Depth mapping and characterisation of the Utsire Sand Saline Aquifer, Central and Northern North Sea

This report summarises the results of detailed mapping of the Utsira Sand in the central and northern North Sea utilising regional seismic reflection surveys combined with well data. It outlines the process of integration of these two datasets and conversion of the seismically-derived TWTT maps to depth. A series of contour maps illustrating the Utsira Sand in terms of porosity and proportion of clean sand are presented, based on determinations using available geophysical logs, and these are used to estimate the total pore volume of the reservoir. The Utsira Sand is a basinally restricted Mio- Pliocene sand unit deposited in a N-S elongate area overlying the Viking Graben of the central and northern North Sea. Although dominated by sand, there are locally numerous shale interbeds. It has a maximum N-S extent of 400 km and E-W extent of up to 100 km. There are three depocentres, with maximum thicknesses in excess of 300 m occurring in the south, and ranging up to 200 m in the depocentres to the north. The depth to the top of the reservoir ranges from 550 to 1500 m and the base from 600 to 1700 m. The basal surface is more complex than the top, due to the presence of mud volcanoes combined with channelling and loading features. The form of the top surface suggests that long-term buoyancy-driven migration of injected CO2 will be from the injection site towards to the north and then the northwest towards the margins of the reservoir. Determinations of porosity from the Utsira core are in the range 30 – 42%; determinations from logs at the Sleipner field are in accord with this, averaging some 38%. However away from the Sleipner field there is substantial variation in porosity of the Sand unit, ranging from 30 to 40%. The proportion of clean sand in the Utsira Sand varies greatly, ranging from as low as 30% locally to 100%. Variations are due to both widespread thin shale interbeds and locally thick shale units. An estimate of a total pore volume of 6.05 x 1011 m3 was obtained by integrating the results of the above analyses. This is close to an earlier estimate made based on assumed porosity and thickness variations.