The reservoir interval in the Endicott field, located under the delta of the Sagavanirktok River east of Prudhoe Bay, is the Mississippian Kekiktuk formation. The rocks are quartzose, and lithofacies and sequence analysis of core material shows that the Kekiktuk in that area can be subdivided into three sedimentologically distinct intervals, each of which reflects a different fluviatile depositional environment. The lowermost interval (zone 1) rests directly on metamorphic basement and comprises interbedded coal, mudstones, siltstones, and fine-grained sandstones. These rocks were deposited in a very low-lying swamp plain containing local lakes and sluggish, highly sinuous streams. Zone 1 is overlain sharply by zone 2, which comprises medium to coarse-grained, multistory andstone that was deposited within an unconstrained braided river system. The braid plain contained both ephemeral and permanent lakes, and periodic gravity flows deposited coarser sediment into the latter. That sequence passes gradationally upward into zone 3, which is composed of coarse-, medium-, and fine-grained sandstones, as well as siltstones, mudstones, and coals. The lower part of zone 3 is dominated by upward-fining sandstone sequences, interpreted as channel bars deposited within a moderately sinuous fluvial system. They pass gradationally upward into other, distinct upward-fining sequences, which differ in having higher proportions of siltstone, mudstone, and coal. Those rocks were deposited in a more highly sinuous (meandering) fluvial environment. The rocks grade upward int shallow marine sediments of the Kayak/Itkilyariak formations. The vertical sequence within the Kekiktuk in this area permits an interpretation of the structural history of the basin during Kekiktuk time.

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