Geologic controls on gas hydrate occurrence in the Mount Elbert prospect, Alaska North Slope

Abstract

a b s t r a c tData acquired at the BPXA-DOE-USGS Mount Elbert Gas Hydrate Stratigraphic Test Well, drilled in the Milne Point area of the Alaska North Slope in February, 2007, indicates two zones of high gas hydrate saturation within the Eocene Sagavanirktok Formation. Gas hydrate is observed in two separate sand reservoirs (the D and C units), in the stratigraphically highest portions of those sands, and is not detected in non-sand lithologies. In the younger D unit, gas hydrate appears to fill much of the available reservoir space at the top of the unit. The degree of vertical fill with the D unit is closely related to the unit reservoir quality. A thick, low-permeability clay-dominated unit serves as an upper seal, whereas a subtle transition to more clay-rich, and interbedded sand, silt, and clay units is associated with the base of gas hydrate occurrence. In the underlying C unit, the reservoir is similarly capped by a clay-dominated section, with gas hydrate filling the relatively lower-quality sands at the top of the unit leaving an underlying thick section of high-reservoir quality sands devoid of gas hydrate. Evaluation of well log, core, and seismic data indicate that the gas hydrate occurs within complex combination stratigraphic/ structural traps. Structural trapping is provided by a four-way fold closure augmented by a large western bounding fault. Lithologic variation is also a likely strong control on lateral extent of the reservoirs, particularly in the D unit accumulation, where gas hydrate appears to extend beyond the limits of the structural closure. Porous and permeable zones within the C unit sand are only partially charged due most likely to limited structural trapping in the reservoir lithofacies during the period of primary charging. The occurrence of the gas hydrate within the sands in the upper portions of both the C and D units and along the crest of the fold is consistent with an interpretation that these deposits are converted free gas accumulations formed prior to the imposition of gas hydrate stability conditions.Published by Elsevier Ltd.

Fault interactions and reactivation within a normal-fault network at Milne Point, Alaska

“…The region is of particular interest because of the presence of numerous major gas and oil fields including the Prudhoe Bay, Milne Point, and Kuparuk River oil fields (Carman and Hardwick, 1983;Collett, 1993;Bird, 1999;Boswell et al, 2011). Large quantities of oil have been produced, since the discovery of the first field in 1968, from complex structural/stratigraphic traps within Permian to Cenozoic reservoirs at production depths >2000 m (>6562 ft) .…”

“…The principal structural features of the region ( Figure 1A) are the Barrow Arch to the north, and the Colville Basin and Brooks Range to the south (Carman and Hardwick, 1983;Bird, 1999;Boswell et al, 2011). The Barrow Arch is an eastwest-trending rift shoulder and the Brooks Range is a fold and thrust mountain belt related to continentcontinent collision (Bird, 1999).…”

Physical properties of sediment from the Mount Elbert Gas Hydrate Stratigraphic Test Well, Alaska North Slope

“…The base of permafrost on the ANS ranges from 220 m to 660 mbgs (meters below ground surface) (Collett et al, 1988), but at the Mount Elbert well site ice-bearing permafrost currently extends to a depth of about 536.4 m RKB. Another hypothesis suggests that hydrate formation may have preceded the full development of permafrost (Collett et al, 2011b;Boswell et al, 2011;Dai et al, 2011).…”

Methane Hydrate Dissociation by Depressurization in a Mount Elbert Sandstone Sample: Experimental Observations and Numerical Simulations

“…Recent studies have concluded that methane hydrate (hereafter hydrate), a naturally occurring clathrate compound consisting of molecular water cages surrounding gas molecules (primarily methane), contains large quantities of methane in shallow sediments throughout the world's coastal margins and polar regions [Boswell et al, 2010;Milkov, 2004;Sloan et al, 1998]. Each cubic meter of hydrate can hold approximately 160 m 3 of natural gas at standard temperature and pressure [National Resource Council, 2004].…”

“…Known deposits of methane hydrate are present in regions beneath the north slope of the Brooks Range where conditions are suitable for methane hydrate occurrence. These hydrate-bearing strata are confined by low-permeability layers [Boswell et al, 2010], making depressurization the apparent method for production. …”