Carbon Dioxide Storage Potential in the Carbonate and Siliciclastic Rocks of the Lower Ellesmerian Sequence North Slope, Alaska

The U.S. Geological Survey is assessing the North Slope of Alaska as part of a national assessment of potential carbon dioxide (CO₂) storage reservoirs, called storage assessment units (SAUs), which are defined using geologic properties such as reservoir porosity, depth, and the presence of a regional seal. Standard SAUs have an upper depth limit of ~900 meters (3,000 feet) below land surface and a lower limit of 3,960 meters (13,000 feet). Deep SAUs are created when reservoirs display additional storage potential at depths below 3,960 meters (13,000 feet). There are 14 Paleozoic to Paleogene age SAUs in the North Slope. This presentation focuses on the three SAUs defined in the Carboniferous-Permian Lisburne Group and Permian Echooka Formation of the Lower Ellesmerian Sequence: 1) Lower Ellesmerian SAU, 2) Lower Ellesmerian Deep SAU, and 3) Lower Ellesmerian-LCU SAU.

The Lisburne Group is a thick, heterogeneous carbonate-succession consisting of limestone, dolostone, calcareous sandstone, and shale. The Echooka Formation contains both calcareous and quartzose sandstone with variable amounts of bioclastic limestone. Throughout much of the North Slope, the Triassic Kavik Shale of the Ivishak Formation forms the regional seal for the standard and deep Lower Ellesmerian SAUs; however, at Prudhoe Bay beneath the Lower Cretaceous unconformity (LCU), the Kavik Shale is eroded and the Cretaceous Pebble Shale Unit forms the regional seal (Lower Ellesmerian-LCU SAU).

Reservoir porosity varies between the SAUs due to differences in marine depositional environments and diagenetic alterations probably associated with marine, sub-aerial, meteoric, burial and compaction, and/or exposure diagenesis. Average porosity in the porous intervals decreases with depth from 5-18% in the standard SAU to 3-10% in the deep SAU. The Lower Ellesmerian-LCU SAU has greater average porosity values (10-25%) due to increased dolomitization associated with the LCU. The Lower Ellesmerian SAUs are bound by the Brooks Range and Alaskan state-water boundary, and have an average gross thickness of ~460-885 meters (1,500-2,900 feet), resulting in sizable SAUs with significant CO₂ storage potential. Because data availability is most abundant in the northern portion of the North Slope and decreases considerably to the south up to the Brooks Range, a variety of predictive methods were employed to address data availability and geologic complexity throughout the region.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California