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

Philip V. Woodward

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_w) calculated from the SP log. The R_w value was calculated from the relation R_w = R_mf/10^-(SPk)/, where R_mf is the resistivity of the mud filtrate, SP is the S 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 11 ppt observed only in the area of the Kavik gas field, indicating replacement of connate seawater 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 nor h of the Sadlerochit Mountains. Subsequently, the formation has been sealed from surface-water infiltration by thick, overlying shale sequences. Stable hydrogen and oxygen isotope analyses of formation fluids indicate that the mean annual temperature on the North Slope at the time of recharge was 15° to 20°C higher than today--climatic conditions that prevailed throughout most of the Mesozoic and as recently as the Miocene. An exception to this model appears to be the Kavik gas field area, which was uplifted and thrust faulted during Tertiary time and is only 13 mi downplunge of Ivishak outcrops in the Sadlerochit Mountains. The observed low salinities probably result from relatively recent meteoric recharge.

AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.