**2.1 Calcite 'beef' and its significance**

Calcite 'beef', first reported in 1826 by Webster [31], consists of bedding-parallel veins of diagenetic calcite (e.g., [32, 33]). In 1835, Buckland and De la Beche [34] adopted this nomenclature for veins of fibrous calcite within claystone beds in what is now known as the Purbeck Group in Dorset, the term 'beef' having originally

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

#### **Figure 6.**

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

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,


given the estimated ≥2 km of burial during the Cretaceous, before the Cenozoic denudation (e.g., [18]). The idea that the properties of calcite 'beef' enable the Newdigate Fault and neighbouring oil reservoirs to be hydraulically connected was suggested by Geosierra [44], but the present study proposes a different physical

In southern England, calcite 'beef' is best known in the Early Jurassic Shales-With-Beef Member (https://www.bgs.ac.uk/lexicon/lexicon.cfm?pub=SHWB) of the Charmouth Mudstone Formation, part of the Lias Group, which crops out around Lyme Regis in Dorset (e.g., [36, 45–47]). Calcite 'beef' is also well known in the Late Jurassic of the Weald Basin from both outcrop and borehole sections (e.g., [48]). In the Howett [48] stratigraphy, this fabric occurs within the 'Shales with Beef and Clay-ironstone' unit, which occurs at the top of the Middle Purbeck

This fabric (reported as 'calcite veining') is also known from older Late Jurassic

deposits, for example in core recovered between 701 and 710 m depth (below ground level 80.3 m O.D., so at 621–630 m TVDSS) in the Collendean Farm borehole near the Horse Hill site (**Figure 1**), in glauconitic sandstone forming the lower part of the Portland Group. Gallois and Worssam [15] placed this stratigraphic level in what they regarded as the sandy upper part of the underlying Kimmeridge Clay Formation. Nonetheless, in recent petroleum exploration reports (e.g., [26]), as in **Table 1**, this glauconitic sandstone with calcite 'beef' is reinstated within the Lower Portland Sandstone. Its inclusion within the Portland Group explains why this group is portrayed as much thicker in the recent petroleum-oriented literature (e.g., 130 m thick in **Table 1**) than by Gallois and Worssam [15], who stated its thickness as only 54 m at Collendean Farm. As these latter authors noted, the Portland Group in the Weald Basin is not well correlated with the 'type' Portlandian of the Portland area of Dorset, which is in the Portland – South Wight Basin (e.g., [21]). The 'type' Portlandian includes the Portland Limestone (now known as the Portland Stone Formation), an important building stone, the sediments of this age

The significance of all the above for the present study is as follows. It has previously been noted that the processes responsible for 'beef' formation will create permeability anisotropy, permeability being far greater parallel to the fabric and bedding than in the perpendicular direction (e.g., [39, 49]). Various workers have estimated the permeability of such bedding-parallel fractures, the highest estimate

et al. [50] for the Ordovician Utica Shale of eastern North America. This is many orders-of-magnitude higher than the expected nanodarcy permeability of shale

The study area has been illustrated using the map (**Figure 1**), and seismic cross-

section (**Figure 2**) from Hicks et al. [1]. However, the original versions of both these figures have required significant amendment regarding accuracy issues. This map was originally geolocated using geographical co-ordinates; to make it easier to use British National Grid (BNG) co-ordinates have been added. This map also shows seismic lines and faults. The information source for seismic lines, including line TWLD90–15 that is illustrated in **Figures 2** and **3**, was not reported by Hicks et al. [1]; it is evident that they are from the UKOGL location map. Hicks et al. [1] also explained that (in lieu of using the existing literature) they identified faults in the study area through their own interpretation of seismic lines. As already

perpendicular to bedding, and is quite a high value for rocks in general.

), being by Carey

mechanism as the cause of the hydraulic connection (**Figure 5**).

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

succession and is typically 20 m thick.

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

not being sandstone-dominated as in the Weald Basin.

**2.2 Geolocation**

**71**

identified during the present work, 900 mD (<sup>9</sup> <sup>10</sup><sup>13</sup> <sup>m</sup><sup>2</sup>

*Data for tops of stratigraphic subdivisions (as used by UKOGL; not all expressed using modern formal stratigraphic nomenclature, which is available from https://www.bgs.ac.uk/lexicon) are from the online well log (https://ukogl.org. uk/map/php/pdf.php?subfolder=wells\tops& filename=3041.pdf), supplemented by values from Pullan and Butler [13], NR indicating 'not reported'. Measured Depth (MD) is measured below a datum at 66.9 m O.D., below the local ground level of 74.5 m O.D. at the wellhead, at TQ 25254 43600. TVDSS is True Vertical Depth below O.D.; TWT is echo time. Values of interval velocity, VI, are determined in this study, ND indicating 'not determined'. Notes: 1. The Durlston Beds (or Durlston Formation) are nowadays regarded as earliest Cretaceous; the rest of the Purbeck Group is Late Jurassic. 2. The Cornbrash Formation is too thin here for its interval velocity to be reliably determined. 3. Interval velocities for the Mercia Mudstone and the Dolomitic Conglomerate combined. 4. TVDSS for the top Devonian estimated given the vertical orientation of the deepest part of the well. 5. The well bottoms (at TD) in Upper Devonian mudstone.*

#### **Table 1.**

*Stratigraphy of the Horse Hill 1 borehole.*

given the estimated ≥2 km of burial during the Cretaceous, before the Cenozoic denudation (e.g., [18]). The idea that the properties of calcite 'beef' enable the Newdigate Fault and neighbouring oil reservoirs to be hydraulically connected was suggested by Geosierra [44], but the present study proposes a different physical mechanism as the cause of the hydraulic connection (**Figure 5**).

In southern England, calcite 'beef' is best known in the Early Jurassic Shales-With-Beef Member (https://www.bgs.ac.uk/lexicon/lexicon.cfm?pub=SHWB) of the Charmouth Mudstone Formation, part of the Lias Group, which crops out around Lyme Regis in Dorset (e.g., [36, 45–47]). Calcite 'beef' is also well known in the Late Jurassic of the Weald Basin from both outcrop and borehole sections (e.g., [48]). In the Howett [48] stratigraphy, this fabric occurs within the 'Shales with Beef and Clay-ironstone' unit, which occurs at the top of the Middle Purbeck succession and is typically 20 m thick.

This fabric (reported as 'calcite veining') is also known from older Late Jurassic deposits, for example in core recovered between 701 and 710 m depth (below ground level 80.3 m O.D., so at 621–630 m TVDSS) in the Collendean Farm borehole near the Horse Hill site (**Figure 1**), in glauconitic sandstone forming the lower part of the Portland Group. Gallois and Worssam [15] placed this stratigraphic level in what they regarded as the sandy upper part of the underlying Kimmeridge Clay Formation. Nonetheless, in recent petroleum exploration reports (e.g., [26]), as in **Table 1**, this glauconitic sandstone with calcite 'beef' is reinstated within the Lower Portland Sandstone. Its inclusion within the Portland Group explains why this group is portrayed as much thicker in the recent petroleum-oriented literature (e.g., 130 m thick in **Table 1**) than by Gallois and Worssam [15], who stated its thickness as only 54 m at Collendean Farm. As these latter authors noted, the Portland Group in the Weald Basin is not well correlated with the 'type' Portlandian of the Portland area of Dorset, which is in the Portland – South Wight Basin (e.g., [21]). The 'type' Portlandian includes the Portland Limestone (now known as the Portland Stone Formation), an important building stone, the sediments of this age not being sandstone-dominated as in the Weald Basin.

The significance of all the above for the present study is as follows. It has previously been noted that the processes responsible for 'beef' formation will create permeability anisotropy, permeability being far greater parallel to the fabric and bedding than in the perpendicular direction (e.g., [39, 49]). Various workers have estimated the permeability of such bedding-parallel fractures, the highest estimate identified during the present work, 900 mD (<sup>9</sup> <sup>10</sup><sup>13</sup> <sup>m</sup><sup>2</sup> ), being by Carey et al. [50] for the Ordovician Utica Shale of eastern North America. This is many orders-of-magnitude higher than the expected nanodarcy permeability of shale perpendicular to bedding, and is quite a high value for rocks in general.
