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 The Anadarko Hot Ice 1well was cored with a continuous coring system that recovered a 3.25-in.-diameter (8.25-cm-diameter) core from 107 to 2300 ft (33 to 701 m) below the surface. Core recovery was 94.5%. The coring proceeded in two phases, separated by a summer of noactivity. In phase I, the core was acquired from 107 to 1400 ft (33 to 427 m). With the exception of the bottom 140 ft (43 m), the phase I core was from the permafrost interval. The base of the permafrost interval was identified at a depth of 1260 ft (384 m). The core from the permafrost zone was recovered in a frozen state. The recovered core was described and characterized at the drill site. The observations and measurements made at the drill site along with the subsequent analysis are described in five individual reports published in this Memoir. This Project Summary and Geological Description of the Core report contains a detailed project review and description of the Hot Ice project and highlights from each of the five technical reports included in this volume. The Hot Ice core penetrated 7 ft (2 m) of surface gravel and then entered the Sagavanirktok Formation. The Hot Ice well cored through the early Tertiary age sediments of the Sagavanirktok Formation, the Tertiary and Upper Cretaceous Prince Creek Formation (which includes the informally named Ugnu sandstones), and the 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 sediments. The sediments above the marine section were identified from the visual core description as deposited in environments that ranged from fluvial to marginal marine and upper deltaic. Alternating sandstone, mud-stone, conglomerate, and coal formations form sequences that indicate an overall progradation and shallowing of environments of deposition with time. The core from the first phase of the project, phase I, after the melting of the pore ice was completely unconsolidated. Core recovery through the bottom of the mudstone in which phase I drilling was terminated was 96.3%. The core recovered from this interval contains 12.4% conglomerate, 43.50% sandstone, 37.8% mud-stone, and 6.3% coal. The second phase of the coring program, phase II, recovered core from 1403 ft (428 m) subsurface to 2300 ft (701 m). The bottom of the hole was estimated to be below the base of the hydrate stability zone at the Hot Ice location. The first 59 ft (18 m) of the core from phase II consisted of the marine sediments in which the phase I drilling program ended. The remainder of the core was composed of sediments from the Schrader Bluff Formation (West Sak section). The sediments cored during phase II are not as variable in character as those encountered during phase I and are finer grained. Layers of shell fragments and whole bivalve shells are common. The sandstones and mudstones in this interval form upward-coarsening sequences and have gradational contacts with the sediments above and below. These features suggest that the sediments in this interval may have been deposited in a shallow marine shelf environment. Unlike the phase I core, the phase II core contains no conglomerate or coal. The core recovered below the marine sand in which phase I terminated contains 68% mudstone, 26.3% sandstone, and 5.7% siltstone. In general, the sandstones were finer grained than those of the Ugnu sands. Gas hydrate was not encountered at the site of the Hot Ice 1 well; however, the recovered cores and associated laboratory core observations and analysis provide a critical data set for understanding the nature of the sediments that have been shown to be gas-hydrate-bearing in nearby wells.

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