Journal of Petroleum Geology, Vol.14,
No.2, pp. 143-160, 1991
©Copyright 2000 Scientific Press,
Ltd.
HYDROCARBON POTENTIAL OF
NORTHERN IRELAND:
Part II. Reservoir potential of the Carboniferous
J. Parnell*
*School of Geosciences, The
Queen's University of Belfast, Belfast BT7 1NN.
Abstract
Carboniferous sandstones occur in several
basins in the north of Ireland, deposited predominantly in
fluvio-deltaic environments. The Carboniferous is thickest in the
west of the region. Primary compositional variations are evident;
in particular, the sandstones are more arkosic in the west.
Variations in authigenic mineralogy also occur; sandstones in the
west contain greater volumes of authigenic quartz, and exhibit
hairy/fibrous illite which is lacking in the east. The
distribution of carbonate and sulphate cements is more irregular,
and is related to the distribution of marine limestone and
evaporite beds. Porosity in the sandstones is secondary after
dissolution of carbonate and sulphate cements and, to a lesser
degree, dissolution of unstable grains. Porosities are generally
low (less than 5%) in the west, but sandstones there may be
comparable with those in tight gas reservoirs elsewhere.
Sandstones in the east exhibit very high porosities in boreholes.
Kaolinite is a late phase occupying pore space in the upper parts
of most successions, deposited by downward-migrating meteoric
fluids, possibly during Permian times. Late iron oxides in some
sandstones were similarly precipitated during deep weathering
below the sub-Permian unconformity. Sandstones exhibit very
patchy bitumen residues in the west, demonstrating that oil has
migrated through these rocks.
