Geologic interrelations relative to gas hydrates within the North Slope of Alaska

Abstract

Figure H-2. Map of the depth to the base of the deepest ice-bearing pennafrost on the North Slope of Alaska as determined from analysis of well log responses. Table II-4 lists wells and depths used in constructing this map.

Permafrost-associated natural gas hydrate occurrences on the Alaska North Slope

“…The analysis of mud log gas-chromatographic data from industry exploratory wells generally indicates that methane is the dominant hydrocarbon gas in the near-surface (0-1500 m) sedimentary rocks of the North Slope (Collett et al, 1988). Analysis of gas evolved from recovered cored gas hydrate-bearing sedimentary sections in the Prudhoe Bay and Milne Point fields confirm that the in situ gas hydrates are composed mostly of methane in this portion of the North Slope (Collett, 1993;Lorenson et al, 2011).…”

“…Subaerial emergence of portions of the Arctic continental shelf to current water depths of 120 m ( Bard and Fairbanks, 1990) during repeated Pleistocene glaciations, subjected the exposed shelf to temperature conditions favorable to the formation of permafrost and gas hydrate. Thus, it is speculated that ''relic'' permafrost and gas hydrate may exist on the continental shelf of the Arctic Ocean to present water depths of 120 m. However, the analysis of downhole log data from industry exploratory wells drilled on the Alaska Beaufort continental shelf suggests that the occurrence of permafrost and gas hydrate beneath the continental shelf of the Arctic Ocean may be limited to a maximum water depth of only w50 m or less (Collett et al, 1988). The reason for the discrepancy between the model-derived and directly measured permafrost and gas hydrate stability conditions on the Beaufort continental shelf is uncertain; but it may be because of the limited number of direct measurements of permafrost occurrence on the shelf.…”

“…Subsurface pore-pressure gradients calculated from shut-in pressures recorded during drill-stem testing in wells from the North Slope range from 9.3 kPa/m to 11.2 kPa/m, with an average gradient of 9.7 kPa/m (0.43 psi/ft), near hydrostatic (Collett et al, 1988). To further evaluate pore-pressure conditions, Collett (1993) used gamma ray and density well logs to study overburden compaction profiles.…”

“…It is also possible that gas hydrates occur on the Norwegian island of Svalbard, but the evidence is inconclusive (Landvik et al, 1988). The combined information from Arctic gas hydrate studies shows that, in permafrost regions, gas hydrates may exist at subsurface depths ranging from about 130 m to 2000 m. Collett et al (1988) and Collett (1993) included extensive analyses of gas hydrate stability conditions in northern Alaska. More recently, Collett et al (2008a) and Lee et al (2008) used log data from wells drilled since these earlier works to update the methane hydrate stability zone maps in northern Alaska as reviewed below in this report.…”

“…However, specific evaluation of subsurface temperatures at any one particular site on the North Slope is subject to error because of the vastness of the region and the limited number of equilibrated well-bore temperature surveys. To augment the limited North Slope temperature database, Collett et al (1988) developed a new method to evaluate local geothermal gradients. In this method, well log picks for the base of the icebearing permafrost from 102 wells were combined with regional temperature constants derived from the high-resolution surveys to extrapolate temperature data.…”