Title: The Origin of Jonah Field: Evidence for Pressure Leakage from Sonic and Resistivity Log Anomalies in the Northern Green River Basin, Wyoming
Speaker: Robert M. Cluff, The Discovery Group
Date: June 7, 2002
Publication: The Outcrop, June 2002, p. 4
The prolific production at Jonah Field and many other fields in the Green River basin is dependent on the presence of overpressured cells. In the Gulf Coast, resistivity and sonic vs depth profiles have successfully identified overpressured zones. The same technique has been proposed to map overpressure compartments and boundaries in the Rocky Mountain region. At Jonah Field, the top of overpressure (determined by continuous flaring during drilling) correlates within a few hundred feet of a drop in shale resistivity and increase in shale transit time. However, detailed cross-sections through the field and many nearby normally pressured wells show that the log anomalies in fact extend significantly beyond the overpressured area. The velocity and resistivity change tends to follow a stratigraphic boundary near the base of the Tertiary Ft. Union Fm. instead of tracking the top of flare surface.
The log data clearly demonstrate that the sonic and resistivity anomalies are not directly related to present day overpressure conditions. The conventional interpretation of this area is that Jonah Field was formed by gas leaking upwards from regional overpressure conditions some 2000 to 3000 feet deeper. The leakage is presumed to be controlled by extensive microfracturing in a chimney bounded by large sealing faults. The log data, however, suggest the situation may be exactly the opposite. The regional log anomaly was formed by regionally extensive overpressure conditions up to the base of the Ft. Union. Subsequent relative uplift resulted in slow leakage of the overpressure conditions anywhere the system was not tightly sealed. The top of overpressure surface is thus dropping with time. In this model Jonah Field represents a high remnant of the former regional top of overpressure rather than a leakage chimney upwards from the present-day top of pressure. If correct, exploration methods should focus on the top seal conditions that prevent leakage rather than fracture models that promote leakage.