SALINITY VARIATIONS IN NORTH-SEA FORMATION WATERS - IMPLICATIONS FOR LARGE-SCALE FLUID MOVEMENTS

Sodium chloride is the major salt dissolved in formation waters, even at a considerable distance away from any evaporites. Chloride anions are not consumed to any significant extent by diagenetic reactions outside the salt deposits. Chloride ions are therefore preserved in the pore waters, and the distribution of pore water salinities in sedimentary basins may help to constrain pore water flow and the transport of dissolved species of silica and carbonate minerals. In the North Sea Basin and Haltenbanken (mid-Norway), salinity gradients can be inferred from formation water samples. Measurements of pore water salinity from wireline logs are less accurate than formation water samples, but have the advantage that a much larger database can be obtained. Data from 150 wells in the North Sea and Haltenbanken show that the salinities measured from well logs range from 20000 to 300000 ppm. In the southern part of the Norwegian North Sea and Haltenbanken, there is a clear trend towards higher salinities with greater burial depth. There is a pronounced salinity increase in the 500 m of section closest to the evaporites. In the northern North Sea, where underlying evaporites are not known, there is only a slight increase in the measured salinities with depth. There are, however, no measurements of high salinity pore water (>100 000 ppm) at depths shallower than 2200 m. This pattern of salinity distribution 'in the pore water precludes large-scale convection of the pore water during diagenesis. It also indicates limited compaction-driven flow. Log-derived salinity data from the North Sea suggest that the pore waters in Upper Jurassic and Tertiary reservoirs are generally more saline than those of Middle and Lower Jurassic reservoirs at the same depth. The constraints on pore water flow that can be deduced from salinity variations puts severe limitations on the mechanisms of transport of dissolved solids in sedimentary basins and thus also on aspects of diagenetic models.