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Abstract: Jonah Field: A Shallow Sweetspot in the Basin-centered Gas Accumulation of the Northern Green River Basin


Recently acquired data are beginning to illuminate the nature of the large, overpressured, basin-centered gas accumulation in the northern Green River Basin, Wyoming. The shallow, compartmentalized gas accumulation at Jonah field is an anomaly within the larger basin-centered accumulation.

The upper portion of the regional gas cell is within the Upper Cretaceous strata of the Lance Formation and Mesaverde Group (Ericson Sandstone and Rock Springs Formation). These units are composed of fluvial-channel sandstones and siltstones, floodplain shales and minor coals that were all deposited between the Wyoming Thrust Belt and the then-emerging Wind River Mountains. Jonah field is located on the gentle northeast-dipping flank of the basin. Dispersed carbonaceous material and discontinuous coal beds within the Upper Cretaceous section are the sources of the hydrocarbons.

The upper boundary of the gas accumulation at Jonah coincides closely with the top of the Lance (plus or minus 8500 ft), whereas in other portions of the basin the top of the gas cell is near the top of the Mesaverde (plus or minus 10,500 ft). The origin of the sweetspot at Jonah is not clearly understood, but it doesn't appear to be caused by a thermal hotspot, depositional or stratigraphic reasons, or by structural folding. The abrupt southern boundary of the accumulation may hold the key to understanding the field. Fractures, formed in response to a left-lateral shear zone located, at the southern boundary, may be the reason for the shallower accumulation and apparent higher permeability at Jonah verses other portions of the basin.

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