ABSTRACT

We estimated the pore water salinity of reservoir sands near a major fault in the Eugene Island Block 330 Field using Spontaneous Potential (SP) logs from 50 wells. Pore water salinities estimated from SP logs are generally consistent with salinities measured from recent well head samples in the region. Our results show: (a) shallow reservoir sands (depth < 1600 m) have higher salinities than deeper sands (depth > 2000 m); (b) salinity systematically increases with distance from the fault zone; (c) salinity estimated from SP logs taken after 1985 are higher than salinities calculated from SP logs taken in the early 1970s in shallow reservoir sands; and (d) salinity estimates for deeper reservoir sands do not vary with the age of the SP log. We interpret these results as evidence for recent expulsion of low salinity geopressured fluids along faults into sand layers in the overlying hydropressured Plio-Pleistocene section. Lower salinities and the absence of changes in salinity with age of the SP log in deeper reservoir sands implies that these sands have been in persistent hydrologic contact with the geopressured zone or have experienced an infusion of low salinity fluids during the production history of the South Eugene Island Block 330 Field.