EARLY CARBONIFEROUS BASINS AND PLATFORMS OF CENTRAL BRITAIN*
Lake District massif
Shale gas study
BGS/DECC study area
East Midlands shelf
*CLH = Central Lancashire high; HH = Holme high. The presence of Early Carboniferous basins beneath the Permo- Triassic Cheshire basin (Smith et al., 2005, cf Waters et al., 2009) and a putative Humber basin (Kent, 1966,
and Hodge, 2003) are both debatable.
Source: Modifed after Fraser et al., 1990, Kirby et al., 2000
shales is typically in the range of 1-3% but can reach 8%.
The maturity of the Bowland-Hodder shales is a function
of burial depth, heat flow, and time, but subsequent uplift
complicates this analysis. Where they have been buried to
sufficient depth for the organic material to generate gas, the
Bowland-Hodder shales have the potential to form a shale
gas resource analogous to the producing shale gas provinces
of North America (e.g. Barnett shale, Marcellus shale). Where
the shales have been less-deeply buried, there is potential for
a shale oil resource (but, as yet, there is inadequate geotech-nical data to estimate the amount of oil in place).
In this study, shales are considered mature for gas generation (vitrinite reflectance > 1.1%) at depths greater than
c. 9,500 ft ( 2,900 m) (where there has been minimal uplift).
However, central Britain has experienced a complex tectonic
history and the rocks here have been uplifted and partially
eroded at least once since Carboniferous times. Because of
this, the present-day depth to the top of the gas window is
dependent on the amount of uplift, and can occur significantly shallower than 9,500 ft.
The total volume of potentially productive shale in central
Britain was estimated using a 3D geological model generated
using seismic mapping, integrated with outcrop and deep
borehole information. This volume was truncated upwards
at a depth of 5,000 ft ( 1,500 m) below land surface (a suggested US upper limit for thermogenic shale gas production)
or the depth at which the shale is mature for gas generation
(whichever was the shallowest).
The volume of potentially productive shale gas was used
as one of the input parameters for a statistical calculation (
using a Monte Carlo simulation) of the in-place gas resource.
For the purposes of resource estimation, the Bowland-Hodder unit is divided into two units: an upper postrift
unit in which laterally contiguous, organic-rich, condensed
zones can be mapped, even over the platform highs, and an
underlying synrift unit, expanding to thousands of feet thick
in fault-bounded basins, where the shale is interbedded with
mass flow clastic sediments and redeposited carbonates.
The upper unit is more prospective, primarily due to the
better well control which demonstrates its closer resemblance to the prolific North American shale gas plays, in
which the productive zones are hundreds of feet thick. The
lower unit is largely undrilled, but where it has been penetrated it contains organic-rich shale intervals whose lateral
extent is unknown.
This study offers a range of total in-place gas resource
estimates for the upper Bowland-Hodder unit shales
across central Britain of 164-264-447 tcf ( 4. 6-7. 5-12. 7 tcm)
(P90-P50-P10). It should be emphasized that these ‘
gas-in-place’ figures refer to an estimate for the entire volume of
gas contained in the rock formation, not how much can be