Abstract: Electrical Resistivity Image Reservoir Description of the Lance Formation, Jonah Field, Sublette Co., Wyoming

EVANS, LESLEY W.* and LAURA S. FOULK

In September, 1996 Snyder Oil Company drilled the Stud Horse Butte 11-27 and captured two-thousand feet of electrical borehole images in the Upper Cretaceous Lance Formation. The Lance section in Jonah Field averages about 2,500 ft. of continental deposits with between 20% and 30% sand.

Bedding styles, sedimentary structures and vertical stacking sequences identified in the electrical images provide a description of depositional environment. The sands are composed of lag deposits, massive and planar beds, and planar and trough cross beds. Internal structures show sharp bases, multiple scour surfaces, soft sediment deformation, and some rooted surfaces. The rooted surfaces are confirmed by core. The majority of the shales are washed out, loosing bedding character in the electrical images.

Prior to image analysis, variations in channel sand stacking arrangements in the Lance were thought to correspond to changes from braided to meandering stream environments. Analysis of the electrical images, however, reveals the Lance to be deposited dominantly by braided stream and alluvial processes. The thickest, stacked bar sands show the same internal bedding as the thin channel sands. The base of these channels is characterized by a scour and lag deposit which is capped by medium and fine grained sands with multiple internal scour surfaces and lags within the channel unit. Many of the beds are disturbed by soft sediment processes. Slumps and overturned beds are common. The change in stacking arrangements is now thought to be related to rate of sediment supply and accommodation space within the braided stream environment. This discrimination is important to the prediction of reservoir quality from fluid flow processes.

AAPG Search and Discovery Article #90946©1997 AAPG Rocky Mountain Section Meeting, Denver, Colorado