ZAHM, CHRIS K., Colorado School of Mines, Dept. of Geology and Geological Engineering, Golden, CO

ABSTRACT: Tear Faulting as a Result of Forelimb Attenuation, Thermopolis Anticline, Wyoming

A differential global positioning system (GPS) was used to map the Triassic-age Alcova Limestone which outcrops along the Thermopolis Anticline in the southern Bighorn basin. Of interest, more than one hundred tear faults with at least one meter or greater offset occur on the forelimb of this basement-involved fold and have a strike that is roughly perpendicular to the fold axis. In a nearby reservoir, high permeability anisotropy nearly perpendicular to the fold axis occurs within the field creating significant reservoir compartmentalization, differential water-cut rates, and erratic oil. It is believed that faults similar to those that outcrop at Thermopolis anticline may be responsible for the high anisotropy in the nearby reservoir.

The kinematics of how the faults form is poorly understood, but detailed mapping of the geology has revealed information that may illuminate the mechanics involved with the fault development. Backlimb measurements of the thickness between two key marker units: the Alcova Limestone and an unnamed limestone within the Jurassic Gypsum Springs--is constant. However, thickness measurements between these two units on the forelimb of the fold have shown thinning or attenuation that ranges from 10 to 40% within the siltstones and shales of the Upper Chugwater Formation. There is also strong correlation between the amount of thinning and the dip of bedding, which varies from 250 upright to 600 overturned. In general, the greater the dip the more the unit is thinned. It is believed that the differential attenuation of the Upper Chugwater Fm. may be partly or wholly responsible for the development of the numerous tear faults mapped within the Alcova Limestone.

AAPG Search and Discovery Article #90909©2000 AAPG Foundation Grants-in-Aid