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Abstract

Prolific production at Jonah field and many other fields in the Green River basin is dependent on the presence of overpressure. In the Gulf Coast and other areas, plots of shale resistivity and shale sonic transit time versus depth have been used to identify overpressured zones. The same technique has been proposed to map overpressure compartments and their boundaries in the Rocky Mountain region using well logs or, alternatively, interval velocities determined from seismic data. At Jonah field, the top of the overpressure (determined by continuous gas flaring during drilling) correlates within a few hundred feet to a drop in shale resistivity and increase in shale transit time. However, studies of nearby wildcat wells and detailed cross sections through both overpressured and normally pressured wells show that the log anomalies extend significantly beyond the overpressured area. Velocity and resistivity changes in the area around Jonah tend to follow a stratigraphic boundary near the base of the Tertiary Fort Union Formation instead of tracking the top of the overpressured volume.

Early studies of Jonah field considered the hydrocarbons in the field to be derived from vertical migration of gas from regional overpressure conditions 2000–3000 ft (610–915 m) deeper. The upward migration was presumed to be controlled by the presence of extensive microfractures that form a leakage chimney between large sealing faults. This study suggests that the log anomaly both within and surrounding Jonah provides an alternative interpretation. Until the middle Tertiary, overpressure conditions extended up to the base of the Fort Union and resulted in undercompaction of Cretaceous shales, as reflected by resistivity and velocity anomalies. Late Tertiary relative uplift initiated slow leakage of the overpressure conditions wherever the system was not tightly sealed. As a result, the top of the overpressure has been dropping with time over most of the northern Green River basin. The sonic and resistivity anomalies are irreversible, and the logs reveal the signature of the former overpressured and undercompacted conditions.

Based on this new model, Jonah field represents a high remnant of the former regional top of overpressure instead of a leakage chimney from a deeper overpressured generation cell. If this model is correct, exploration methods should focus on the seal conditions that prevent leakage instead of fracture models that promote leakage.

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