Abstract

Sandstone petrography and diagenetic analysis within a sequence stratigraphic framework provides a better understanding of the reservoir characteristics in the Eocene Tyee basin, an accretionary and forearc sequence, southern Oregon Coast Range. Detailed comparison of relative abundance of major detrital framework grains documents a marked difference of sandstone composition in each depositional sequence. Such a difference is mainly due to an abrupt change in provenance from a local Klamath Mountains metasedimentary source to a more distant extrabasinal Idaho Batholith-Clarno volcanic arc source. Furthermore, the composition of framework grains varies systematically from the lowstand systems tract to the highstand systems tract within a depositional sequence. This suggests that the patterns of sedimentation and sandstone composition can be affected by relative changes in sea level and tectonic uplift in the source area. In addition, the Eocene Tyee basin sandstones 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 burial depth. The down-section distribution of authigenic minerals is also causally 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 porosities and permeabilities of reservoir quality (averaging 10.80%; 2.76 md), however, are present locally in some highstand delta-front sandstone facies in the southern part of the basin as well as in lowstand turbidite sandstones in the deeper part of the basin to the north. The development of these reservoir-quality sandstones within the Eocene Tyee basin sequence is due to a complex burial diagenesis, which is directly related to temporal and spatial variations in original detrital mineralogy, in sedimentation pattern, and in burial temperature in the basin.

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