ABSTRACT: Integrated Analysis of Natural Fractures in Big Horn Basin Anticlines

N. F. Hurley, T. R. Garfield, S. P. Simmons

The Big Horn basin, Wyoming, contains numerous elongate (2-20 mi or 3.5-35 km), asymmetric, doubly plunging anticlines around its margins. Many anticlines produce hydrocarbons from fractured Phosphoria Formation (Permian), Tensleep Sandstone (Pennsylvanian), and Madison Limestone (Mississippian). Fractures are evident in cores and wireline logs, and they influence production by way of high water cuts and rapid breakthrough of injected fluids. This integrated analysis of natural fractures combines well log, interference test, and production data with results from outcrop examination, air photo interpretation, and core description.

Outcrop work is focused on Sheep Mountain anticline, eastern Big Horn basin. New results from the Phosphoria Formation have been combined with published data from other units to characterize fracture intensities and orientations. Fracture intensity is highest in cherty carbonates, dolomites, and limestones, respectively. The most intense fracturing is along the steep, asymmetric limb of the fold. In all formations, fracture orientations lie approximately parallel (north-northwest) and perpendicular (east-northeast) to the fold axis.

Subsurface work is concentrated on Garland (Madison) and Little Sand Draw (Phosphoria) fields. Core descriptions show that fracture intensity is high in both reservoirs, with highest intensity along the steep, asymmetric limbs. Oriented cores suggest that fracture azimuths are not highly systematic. However, Formation MicroScanner (FMS) logs and pressure-interference tests show that the fractures which control flow in the reservoir have a preferred east-northeast orientation, roughly perpendicular to fold axes. East-northeast fractures may be open because they are oriented roughly parallel to the present-day maximum horizontal stress.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990