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ABSTRACT: Geostatistical Characterization of Snake River Plains Basalts

C. F. Knutson, K. A. McCormick

Eastern Snake River Plains vadose zone basalts at RWMC (Radioactive Waste Management Complex) were characterized via a laboratory, core, and field study. The geostatistical data were obtained from measurements made on core from 15 RWMC wells and from field studies at Box Canyon and Hell's Half Acre.

The medial and distal portions of flows are typified by lobes and viscous fingering. The median length/width/thickness from the field studies is 130:69:15 ft (40:21:4.6 m). Flows commonly have pahoehoe surfaces. Flow bases rest on surfaces characterized as smooth, 51%; fracture/fissures, 19%; rubble, 17%; and bouldery/blocky/broken, 13% of the time.

The individual flow lobes are vertically differentiated into four elements: (1) substratum, (2) lower vesicular, (3) central nonvesicular, and (4) upper vesicular.

The permeability is generally matrix controlled and the vesicular elements with lower crystallinity have lower matrix permeability. However, the highest permeabilities (>5000 md) were found in very porous crustal layers. The lowest permeabilities (<0.05 md) were found in very dense central element cores.

Porosities and bulk densities of vesicular elements are controlled by the fraction of vesicles. Medium grain density is 3.05 + 01 gm/cc. The slightly lower median grain density of vesicular elements, 3.04+ gm/cc, compared to the 3.05+ gm/cc of nonvesicular elements, is a function of higher glass and oxide content of the former.

AAPG Search and Discovery Article #90097©1990 Fifth Circum-Pacific Energy and Mineral Resources Conference, Honolulu, Hawaii, July 29-August 3, 1990