--> ABSTRACT: Potential Petroleum Reservoir Rocks of North-Central Oregon, by Alfred J. Riddle and Lanny H. Fisk; #91040 (2010)

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Potential Petroleum Reservoir Rocks of North-Central Oregon

Alfred J. Riddle, Lanny H. Fisk

The petroleum potential of north-central Oregon has been interpreted by many to be low. One of the dominant factors contributing to this interpretation is the assumption of poor reservoir rock quality. Our porosity and permeability measurements, scanning electron microscopy (SEM) studies, and petrographic analyses instead indicate that potentially good reservoir rocks exist. Well log responses also indicate that some subsurface sandstones have excellent porosities.

Cretaceous turbidite sandstones of the "Mitchell Formation" in outcrop are poorly sorted, clay rich, tightly cemented, and have low porosities and permeabilities (range 3 to 11%, average 7%, and 0.01-9.8 md, average 1 md). However, in the subsurface where not tightly cemented with calcite, such as in reservoirs that accumulated petroleum prior to cementation, these submarine-fan sandstones may contain adequate porosity and permeability. Important secondary porosity could also be present, created by dissolution of calcite cement and detrital framework grains and by fracturing.

Volcanic rocks and associated lithic sandstones make up most of the Tertiary stratigraphic sequence in the area. Fluvial channel sandstones of the Paleocene-Eocene "Herren Formation" are potentially excellent petroleum reservoirs, exhibiting good porosities (range 8 to 21%, average 12%) and permeabilities (up to 21 md, average 5 md). Low permeabilities in most samples are a result of authigenic clays plugging pore throats but this may be largely a surface weathering effect. Porosities of volcaniclastic sandstones in the Clarno Formation are surprisingly high (range 4 to 38%, average 17.5%). Permeabilities average 8 md and range from .01 to 43 md. Secondary porosity in these sandstones is a natural consequence of thermal maturation of associated kerogen-rich source rocks and the production of organic acids. The resulting dissolution of feldspars and lithic fragments has enhanced the reservoir quality of these formations.

AAPG Search and Discovery Article #91040©1987 AAPG Rocky Mountain Section Meeting, Boise, Idaho, September 13-16, 1987.