Reservoir evaluation of 8 wells in the Palaeozoic of the Irish Sea area : petrophysical interpretations of clay volume, porosity and permeability estimations

This report details the reservoir evaluation of 8 wells across the Palaeozoic (Carboniferous and Permian age) rocks of the UK Irish Sea for the 21CXRM Palaeozoic project. This reservoir evaluation is based on the petrophysical interpretation of available digital wireline log curve data for 8 wells and associated digitised core porosity and permeability data (available for 6 of the 8 wells interpreted, with 7 to 20 measurements per well) across the Palaeozoic interval (according to reinterpreted stratigraphic formations defined and correlated for this project, documented in Wakefield et al., 2016). Outputs of this part of the project include continuous (along borehole) interpretations of porosity, clay volume, and include basic permeability estimations. These interpreted curves were used to calculate Net to Gross (NTG) values and average porosities and permeabilities for each formation in each well analysed. The highest average porosities were found in the Permian aged Appleby Group (19%; previously termed the Collyhurst Sandstone). This unit also had the highest NTG and second highest average permeabilities of the units examined. Although the highest average permeability is low (0.13 mD) for the Appleby Group, maximum values in the 50-100 mD range are recorded for several wells. The Cumbrian Coast Group (Upper Permian), Pennine Lower Coal Measures (Carboniferous) and Millstone Grit Groups all had reasonable porosities averaging 11-14%, although they have low net to gross values (7-13%). The Cumbrian Coast Group (Upper Permian) includes some evaporite deposits of no reservoir potential themselves, but these could potentially act as a barrier (trap) to any hydrocarbons beneath them. Most of the other units in the wells examined show heterogeneous properties with low net to gross. Although the Millstone Grit Group generally has a low net to gross because of its high clay volume, cleaner reservoir intervals with reasonable porosity exist and more study on the permeabilities and distribution of these could be worthwhile. The basal limestones appear cleaner, but have very low matrix porosities and so are not considered to be potential reservoirs unless fractures contribute to their porosity and permeability (not examined here). Note that given the limited number of wells examined and the regional scale of the project, more detailed study of the reservoirs including mapping property trends and identifying prospective intervals was out of scope of this project. A brief examination of the distributions of net to gross and average porosities, both by formation in each well and for the total Palaeozoic interval in each well was not able to highlight any particular property trends or geographic areas with particularly favourable properties.