EVOLVING FLUVIAL STYLE IN THE KEKIKTUK FORMATION (MISSISSIPPIAN), ENDICOTT FIELD AREA, ALASKA - BASE-LEVEL RESPONSE TO CONTEMPORANEOUS TECTONISM

The hydrocarbon reservoir at Endicott field, offshore from the Sagavan irktok delta in northeastern Alaska, is found in the Upper Mississippi an (Visean) Kekiktuk Formation. The trap is bounded by the northwest-s outheast-trending Tigvariak and Mikkelson Bay faults, and the younger east-west-trending Niakuk fault. The Kekiktuk comprises sandstones, si ltstones, and mudstones with locally significant coals and conglomerat es, and is subdivided into three major reservoir zones based on the re lative dominance of these lithologies. Zone 1 (the lowest) comprises m udstones, siltstones, and coals with subordinate isolated sandstones, and represents deposition within a low-lying swamp plain. With very lo w net-to-gross ratio, this zone is considered to be effectively nonres ervoir. Major movement on the Mikkelson Bay and Tigvariak faults creat ed a half-graben with uplift to the north and east and a very signific ant drop in base level. This change resulted in fairly widespread depo sition of zone 2, medium- to coarse-grained sandstones from sandy bed- load braided streams. However, those sandstones thin significantly to the west, away from the faults. Continuing tectonism at this time crea ted local shifts in base level, manifest as diversion of streams wides pread abandonment, and subsidence leading to the creation of a large l ake. The lacustrine deposits are widespread, and effectively divide zo ne 2 into two subzones. Those subzones are mappable sheet sandstones, and constitute the best reservoir at Endicott field in terms of net-to -gross ratio, porosity/permeability, and sandstone connectivity. Base level shows evidence of rising dramatically at the end of zone 2. This rise is attributed to continuing tectonism within the basin, and is p articularly associated with the east-west-trending mid-field fault sys tem at Endicott field, as well as ongoing activity on the Tigvariak/Mi kkelson Bay fault system. Thus, at the base of zone 3 (subzone 3A), in the southern and eastern parts of the field, the rocks are mudstones and widespread coals, and contain at least one unequivocally marine mu dstone. These rocks are lower delta-plain interdistributary bay-fill d eposits, with rare distributary channel sandstones. North and west of the mid-field fault system, the marine mudstone is absent and the faci es changes significantly with the loss of dominant coal and a concomit ant increase of distributary channel sandstones. Reservoir properties of subzone 3A, including connectivity, are significantly different in this area. Subzone 3B is a thick accumulation of fine-grained alluvial overbank deposits within which occur rare, isolated anastomosed chann el sandstones of limited connectivity. This subzone makes minimal cont ribution to Endicott field reserves. Subzone 3C is characterized (part icularly in its upper part) by well-developed fining-upward cycles ind icating mature meandering streams. The relationships between these two highest subzones of the Kekiktuk at Endicott field indicate they repr esent an aggradational episode initiated by tectonically induced avuls ion and set against a background of rising base level. To the west, at some distance from the Tigvariak/Mikkelson Bay fault system, zone 3 i s much thinner than at Endicott field, and is dominated by mudstones a nd coals. This evidence emphasizes the control that contemporaneous te ctonism related to those faults had on sediment (especially sandstone) distribution within the Kekiktuk Formation. Base level changes can be identified within the Kekiktuk Formation in the North Slope subsurfac e by careful lithofacies analysis and an awareness of key constraining indicators, such as are revealed by palynology, paleontology, and str uctural geology. By analogy with shallow-marine rocks, sequence strati graphic concepts may be extended to continental successions. However, one must approach such problems with a full awareness of the impact of tectonism on base level and basin development.