The Lisburne Group: A potential major hydrocarbon objective of the Arctic Slope, Alaska

Abstract

The Lisburne Group, a thick carbonate rock unit of Mississippian and Pennsylvanian age, is one of the most widespread potential reservoir rock units in northern Alaska. A comprehensive review of the Lisburne in the subsurface of the eastern Arctic Slope indicates attractive reservoir characteristics in a favorable source and migration setting where numerous trapping mechanisms appear to be available. Evaluation of this group as a potential exploration objective is particularly timely in view of impending offshore sales in the Beaufort Sea and current exploration programs underway in the Prudhoe Bay area and the Naval Petroleum Reserve. Dolomite and sandstone have been identified as reservoir rocks. Oolitic grainstone is a common rock type, but all observations to date indicate little reservoir potential owing to complete void filling by calcite cement. The most important reservoir rock as judged by thickness, areal extent, and predictability is microsucrosic (10-30 μ) dolomite of intertidal to supratidal origin. It is present throughout the Lisburne and is most abundant near the middle of the sequence. Northward it decreases in thickness from 1,000 feet (300 m) to less than 100 feet (30 m). Porosity of the dolomite as determined in selected wells averages between 10 and 15 percent and attains a maximum of slightly more than 25 percent. Net thickness of reservoir rocks (i.e., rocks with greater than 5 percent porosity) varies in these wells from 140 feet (40 m) to 390 feet (120 m). Oil shows are common, and drill-stem tests have yielded as much as 1,600 bbls/day oil and 22 MMcf/day gas in the Lisburne pool of the Prudhoe Bay Field and as much as 2,057 bbls/day saltwater outside the field area. The occurrence of dolomite over such a large area makes its presence in the offshore Beaufort Sea and adjacent Naval Petroleum Reserve No. 4 fairly certain. The occurrence of sandstone as thick as 140 feet (40 m) in the middle and upper part of the Lisburne in two coastal wells suggests that larger areas of sandstone may be found to the north in offshore areas. Shows of oil and gas and a saltwater flow of 1,470 bbls/day have been recorded from this sandstone facies. Shales of Permian and Cretaceous age unconformably overlie the Lisburne, providing adequate sealing beds above potential reservoirs. Impermeable limestone (completely cemented grainstone) and thin beds of shale may serve as seals within the Lisburne, but the possibility of fractures in these units may negate their sealing capability. The most favorable source rock for Lisburne hydrocarbons appears to be Cretaceous shale that unconformably overlies the Lisburne east of Prudhoe Bay. This shale is reported by Morgridge and Smith (1972) to be a rich source rock and is the most likely source for the entire Prudhoe Bay Field. A source within the Lisburne or within the underlying Kayak Shale is postulated to explain oil shows in the southernmost Lisburne wells. This postulated source may be in a more basinal facies of the Lisburne and may be similar to dark shale in the upper Lisburne found in thrust slides in the Brooks-Range. Coal in the underlying Endicott Group is a possible source for dry gas. It is inferred that at the present time much of this coal is in a gas—generating regime downdip from the Prudhoe Bay Field area. Stratigraphic traps involving the Lisburne Group may exist as a result of widespread Permian and Cretaceous unconformities. Structural traps related to normal faulting may occur along the trend of the Barrow Arch, and faulted anticlines are numerous in the foothills of the Brooks Range. Combination traps are possible along the trend of the Barrow Arch at places where both stratigraphic and structural trap might exist.