WEBB, MICHAEL W., Department of Geology and Geophysics, The University of Wyoming, Laramie, WY

ABSTRACT: Fluvial Sheet Sandstones of the Basal Lance Formation, Bighorn Basin, Wyoming

The Lance Formation of Wyoming is becoming recognized as an important petroleum reservoir unit in North America. Fluvial architectures of the basal Lance Formation were studied along a 40 kilometer transect between Meeteetse and Thermopolis in the southwestern Bighorn Basin of Wyoming. At Gooseberry Creek, braided river deposits of the Lance Formation form several sandstone sheets along an outcrop exposure of about 2 kilometers. Photomosaics and measured vertical sections were employed to study reservoir geometries and heterogeneities at the levels of channel-bar systems, channel belts, and valley fills.

Channel-bar systems form units mainly of trough crossbedded sandstones that vary in thickness from 0.5 to 3 meters and have limited lateral extent. Poorly preserved bars are common due to a combination of rapid channel switching and lack of grain size variation. Channel-bar systems contain no major internal flow barriers. Channel belts are composed of stacked channel-bar systems, vary in thickness from 8 to 22 meters, and cover areas on the order of square kilometers across the Gooseberry Creek study area. Channel belts are separated by significant thicknesses of fine-grained material that were deposited after avulsion events. Valley fills are the largest architectural element present, with probable widths of 2 kilometers and incision depths on the order of tens of meters. Valley fills are evidenced by drastic largescale changes in sand/shale ratios and depositional style along strike.

AAPG Search and Discovery Article #90915©2000 AAPG Rocky Mountain Section, Albuquerque, New Mexico