Regional Evaluation of Formation Fluid Salinity by SP Log, Ivishak Sandstone, North Slope, Alaska: ABSTRACT

Abstract

Formation water salinity in the Triassic Ivishak Sandstone was calculated from the spontaneous potential (SP) log in 72 wells extending 260 mi from the western part of the National Petroleum Reserve in Alaska (NPRA) eastward to the Kavik gas field and from 10 mi north of the coastline southward to the edge of the coastal plane to investigate a fluid-migration model for the North Slope basin. The determination of total dissolved solids (TDS), reported as NaCl equivalents, comes from Schlumberger relations using the resistivity of the formation fluids (R/sub W/) calculated from the SP log. The R/sub W/ value was calculated from the relation R/sub W/ = R/sub mf//10/sup -(SP/k)/, where R/sub mf/ is the resistivity of the mud filtrate, SP is the SP log reading, and k is the temperature coefficient (60 + 0.133 F), where F is the formation temperature, in degrees Fahrenheit. TDS values calculated from the SP log are generally within 20% of those obtained from analysis of the small number of available water samples. The TDS in the Ivishak Sandstone ranges from 8 to 32 parts per thousand (ppt) and averages 23 ppt across the entire North Slope with lower values of 8 to 11more » ppt observed only in the area of the Kavik gas field, indicating replacement of connate sea water by meteoric water. A weak trend of increasing TDS from northwest (14 ppt) to southeast (32 ppt) may be depth controlled. The relatively narrow range of TDS and values no lower than 8 ppt may be due to the long residence time of fluids within the formation and thus allowing for water-rock equilibrium conditions to be established. Basin reconstructions indicate that meteoric recharge of the Ivishak Sandstone last occurred during Early Cretaceous time through subaerial exposure of the formation, extending from north of Harrison Bay to north of the Sadlerochit Mountains.« less