Pore fluid geochemistry from the Mount Elbert Gas Hydrate Stratigraphic Test Well, Alaska North Slope M.E. Torres a, * , T.S. Collett b , K.K. Rose c , J.C. Sample d , W.F. Agena b , E.J. Rosenbaum c a Oregon State University,104 COAS Administration Building, Corvallis, OR 97331, United States b U.S. Geological Survey, Denver Federal Center, MS-939, Box 25046, Denver, CO 80225, United States c U.S. Department of Energy, National Energy Technology Laboratory, 3610 Collins Ferry Road, Morgantown, WV 26507, United States d Department of Geology, Northern Arizona University, S. San Francisco Street, Flagstaff, AZ 86011-4099, United States article info Article history: Received 8 March 2009 Received in revised form 30 September 2009 Accepted 1 October 2009 Available online 21 October 2009 Keywords: Gas hydrate Mt Elbert Well Pore water Water isotopes Permafrost abstract The BPXA-DOE-USGS Mount Elbert Gas Hydrate Stratigraphic Test Well was drilled and cored from 606.5 to 760.1 m on the North Slope of Alaska, to evaluate the occurrence, distribution and formation of gas hydrate in sediments below the base of the ice-bearing permafrost. Both the dissolved chloride and the isotopic composition of the water co-vary in the gas hydrate-bearing zones, consistent with gas hydrate dissociation during core recovery, and they provide independent indicators to constrain the zone of gas hydrate occurrence. Analyses of chloride and water isotope data indicate that an observed increase in salinity towards the top of the cored section reflects the presence of residual fluids from ion exclusion during ice formation at the base of the permafrost layer. These salinity changes are the main factor controlling major and minor ion distributions in the Mount Elbert Well. The resulting background chloride can be simulated with a one-dimensional diffusion model, and the results suggest that the ion exclusion at the top of the cored section reflects deepening of the permafrost layer following the last glaciation (w100 kyr), consistent with published thermal models. Gas hydrate saturation values esti- mated from dissolved chloride agree with estimates based on logging data when the gas hydrate occupies more than 20% of the pore space; the correlation is less robust at lower saturation values. The highest gas hydrate concentrations at the Mount Elbert Well are clearly associated with coarse-grained sedimentary sections, as expected from theoretical calculations and field observations in marine and other arctic sediment cores. Ó 2009 Elsevier Ltd. All rights reserved. 1. Introduction Gas hydrates, crystalline substances composed of water and gas (Sloan, 1998), have long been known to occur in Arctic permafrost areas (e.g. Makogon et al., 1972; Bily and Dick, 1974; Galate and Goodman, 1982; Collett, 1993; Collett et al., 2008). In a recent effort from the U.S. Geological Survey and the Bureau of Land Manage- ment, a detailed geological and geophysical analysis was used to map the spatial distribution of the gas hydrate stability zone and assess the gas hydrate accumulations in the Alaska North Slope, within an area that extends from the National Petroleum Reserve of Alaska (NPRA) on the west, through the Alaska National Wildlife Refuge (ANWR) on the east (Collett et al., 2008). The Alaska North Slope (ANS) is an east–west elongate basin that extends from the Brooks Range to the Arctic Ocean and that has accumulated sediment ranging in age from Mississippian through Quaternary (Fig. 1). Three principal sedimentary sequences have been identified in this region: the Ellesmerian (Mississippian through Triassic); the Beaufortian (Jurassic through early Creta- ceous) and the Brookian (middle Cretaceous to Recent) (Bird, 1987, 1991; Mull et al., 2003). The Ellesmerian sediments were derived from a northern source area and deposited in a passive margin setting. The development of the Brooks Range compressional oro- gen began in late Jurassic and early Cretaceous (Fuis et al., 1997). The Beaufortian sequence is characterized by mud dominated sediment, with interbedded sandstone and shales and marks the end of passive sedimentation; its sediment source was local or from the north. The present Arctic Ocean opened in the mid to late Cretaceous, and clastic input shifted from the north to the south as a result of plate rotation, faulting and uplift of the Brooks Range. The deposition of the Brookian sequence began with proximal fan delta and turbidites, and succeeding prodelta and deltaic systems filled the newly formed Coville foreland basin from the south-west to the north-east. Stratigraphic correlations with previous outcrop * Corresponding author. E-mail address: mtorres@coas.oregonstate.edu (M.E. Torres). Contents lists available at ScienceDirect Marine and Petroleum Geology journal homepage: www.elsevier.com/locate/marpetgeo 0264-8172/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.marpetgeo.2009.10.001 Marine and Petroleum Geology 28 (2011) 332–342