Characterization of a Sediment Core from Potential Gas-hydrate-bearing Reservoirs in the Sagavanirktok, Prince Creek, and Schrader Bluff Formations of Alaska's North Slope

Abstract In the Anadarko Hot Ice 1 well, a continuous core was acquired from 107 ft (33 m) subsurface to 2300 ft (701 m). The Hot Ice 1 well cored through the Tertiary-age sediments of the Sagavanirktok Formation, the Tertiary and upper Cretaceous Prince Creek Formation (which includes the informally named Ugnu sandstones), and Schrader Bluff Formation (which includes the informally named West Sak sandstones); the core ended in 42 ft (13 m) of what appears to be a marine section of fine-grained sediment. The recovered core was described as 37% unconsolidated sandstone. The Ugnu sandstones and shallower section cored during phase I of the project were 44% sandstone, and the West Sak sandstones and deeper zones cored during phase II of the project were 26% sand. These zones represent the primary potential reservoir rock in this area. Core plugs for analysis were removed from all the identified sand zones. Porosity, permeability, grain density, and bulk modulus were obtained for the unconsolidated sands at 57 depths during phase I and 47during phase II. Porosity and permeability were measured at 800, 1200, and 1800 psi (5.5, 8.3, 12.4 MPa) confining stress. The phase I recovered sands had an average porosity at 800 psi (5.5 MPa) confining pressure of 39.3%. At 1800 psi (12.4 MPa), this average had decreased to 37.3%. The Ugnu sandstones exhibited very high permeability. Several samples were close to 20 d. The geometrical mean of the permeability at 800 psi (5.5 MPa) confining pressure was 3.72 d. At 1800 psi (12.4 MPa), it had been reduced to 3.15 d. No discernable correlation between porosity and permeability was observed. The average grain density was 2.64 g/cm 3. The bulk modulus ranged from about 20,000 to 60,000 psi (138 to 414 MPa). The phase II recovered sands were slightly less porous on the average. At 800 psi (5.5 MPa) confining pressure, the average porosity was 37.2%. This was reduced to 35.4% at 1800 psi (12.4 MPa). The phase II recovered sands were considerably lower in permeability than the phase I recovered sands. At 800 psi (5.5 MPa) confining pressure, its geometric mean permeability was 0.633d. This was reduced to 0.486 d at 1800 psi (12.4 MPa). A good correlation between porosity and permeability was observed. The average grain density for the phase II recovered sands was 2.70g/cm 3. The bulk modulus ranged from 20,000 to 100,000 psi (138 to 690 MPa). Five anomalous thin hard cemented sand zones were observed in the phase II recovered core. Three of the zones were sampled. Two of the sampled zones had porosity in the 1to2%range and permeability in the microdarcy range. Porosities in a third zone were in the 10 to 20% range and permeability in the microdarcy to tens of microdarcys range. With one exception, the bulk moduli in these anomalous hard zones were an order of magnitude larger than the unconsolidated sands. The grain densities were similar to the West Sak unconsolidated sandstones.

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