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Abstract: A Structural Model for Shallower Emplacement of Gas in Overpressured Basin Centers: Jonah Field as a Previously Undetected Abnormally Pressured Structural Trap


Jonah Field, located in the northwestern arm of the Green River Basin, Sublette County, Wyoming, produces gas-condensate from abnormally pressured sandstones of the Upper Cretaceous Lance Formation. The field was initially thought to be a basin centered gas accumulation. Recent evidence suggests that the field is a complicated pressure compartmentalized structural-stratigraphic trap bounded by wrench faults. Minor high angle faults may be responsible for upward migration of overpressured gas into the thermally immature Upper Lance section where it is trapped beneath a shale topseal.

The Jonah zone of the Middle Lance, Previously thought to be a widespread braided stream deposit, is now understood to be the coarsest and most continuous sandstones of an alluvial plain system. The alluvial system wedges out to the west of Jonah on the flanks of the LaBarge Platform. Within Jonah Field the productive Upper and Middle Lance varies from 1700 ft on the west to over 2700 ft on the east.

Jonah Field is over 22 square miles and averages 360 net feet of pay. Field boundaries are unknown on the west and east sides of the field. Gas in place is in excess of 6 TCF. There are currently 30 producing wells and over 100 proven undeveloped locations on 80 acre spacing.

Cross-sections, structure maps and net pay isopachs will be used to demonstrate field geometry.

AAPG Search and Discovery Article #90946©1997 AAPG Rocky Mountain Section Meeting, Denver, Colorado