Olatunde Atoyebi 7480184 POSTER

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

[email protected]

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 DEPTH VS 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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VITRINITE REFLECTANCE (%)

UP HOLLAND-1 DEPTH VS VITRINITE

REFLECTANCE

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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.