Salinity structure of the central North Slope foreland basin, Alaska, USA: implications for pathways of past and present topographically driven regional fluid flow

Previous studies of the areal variation in heat flow in the National Petroleum Reserve Alaska (NPRA) support the existence of an active topographically driven regional fluid flow regime in this central part of the North Slope foreland basin. Drilling records and wireline logs for over 30 wells drilled in the NPRA provide additional field information, which can be used to further constrain interpretation of the pattern of regional flow of basinal waters within the NPRA. Hydraulic heads estimated from drilling mud weights show that ground water flow occurs generally from south to north, but with divergence to the north‐east and north‐west away from the central part of the NPRA towards coastal areas of elevated shallow heat flow. Salinities calculated from SP logs range from less than 1 g L^?1, to marine values of 35 g L^?1, to hypersaline values of over 150 g L^?1. The entire upper sedimentary section to a depth of 2 km or more in the eastern part of the NPRA has been preferentially flushed with meteoric water through an area corresponding to the sandiest portion of the Nanushuk group. Deeper areas of low salinity occur within the Sadlerochit and Lisburne sections. The pattern of regional flow in the east is complicated, however, by the presence of a large mass of hypersaline water at depth. It is not known whether these brines are being displaced laterally and upward towards the discharge end of the basin or whether fresher waters are simply riding up over the top. Deep, hypersaline waters also occur in fault slices in the Brooks Range and have survived meteoric flushing. The brines were probably formed at the time of deposition of the Lisburne carbonates. The fluid flow regime to the west is different. Low‐salinity waters may be flowing northward underneath this section through the Ellesmerian section and discharging upward nearer the coast. However, sparse well log control severely limits what can be deduced about the details of flow paths in the central and western parts of the NPRA.