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Reservoir Characterization in Jonah Field, Wyoming: Integration of Offset VSPs, and 3D Surface Seismic Data

House, Nancy1, Pete Wynn1, Dean Dubois1, Brian Fuller2, Marc Sterling2
1 EnCana Oil and Gas, Denver, CO
2 Sterling Seismic Services, Littleton,

Jonah Field, in the Green River Basin in Southeast Wyoming produces from an Upper Cretaceous fluvial section in the depth range of 7,500 to 12,000 ft. Nearly vertical faults define the field boundaries on the south and northwest side of the triangularly shaped field. The upper Cretaceous Lance reservoir section consists of nearly 3,000 feet of stacked, tight, fluvial sands. Major wrench faults on the South and Northwest are believed to be boundaries to production with numerous internal faults that also appear to influence production. 3D surface seismic successfully images the larger field-bounding faults and shows the discontinuous nature of the reservoir section, however, it has too low a frequency range to image the reservoir details.

Several VSP datasets were recorded to enhance the seismic resolution in critical production areas and to further constrain the 3D surface seismic interpretation. A 3D VSP, recorded with EnCana’s partners yielded seismic frequencies as high as 130 Hz in the 3D image area. Three zero-offset and combined zero-offset/offset VSP datasets provided important structural and stratigraphic details within the field. Subtle internal faulting was clearly discernable. In one instance a very subtle fault was interpretable that was only inferred from loss of signal in the surface 3d volume. As the VSPs were recorded with three component geophones, the wave field could be separated into its three components giving additional detail for AVO modeling and interpretation.

The combination of VSP and 3D surface seismic datasets enhanced our understanding of the reservoir by providing geologic details in key areas that could not be obtained from the surface seismic data alone. The 3D surface data provides a large-scale picture of the reservoir that cannot be obtained from the VSP data alone.