Reservoir evaluation of 12 wells in the Devonian-Carboniferous of the Central North Sea : petrophysical interpretations of clay volume, porosity and permeability estimations

This report details the reservoir evaluation of 12 wells across the Devonian-Carboniferous rocks of the UK Central North Sea for the 21CXRM Palaeozoic project. A companion report examines the source rock potential (total organic carbon content) of the non-reservoir intervals (for a different, but overlapping set of wells) (Gent, 2015). This reservoir evaluation is based on the petrophysical interpretation of available digital wireline log curve data for the 12 wells and digitised core porosity and permeability data (1 to 281 measurements available for 7 of the 12 wells) across the Devonian-Carboniferous interval (according to reinterpreted stratigraphic formations defined and correlated for this project, documented Kearsey et al. (2015). Outputs of this part of the project include continuous (along borehole) interpretations of porosity, clay volume, coal presence, and include basic permeability estimations where sufficient data exists to generate these. 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 Yoredale and the Scremerston formations appear to have the most favourable reservoir properties in terms of porosity (up to 19% and 15% respectively), and permeability (up to 45.28 mD and 785.52 mD respectively). However, they have relatively low NTG values (0.27 & 0.18 respectively). The Fell Sandstone Formation has the greatest NTG of the intervals examined (0.61), but porosity and permeability values are lower (0.13 and 42.69mD are the greatest average values from the wells examined). All these reservoirs show heterogeneous character in the geophysical log response, with reservoir intervals interbedded with non-reservoir. Other reports document the stratigraphic extent of these units (e.g. Kearsey et al., 2015). 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 Devonian-Carboniferous interval in each well was not able to highlight any particular property trends or geographic areas with favourable properties.