The Columbia River Basalt Group: A Volcanic Reservoir Analog

Jackson, James

A number of discoveries have been reported in recent years with lava flows acting as the reservoir interval. These include fields in China, India, Indonesia as well as older discoveries in Japan.

Lava flows show less uniformity than most clastic and carbonate reservoirs. This renders appraisal and development more problematic. The seismic response of lava flows also shows less detail than other reservoir lithologies. Knowledge of well-exposed outcrop analogs can help to evaluate these enigmatic discoveries.

The Miocene Columbia River Basalt Group (CRB) crops out in Washington, Oregon, and Idaho. Outcrops along the Columbia River and its tributaries, as well as on the Pacific Coast, provide excellent exposures of this aerially extensive unit.

Individual units occur both as broad sheets and as narrow, linear intra-canyon flows. Within the flows, porosity and permeability are best developed on the flow tops, where brecciation is common and vesicles can form relatively thick, porous zones. The flow base may also be brecciated in a thin zone.

The flow interiors are jointed, typically with a colonnade above the flow base, and an entablature below the flow top. Where present, vesicles do not create significant porosity. The joints appear to create permeability in the flow interior, but this rarely the case. The joints are not laterally persistent and are often filled with post-emplacement minerals.

The best porosity and permeability occurs between the flows in two settings. Soils occur above some flows, inter-bedded with minor fluvial sandstone deposits. These act as aquifers where they occur in the subsurface. Hyaloclastic deltas occur where flows entered rivers, large lakes, or the Pacific Ocean. These can also act as aquifers in the subsurface, though the lacustrine hyaloclastic facies are limited in aerial extent.

The CRB is deformed in the Yakima Fold Belt into long, thrust-related anticlinal ridges. Open fractures are common along the crests of the folds. Minor strike-slip faults also occur with narrow zones of open fractures.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013