OUTCROP TO SUBSURFACE GAMMA-RAY CORRELATION OF MIDDLE JURASSIC UNITS, BIGHORN BASIN, WYOMING

Difficulty in defining lithostratigraphic relationships between the Middle Jurassic Gypsum Spring, Piper and Sundance formations in the Bighorn Basin, WY, has resulted in inconsistent and often contradictory uses of these unit names. These discrepancies are partially derived from attempts to correlate the units in the subsurface using only well logs because no cores exist to compare lithology to well log patterns.

A promising method involves the use of a hand-held gamma-ray spectrometer on outcrops in combination with field descriptions. The objective of outcrop gamma-ray measurements for the Gypsum Spring, Piper and Sundance formations is to permit detailed correlation among outcrop traverses and between these traverses and drill-hole gamma-ray logs. The equipment used for field measurements is a portable 256-channel gamma-ray spectrometer, capable of determining total natural gamma counts as well as concentrations of potassium, thorium, and uranium. For the purposes of this study, total gamma ray measurements were examined. All gamma-ray logging was in the form of up-section traverses coinciding closely with the location of field descriptions. Each gamma-ray traverse consisted of measurements at 0.1 to 0.5 m intervals, depending on access and survey efficiency. Total gamma-ray count at each point was averaged over a 10 second interval.

Outcrop gamma-ray spectrometry logging provides a means of detailed field to subsurface correlations between Middle Jurassic units of the Bighorn Basin, WY. Stratigraphic correlations within this section have proven to be a critical stumbling block in the reconstruction of basin history and regional paleogeographic relationships. This study provides an opportunity to improve correlations through direct comparisons of lithology to well-log patterns.