Abstract: Resistivity Variations in Eocene Sandstones from the Laney Shale Member, Green River Formation, Green River Basin, Wyoming, USA

Robert J. Vinopal, Edward B. Nuhfer

Resistivity variations in two cores from the Laney Shale sandstone interval correlate with differences in the type and morphology of zeolite cements present in the sandstones. Resistivity of the Laney Shale sandstones varies from 12 to 30 ohm-meters. and shows a significant lateral difference in two wells spaced 6 miles apart. Mean sandstone porosity (27%) is the same in both cored intervals and does not correlate with differences in resistivity response. Thin section, x-ray diffraction, scanning electron microscopy, and whole rock chemical analysis show that variation in the content of the zeolites analcime and clinoptilolite is the most significant mineralogical difference between the two cores. The sodium zeolite, analcime, shows a trend of increasing abundance with de th through the higher resistivity sandstone interval. The blocky analcime cement increases sandstone resistivity by decreasing the abundance of conducting pathways. Lower resistivity sandstones contain the sodium-potassium-calcium zeolite, clinoptilolite. Clinoptilolite occurs as a microcrystalline cement in the form of small prismatic crystals that line pores. The clinoptilolite crystals have a much higher surface area to volume ratio than do the larger analcime crystals. This produces a greater abundance of conducting pathways, via ion exchange surfaces, on clinoptilolite crystals. Zeolite cements in the Laney Shale sandstones formed at shallow burial depths, most likely from the interaction of migrating sodium-rich, high pH connate brines with volcaniclastic grain components.

AAPG Search and Discovery Article #90952©1996 AAPG Rocky Mountain Section Meeting, Billings, Montana