Stratigraphy, sedimentology, and hydrocarbon potential of Eocene forearc and subduction zone strata in the southern Tyee basin, Oregon Coast Range

Sequence stratigraphic analysis of the southern Tyee basin, Oregon Coast Range, reveals that the lower to middle Eocene forearc and subduction zone strata comprise four depositional sequences. Sequence I (lower Umpqua Group) represents a partially subducted accretionary wedge. Sequence II (upper Umpqua Group) is a deltaic sequence that filled irregular lows and thinned over submarine highs created by intrabasin, imbricate thrust faulting. Farther north, Sequences I and II rapidly thin and distally onlap oceanic basalt islands and seamounts of the Siletz River Volcanics to form a condensed section and then thicken again northward. These sequences are overlain by a tectonism-forced transgressive systems tract deposited during an onlap caused by tedonic subsidence and clockwise basin rotation approximately 50 Ma. By middle Eocene, sandy submarine fans and wave-dominated deltas of Sequences III and IV (Tyee, Elkton, Bateman, and Spencer formations) prograded northward down the axis of the Tyee forearc basin and across the structural trend of the Umpqua Group.; Organic geochemistry indicates that most units are thermally immature and contain lean, gas-prone Type III kerogen. However, some beds (coals) are sufficiently organic-rich to be sources of biogenic and thermogenic methane discovered in numerous seeps. Reservoir-quality porosities and permeabilities are identified in a few delta front and turbidite sandstones of Sequences II and III, although zeolite, clay, and quartz diagenesis has destroyed most potential sandstone reservoirs.; The overall hydrocarbon potential of the basin is moderately low. Several requirements for commercial accumulations of hydrocarbons, however, probably exist locally within and adjacent to the basin. Three speculative petroleum systems are identified. The first includes the southern part of the basin near the border with the Mesozoic Klamath Mountains and is related to a proposed subduction zone maturation mechanism along thrust faults. The second is centered in the northern part of the study area and may be associated with basin-center gas in an overpressured zone. The third occurs near the eastern border of the basin where maturation is related to local heating by sills and migration of hydrothermal fluids associated with mid-Tertiary volcanism in the Western Cascade arc.