The Prediction of Permeability Enhancing Fracture Patterns within Natural Gas Reservoirs Based on Outcrop Characterization Near Normal Faults; Wind River Basin, Wyoming

Hutch Jobe¹, Randy Herr¹, and Paul MacKay^2
1 Burlington Resources, Midland, TX
² Geoconsultants Ltd, Calgary, AB

The intensity of natural fractures tends to increase near fault planes. These damage zones exist both on the up-thrown and down-thrown sides of normal faults but fracture distribution within the damage zones can vary. Damage zone widths appear to be a function of fault displacement. Faults with at least 100' of vertical displacement can generate damages zones that extend hundreds of feet away from the fault trace. Outcrop evaluation from the Cretaceous and Tertiary aged strata of the Wind River Basin as well as subsurface analysis within the basin supports this conclusion. Understanding fracture trends, intensity, and orientation associated with normal faulting, can help enhance the ability to predict where fractures may occur in the subsurface when dealing with low permeability gas reservoirs. Fractures influence reservoir trap dynamics such that conventional 4-way structure closure may not be necessary.

The upper 350' of the Upper Cretaceous Lower Fort Union Formation (LFU) located in off-structure positions at the Madden field, has had significant gas shows. The shows are located within an inter-bedded sequence of tight dirty sandstones, coals, shales, and siltstones and had been regarded as “coal” or “fracture” gas of limited aerial extent. For the most part, these assumptions were accurate, except where fractures have created drainage areas large enough to provide commercial production. The damage zone architecture associated with the extensional faulting at the top of the LFU has fracture domains that extend the drainage area of the wellbore such that wells have estimated ultimate recoverable reserves (EUR) of up to 5.0 BCFG at depths of less than 7000 feet.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005