Sandstone diagenesis, reservoir potential, and sequence stratigraphy of the Eocene Tyee basin, Oregon

Sandstone petrography and diagenetic analysis within a sequence stratigraph ic framework pro,ides a better understanding of the reservoir characteristi cs in the Eocene Tyee basin, an accretionary and forearc sequence, southern Oregon Coast Range. Detailed comparison of relative abundance of major det rital framework grains documents a marked difference of sandstone compositi on in each depositional sequence, Such a difference is mainly due to an abr upt change in provenance from a local Klamath Mountains metasedimentary sou rce to a more distant extrabasinal Idaho Batholith-Clarno volcanic are sour ce. Furthermore, the composition of framework grains varies systematically from the lowstand systems tract to the highstand systems tract within a dep ositional sequence. This suggests that the patterns of sedimentation and sa ndstone composition can be affected by relative changes in sea level and te ctonic uplift in the source area. In addition, the Eocene Tyee basin sandst ones exhibit a down-section distribution of authigenic minerals, consisting of early-formed zeolites and late-stage quartz as well as a change in the abundance of smectite to mixed-layer chlorite/smectite with increasing buri al depth. The down-section distribution of authigenic minerals is also caus ally linked to the compositional variation of detrital framework grains in each depositional sequence with increasing burial temperature. Much primary porosity has been filled with these authigenic minerals, thus diminishing the permeability of potential reservoir rocks, Secondary porosi ties and permeabilities of reservoir quality (averaging 10.80%; 2.76 md), h owever, are present locally in some highstand delta-front sandstone facies in the southern part of the basin as well as in lowstand turbidite sandston es in the deeper part of the basin to the north, The development of these r eservoir-quality sandstones within the Eocene Tyee basin sequence is due to a complex burial diagenesis, which is directly related to temporal and spa tial variations in original detrital mineralogy, in sedimentation pattern, and in burial temperature in the basin.