The base of the Middle Ellesmerian Megasequence of Hubbard et al. (1987) in the National Petroleum Reserve - Alaska (NPRA) is the non-marine to marginal marine Mississippian Endicott Group. It overlies a highly irregular erosional surface on pre-Mississippian basement complex and is mostly overlain by and transitional with the Mississippian-Pennsylvanian Lisburne Group shallow water carbonates. A late Devonian to early Mississippian breakup of the Basement Complex created the Meade, Ikpikpuk, and Umiat basins in the NPRA. Owing to accommodation within these basins and erosional relief at the top of Basement Complex outside the basins, the distribution of the Endicott Group across the NPRA is highly variable. Seismic correlation and limited well data suggests the basins were filled with as much as 5,400 m (17,700 ft) of clastics and carbonates of the Endicott and Lisburne groups. These same rocks are locally absent between basins and are thin to absent along the coast of NPRA. Core of the Endicott Group in NPRA is limited to four wells, but is only plentiful in the East Simpson 2 well. This core is interpreted to represent the Kekiktuk Formation of the Endicott Group and is comprised of coal and what appear to be crevasse splay sandstone and flood basin mudstone associated with a fan delta system and shallow lacustrine swamp. Limited Endicott core from the Inigok 1 well is tightly quartz-cemented pebbly sandstone and granule conglomerate that appears to be of fluvial origin. Reservoir potential of the Endicott Group within the deep basins is low, but there may be limited potential in the thin, shallower accumulations along the NPRA coast.