1. CARBONIFEROUS HOLYWELL SHALE OF NORTH WALES AS A SHALE GAS PLAY.
FACULTY OF ENGINEERING AND PHYSICAL SCIENCES,SCHOOL OF EARTH, ATMOSPHERIC AND ENVIRONMENTAL SCIENCES, THE UNIVERSITY OF MANCHESTER. M.Sc. PETROLEUM GEOSCIENCE.
AUTHOR: OLATUNDE OYEBANJI ATOYEBI SUPERVISOR: Prof. KEVIN TAYLOR
1. INTRODUCTION: The recent success of the shale gas exploration as an unconventional resource in the U.S has
created a lot of pulses of research and exploration interests in other parts of the world, the U.K has been recently
ranked higher in Europe based on the well test results of the pioneer shale gas exploration in North England and
North Wales and this is due to enormous reserves of shale gas within the Namurian shales.
AIMS: Assessment of the Carboniferous Holywell Shale of North Wales as a shale gas play. Collection of available
data to justify the shale gas prospectively of the Holywell Shale.
2. METHODOLOGY AND DATABASE.
HOLYWELL SHALE
These are Carboniferous (Namurian) units of thick shales of Pendleian to Yeadonian age (fig.2), known from outcrop
in North Wales and Bowland Basin and from wells in the Southern part of East Irish Sea basin. The Namurian basinal
marine oil-and gas prone shales , which form the main
source for the Triassic play in the East Irish Sea Basin
(Floodpage et al, 2009). Figure 1 shows the main part
North Wales where Holywell Shale outcrop and borehole
samples have been used for the assessment.
–Mainly dark grey fissile and blocky mudstone, weakly calcareous, with subordinate sequences of interbedded lime-
ston and sandstone, fossiliferous in more-or-less discrete bands. Thickness between 120m and 620m. The formation
thickens north-eastwards along the axis of the Central Lancashire High as shown in figure 2. It is accumulated in pro-
delta environment with increased in marine influence northwards (Fig. 2), mudstones with thin sandstone, impure
limestones and impersistent coal seams.
Geochemical: Data from outcrops and wells are used to assess the shale gas features of the Holywell shales as a
shale gas play in the U.K. Figure 3 shows some of the fields sourced by Holywell Shale.
Figure 1: Map showing the areal distribution of the Bowland Shale group (Holywell shales) within the North Wales. Bubble points show few bore-
holes explored to describe the variation of Holywell Shales.
These include CARPLOT , Whole Oil GC samples, TOC data(Fig.5) Pie Chart,
Vitrinite Reflectance Excel plots in figure 6 . These indicate the shale gas pro-
spect of the Holywell Shale (Armstrong et al, 1997).
Mineralogy and Petrology: The relative amount of clay minerals to quartz de-
termines the ease to fracture a shale gas reservoir, Holywell Shale has about
56% clay while possessing other mineralogical qualities such as fissibility, ce-
mented weak fractures areas. However the rock physical and geomecchanical
properties need to be understudied and understood prior to development and
production.
Geophysics &Petrophysics: The Holywell Shale (Bowland Shale Group) as
shown in the Thistleton-1 log below (Fig. 5) indicates “gas show”. Other logs
indicate high gas content as well seismic gas effect within the Holywell Shale.
BGS borehole data (Figure 1&2) indicate the distribution across the North
Wales with depth, thickness and lithofacies variations.
Analogue data: The Holywell Shale has been geochemically matched to the
oils recovered from Lennox, Douglas and Hamilton fields (Armstrong et al
1997). The Holywell Shale and Barnett Shale of the U.S share many geologic
similarities in terms of : structural histories and basin evolution, Geochemical
features, sedimentology and sequence stratigraphy however they are not ab-
solute as each shale play has its uniqueness.
3.CARBONIFEROUS BASIN EVOLUTION AND STRUCTURAL SETTING
Structures inherited from the Caledonian Orogeny are the main controlling fea-
tures during early Carboniferous (Fig.7). In North Wales the combinations of
these structures and North-South extensional rifting let to the asymmetric gra-
ben with relative subsidence. In late Dianantian times, uplift source areas led
to southwards prodeltaic deposition of a giant Clastic delta complex which rap-
idly filled the basinal areas. The final closure of the Rheic Ocean led to Veri-
scan orogeny in Late Westphalian and Permian times. Figure 7 shows a re-
gional cross section of the Bowland Shale Formation (Holywell shale) inter-
tongued predominantly within Clastic sediments.
4. RESULTS & INTERPRETATION: Holywell Shale exhibits heterogeneity in
term of geochemistry and sedimentology, this is confirmed in varieties of four
phases of hydrocarbon generated as identified by Armstrong et al, however it
has high gas content both thermogenic and biogenic (R.C Selley, 2010). The
Whole oil GC samples in figure 5 shows relative abundance of the lighter hy-
drocarbons (<C15) while other data (Pie Chart & CARPLOT) confirmed the
source richness, quality and thermal maturation of the shale.
5. CONCLUSION: Holywell Shale satisfied all the necessary conditions for a
shale gas play assessment besides, the current well test results of Island Gas
indicates more reserves of shale gas than the pre-drill estimate, and favoura-
ble legislation, gas price and availability of the latest technologies are critical
for its exploration success in the UK.
There is need to evaluate the Holywell shale physical rock properties and
stress-strain relationship in order to effectively develop and produce the shale
gas.
ACKNOWLEDGEMENT: I would like to express my gratitude to Prof. Kevin and Jim for their support and supervision.
REFERENCES: Armstrong, J.P., Smith, J., D’Elia, V.A.A., Trueblood, S.P., 1997. The occurrence and correlation of oils and Namurian
source rocks in the Liverpool Bay-North Wales Area. Geological Society of London Special Publication No. 124. pp. 195-211.
Jonathan Floodpage, Phil Newman & Jason White. (2001). Hydrocarbon Prospectivity in the Irish Sea Area: Insight from Recent Explo-
ration of The Central Irish Sea, Peel and Solway Basins. Geological Society, London, Special Publications; v.188; 107-134.
Harvey, T., Gray, J., 2010. The Unconventional Hydrocarbon Resources of Britain’s Onshore Basins Shale Gas. Department of Energy
& Climate Change.
British Geological Survey website. 2012.http://www.bgs.ac.uk/data/boreholescans/home.html
colatunde@gmail.com
Figure 2: Lithostratigraphy , Biostratigraphy and distribution of the Holywell Shale within the Millstone Grit Group Hampson et al, 2007.
Figure 3: Hydrocarbon fields in North Wales sourced from Holywell Shale of the Mill-
stone Grit Group .
Figure 4: Thistleton-1 well
log showing gas within
Holywell Shale.
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Borehole Vitrinite Reflectance (%)
BOREHOLE DEPTHVS VITRINITE REFLECTANCE
UpHolland-1
Knutdford-1
South Lancanshire
Linear (UpHolland-1)
Linear (Knutdford-1)
Linear (South Lancanshire)
Figure5:Geochemical data: CARPLOT of Well 110/13 showing petrophysical data and the TOC and S2 of Holywell Shale.
The Pie Chart below shows the TOC of samples results of Holywell Shale.
Figure 7: Regional Cross section of structural and stratigraphic settings showing the Namurian shale distribution, more thicker
towards the North between the Microcraton and Caledonide belts (Harvey & Gray, 2010).
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UP HOLLAND-1 DEPTH VS VITRINITE
REFLECTANCE
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SOUTH LANCASHIRE DEPTH VS VITRINITE
REFLECTANCE
Figure 6: Geochemical Vitrinite Reflectance data plots to demonstrate the thermal maturity of the Holywell Shale.
Whole Oil GC samples of
Holywell Shale showing
abundance of lighter hy-
drocarbons <C15 Alkanes.