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Physical Properties |
Department of Geology, University of Oslo, P.O. Box 1047, Blindern, 0316 Oslo 3, Norway
The salinities of porewaters in sandstones and carbonate reservoir rocks from the North Sea and Haltenbanken basins have been calculated using resistivity logs. Applying Archie’s equation, hydrocarbon containing intervals were avoided and a 100% water saturation assumed in the calculations of the formation water resistivity. The salinities obtained vary from 20 000 to 300 000 ppm NaCl equivalents. The highest salinities, which approach saturation with halite, are found in the Southern North Sea, close to the Upper Permian evaporite sediments. There is a marked upwards decrease in the salinity values away from the evaporites. The salinities seem to be a function of the vertical distance to the evaporites and are independent of stratigraphic horizon. In the Northern North Sea, where Permian salt is absent, the salinity values are much lower (20 000–50 000 ppm NaCl eq.). This salinity stratification strongly indicates that large scale vertical mixing of porewater by convection or compaction driven pore water flow has not taken place. We can therefore conclude that saline pore water has not been transported from the deeper parts of the basin to the shallower parts. This also implies that there is no transport of carbonates or silicates in solution over the same distances. Calculation of salinities from well logs is subject to considerable uncertainties. However, the values obtained in this study agree well with reported values from formation water analyses. Mapping the distribution of salinity in the pore waters of sedimentary basins is very important and the information obtained can be used to constrain our models for porewater flow.
