Jonah Field

摘要: Prolific production at Jonah field and many other fields in the Green River basin is dependent on presence of overpressure. In Gulf Coast areas, plots shale resistivity sonic transit time versus depth have been used to identify overpressured zones. The same technique has proposed map overpressure compartments their boundaries Rocky Mountain region using well logs or, alternatively, interval velocities determined from seismic data. At field, top (determined by continuous gas flaring during drilling) correlates within a few hundred feet drop increase time. However, studies nearby wildcat wells detailed cross sections through both normally pressured show that log anomalies extend significantly beyond area. Velocity changes area around tend follow stratigraphic boundary near base Tertiary Fort Union Formation instead tracking volume. Early considered hydrocarbons be derived vertical migration regional conditions 2000–3000 ft (610–915 m) deeper. upward was presumed controlled extensive microfractures form leakage chimney between large sealing faults. This study suggests anomaly surrounding provides an alternative interpretation. Until middle Tertiary, extended up resulted undercompaction Cretaceous shales, as reflected velocity anomalies. Late relative uplift initiated slow wherever system not tightly sealed. As result, dropping with over most northern basin. are irreversible, reveal signature former undercompacted conditions. Based this new model, represents high remnant deeper generation cell. If model correct, exploration methods should focus seal prevent fracture models promote leakage.