault; LHF, Leigh fault; NGF, Newdigate fault; OKF, Ockley fault; WB1F, and Whiteberry-1 fault. Most of these structures are depicted as shown by Hicks et al. [1], although some are misplaced or misidentified, as discussed in the text. The Crawley and Holmwood faults, not recognised by Hicks et al. [1], are shown schematically (from [15]) where they cross seismic line TWLD-90-15, the southward continuation of which (beyond the excerpt shown in Figure 2) is also shown schematically. The 'Faygate fault' is a mistaken concept by Hicks et al. [1], so is shown 'greyed out' (see text and online supplement). Horse Hill 1 well track is from https://ukogl.org.uk/map/php/well\_deviation\_survey.ph p?wellId=3041. Positions of seismic lines, including line TWLD-90-15, are from the schematic location map provided by the UK onshore geophysical library (https://ukogl.org.uk/), which is indexed to the BNG, and was transformed to geographical co-ordinates by Hicks et al. [1]. Seismograph station GATW ceased operation on 17 May 2019 due to equipment theft. It was replaced by station GAT2, 230 m northwest, from 6 June.*

shale gas (e.g., [11]).

**61**

#### **Figure 1.**

*Map of the study area, modified from Figure 2(a) of Hicks et al. [1]. The original geographical (latitudelongitude) co-ordinate system has been retained, but 'greyed out', with a new co-ordinate system added, indexed to the British National Grid (BNG). Inset shows location. As is discussed in the main text, the original scale bar by Hicks et al. [1] is much too small and has also been 'greyed out'. Faults are identified thus: BF, Brockham fault;*

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

local oilfield activities, in the nearby Brockham-X2Y (BRX2Y) and Horse Hill-1 (HH1) wells, was immediately suspected, but dismissed by petroleum developers (e.g., [2, 3]). Concerns about the possibility that activities in these oilfields were indeed causing these earthquakes were raised through correspondence in The Times newspaper in August 2018 [4]. A workshop, convened by the Oil & Gas Authority (OGA), followed on 3 October 2018, the OGA being a UK government body with responsibilities that include the licencing of exploration and development of onshore oil and gas resources in England, including managing the risk of seismicity from such operations. A summary of the proceedings of this workshop was reported [5], including the statement that 'the workshop participants concluded that, based on the evidence presented, there was no causal link between the seismic events and oil and gas activity although one participant was less certain and felt that this could only be concluded on "the balance of probabilities" and would have liked to see more detailed data on recent oil and gas surface and subsurface activity' ([5], p. 1). It has subsequently been argued that there is indeed no such cause and effect connection (e.g., [1, 6]), developers having repeatedly issued strong public statements to this effect (e.g., [3, 7, 8]). However, a major issue, not noted in any of the above-mentioned works, is the clear temporal pattern of earthquake occurrence (**Figure 4**), with earthquakes strongly concentrated at times when oil is being produced from the Upper Portland Sandstone via the HH1 and/or BRX2Y wells. Production will reduce the fluid pressure in the reservoir being pumped. Fluid pressure changes within faults are well known as a cause of anthropogenic seismicity (e.g., [9, 10]); however, rather than a decrease, the causative change is usually an increase in fluid pressure as, for example, for the Preese Hall earthquake sequence in 2011, caused by injection of water under pressure during 'fracking' for shale gas (e.g., [11]).

Given the geology of the Weald Basin [12, 13], a conceptual model can be envisaged whereby pressure reduction the Portland reservoir might bring nearby faults to the Mohr-Coulomb condition for slip, as illustrated in **Figure 5**. As summarised in the figure caption, this model also provides a natural explanation for why production from the deeper Kimmeridge reservoir does not have an equivalent effect. Nonetheless, testing this model is difficult, for several reasons. The map and cross-section reported by Hicks et al. [1] provide the most detailed documentation of the Newdigate seismicity available (**Figures 1** and **2**), and thus serve as a basis for further discussion. However, a first reason why model testing is difficult is that use of these outputs is problematic because of mistakes in their preparation; before they can be used their geolocation has to be improved (as discussed in the present online supplement, also below). A second reason is uncertainty in the hydraulic properties of elements of the proposed model; this includes the distribution of the permeable 'calcite "beef"' fabric within clay-dominated lithologies that are otherwise impermeable. Each of these aspects will be investigated in this study. A third reason why

*BHF, Box Hill fault; CF, Crawley fault; COF, 'Collendean fault'; FGF, 'Faygate fault'; HF, Holmbush fault; HHF, Horse Hill fault; HKF, Hookwood fault; HWF, Holmwood fault; KFF, Kingsfold fault; LHF, Leigh fault; NGF, Newdigate fault; OKF, Ockley fault; WB1F, and Whiteberry-1 fault. Most of these structures are depicted as shown by Hicks et al. [1], although some are misplaced or misidentified, as discussed in the text. The Crawley and Holmwood faults, not recognised by Hicks et al. [1], are shown schematically (from [15]) where they cross seismic line TWLD-90-15, the southward continuation of which (beyond the excerpt shown in Figure 2) is also shown schematically. The 'Faygate fault' is a mistaken concept by Hicks et al. [1], so is shown 'greyed out' (see text and online supplement). Horse Hill 1 well track is from https://ukogl.org.uk/map/php/well\_deviation\_survey.ph p?wellId=3041. Positions of seismic lines, including line TWLD-90-15, are from the schematic location map provided by the UK onshore geophysical library (https://ukogl.org.uk/), which is indexed to the BNG, and was transformed to geographical co-ordinates by Hicks et al. [1]. Seismograph station GATW ceased operation on 17 May 2019 due to equipment theft. It was replaced by station GAT2, 230 m northwest, from 6 June.*

testing the proposed model is difficult is that key operational data, such as pressure variations in oil wells and logs of wellsite activities that might affect reservoir conditions, have been found to be unavailable. Indeed, preparation of this manuscript was delayed pending an attempt to obtain such data from the OGA under UK law using a Freedom of Information (FOI) request; this request was unsuccessful on the basis that the OGA did not hold such data, notwithstanding the scope of their statutory duties. In the absence of pressure data, testing the proposed model will be limited to investigating the magnitudes of pressure variations (and associated changes to the state of stress) that can be anticipated in the model fault and in each

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

well, as a result of operations in the other well, and the time delays for their propagation, subject to the assumed hydraulic properties for each lithology, which

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.*

**63**

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

are informed by the limited available data.

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

**2. Geological structure and stratigraphy**

#### **Figure 2.**

*2-D seismic section along seismic line TWLD-90-15, modified from Figure 6 of Hicks et al. [1]. The original was indexed by Hicks et al. [1] to the British National Grid (BNG), rather than the geographical co-ordinates used for Figure 1; it has been geolocated for this study using documentation provided by the UKOGL.*

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

testing the proposed model is difficult is that key operational data, such as pressure variations in oil wells and logs of wellsite activities that might affect reservoir conditions, have been found to be unavailable. Indeed, preparation of this manuscript was delayed pending an attempt to obtain such data from the OGA under UK law using a Freedom of Information (FOI) request; this request was unsuccessful on the basis that the OGA did not hold such data, notwithstanding the scope of their statutory duties. In the absence of pressure data, testing the proposed model will be limited to investigating the magnitudes of pressure variations (and associated changes to the state of stress) that can be anticipated in the model fault and in each well, as a result of operations in the other well, and the time delays for their propagation, subject to the assumed hydraulic properties for each lithology, which are informed by the limited available data.
