SEQUENCE STRATIGRAPHY, TECTONIC SETTING, AND NATURAL GAS POTENTIAL AND PETROLEUM SYSTEMS OF EOCENE UMPQUA ACCRETIONARY COMPLEX AND TYEE FOREARC BASIN, S. OREGON COAST RANGE

Four depositional sequences (seq. I-IV) fill the 8,000 m thick Umpqua/Tyee basins. Syntectonic lower Eocene fan delta, slope, and submarine fan lithic(meta) lithofacies of seq. I (lower Umpqua Gp) represent a partially subducted accretionary wedge derived from the Klamath Mts. (Kmts) during a soft collision. Deltaic lithic arkosic ss, congl., coal, and shelf-slope mdst of seq. II (u. Umpqua Gp) filled irregular lows and thin over submarine highs of seq. I created by imbricate thrust faults (e.g., Reston high). Farther basinward, seq. I and II onlap the Umpqua arch, an older constructional high of l. Eocene Siletz River Volcanics (economic basement) and then thicken into the Smith River subbasin. By m. Eocene, continental micaceous arkosic/volc. arc-derived deltas and deep-sea sandy fans of seq. III and IV prograded down the Tyee forearc basin, unconformably across the Umpqua basin structural trend. Tertiary convergence continued with clockwise basin rotation, minor l. Eocene thrusting, Neogene extension/oblique slip faulting and gentle Coast Range folding.

Diagenesis has diminished most primary ss porosity and permeability with smectite-corrensite clay, zeolite and qtz cement and compaction. Reservoir-quality porosity and permeability occur in a few seq. II, III, and IV delta front and turbidite ss. Organic geochem. indicates that most mdst and ss units are thermally immature with lean, gas-prone Type III/IV kerogen. However, coals, carbonaceous mdst and some adjacent Mesozoic KMts mélange units may be organic-rich sources of microbial and meta/thermogenic methane in numerous seeps and wells. Computer models predict some deeply buried seq. I and II units had matured by the Oligo/Miocene.

Petroleum system 1 (PS-1) is related to proposed subduction zone maturation with KMts and gas/fluid migration along thrust faults into seq. III reservoirs (e.g., White Tail Ridge Fm. [WTR]) with mdst seals. PS-2 is unconventional plays associated with overpressured basin-center gas and secondary porosity in turbidites (Tyee Mtn. Mbr.) or coal bed methane in the deltaic units of seq. III and IV (e.g., WTR). PS-3 occurs due to thermal maturation of seq. III and IV units (e.g., WTR, Spencer Fm) by local sills and mid-Tert. W. Cascade arc volc. Plays include Coast Range anticlines, fault propagation folds, and strat. pinchouts.