The Sag River Formation is a minor hydrocarbon reservoir in the Prudhoe Bay field, Alaska. It comprises bioturbated, glauconitic, argillaceous, quartzose fine to very fine-grained sandstone and siltstone and varies from 55 ft (17 m) to 20 ft (6 m) in thickness in the field. The formation is the upper part of a very fine-grained, upward coarsening, terrigenous, clastic-dominated sequence deposited in Late Triassic time. This sequence includes the upper part of the subjacent Shublik Formation. Lithofacies variation within the Sag River is minimal with stratigraphic thinning from the north-northeast to a south-southwest shaleout. The formation was deposited in a low-energy, offshore, marine-shelf environment basinward of a low-relief source area. Upward coarsening, as well a slightly older Sag River facies in more proximal areas, suggests regionally significant marine regression during deposition.

In the Prudhoe Bay field, diagenesis along with abundant primary detrital matrix significantly diminishes reservoir quality. Ductile grain deformation, authigenic clay-grain coatings, quartz overgrowths,and carbonate cementation have resulted in microporosity and associated low permeability, which is the primary shortcoming of the Sag River reservoir. Larger, interconnected secondary pores (and associated improved reservoir quality) were produced by the dissolution of carbonate cement and possibly other mineral phases.

Reservoir quality in the Sag River Formation is strongly influenced by proximity to its subcrop with the overlying Lower Cretaceous "Highly Radioactive Zone." Mineral leaching probably resulted from aggressive fluid incursion at that truncation surface. The source of those fluids is not presently known.

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