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Structural Geology, Seismic Imaging, and Genesis of Jonah Field

Hanson, William B., Victor Vega, Dennis Cox, and Terry J. Young
BP America Production Company, Houston, TX

Jonah Field is a large, complicated structural trap located in the northwestern part of the Green River Basin. The structural components of the trap consist of a combination of fault zone deformation and broad, open folding. The two field-bounding faults, intersect up dip to form the overall wedge-shaped trap and the field is an aggregate of at least four fault compartments each comprised of a subtle, northeast-plunging faulted nose bounded on the west and south by sealing faults. The interior faults terminate at the south field bounding fault and production data indicates that these faults are at least partially sealing. All of the major faults are nearly vertical and extend to the basement although displacement appears to decrease with depth. The south field bounding fault is probably a left lateral wrench fault zone as discussed by Warner (1997); this fault zone locally displays extensional and possibly compressional structures and has a complex and protracted displacement history. The fault was active concurrently with Lance Formation deposition creating thicker Lance Formation locally on the north side of the fault (north-side down during Maastrictian time). Subsequently, post-Paleocene uplift of the thick Lance Formation package (north side displaced upward) may be responsible for the broad open folding. Detailed analysis of the west field-bounding fault suggests that it is a composite of two faults that have merged. This explains the apparent dip reversal along the length of these features and contrary to previous reports, is not a scissor fault.

Seismic imaging of Jonah Field is challenging because of complex fault throws, and generally dim and discontinuous reflections within the producing interval. Advanced seismic techniques have been used to define the trap boundaries and to optimally locate development wells.