Structural Geology, Seismic Imaging, and Genesis of Jonah Field:

A Giant Tight Gas Accumulation, Wyoming, USA

By

William B. Hanson, Victor H. Vega, and Dennis L. Cox

BP America Production Company, Houston, TX

Jonah Field is a large, complicated structural trap located in the Green River Basin. Gas and condensate production is from innumerable Late Cretaceous and early Tertiary fluvial sandstone bodies at 7,300 feet to 12,800 feet. This field is developed on 40-acre spacing and now has more than 350 wells.

The gas producing volume,as great as 4000 feet thick, is strongly layered-not a single gas column. The field shows evidence of early Tertiary paleostructure growth which, in combination with the “bottoms-up” origin of the gas may explain the enigmatic charging of these tight gas sandstones according to structural position, i.e. buoyancy. Gas emplacement occurred prior to complete reduction of sandstone permeability.

The two field-bounding faults intersect up dip to form the overall wedge-shaped trap but the field is an aggregate of four compartments each comprised of a northeast-plunging faulted anticline bounded by sealing faults. These compartments juxtapose areas of high and low reserve wells. The major faults are nearly vertical and extend to the basement with throws commonly less than 200 feet. South Jonah Fault is probably a left lateral wrench as discussed by Warner (1997); this fault zone has transtensional and transpressional features and a complex and protracted displacement history. Jonah Field displays structural inversion. The South Jonah Fault was active concurrently with Lance Formation deposition creating thick Lance on the north side (north-side down). Subsequent post-Paleocene reversal of displacement, formed the Stud Horse Butte Anticline. Both the West Fault and the Antelope Fault are composites of two faults that have merged. This explains the apparent dip reversal along the length of these features; neither is actually a scissor fault.

AAPG Search and Discovery Article #90004©2002 AAPG Rocky Mountain Section, Laramie, Wyoming