Print this pageZAHM, 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