Reservoir evaluation of 3 wells in the Palaeozoic of the Orcadian Basin (UK North Sea) : petrophysical interpretations of clay volume, porosity and permeability estimations

This report details the reservoir evaluation of 3 wells across the Palaeozoic rocks of the Orcadian Basin of the UK North Sea for the 21CXRM Palaeozoic project. This reservoir evaluation is based on the petrophysical interpretation of available digital wireline log curve data for 3 wells and associated digitised core porosity and permeability data (14 to 67 measurements available for each well) across the Palaeozoic interval (according to reinterpreted stratigraphic formations defined and correlated for this project, documented in Whitbread and Kearsey (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 3 wells were selected based on availability of core data (to allow calibration of log-derived porosities and the estimation of permeabilities) to determine reservoir quality potential in the Devonian interval. The results complement the Hannis (2015) study on Carboniferous and Devonian reservoirs of the Central North Sea. Other reports document the stratigraphic extent of these units (e.g. Whitbread and Kearsey, 2016). 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 are not included in this report. The best reservoir properties appear to be found in the Middle Eday Sandstone Formation, which, in well 13/19-1, has a NTG of 1, an average porosity of 14% and the highest permeabilities recorded of the 3 wells (an average of 20 mD with values up to 174 mD estimated from logs calculations derived from associated core data). The Permian Rotliegend Group, and Zechstein Group also show favourable properties, slightly lower NTG and porosities than the Middle Eday Sandstone Formation (Tables 1 and 3) and although no core was available in the 3 wells examined to derive specific permeability measurements from them, log derived estimates from deeper core were up to 208 mD. The Buchan Formation also shows favourable properties, particularly in one well (13/19-1) where NTG was 1, porosity 12% and permeabilities estimated as up to 110 mD. There may also be potential reservoir in the Upper and Lower Strath Rory Formations, as they have good NTG (0.92 and 0.56 respectively) and average porosities (16% and 10% respectively). However, from the data examined, their permeabilities appear comparatively much lower (averages estimated as less than 1 mD, with the highest values estimated (and measured on core) around 5 mD. Over these potential formations of interest, log responses suggest that there are relatively thick intervals of clean “good” reservoir intervals, in comparison to the CNS reservoirs studied which were dominated by a heterolithic succession (Hannis, 2015). The Eday Flagstones, Lower Eday Sandstone and Kupfershiefer formations appear to have poor reservoir properties. The Orcadia and Struie formations and the Granitic basement are not considered to have any in-matrix reservoir potential.