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The Medicine Lake field of Sheridan County, Montana, was discovered in March 1979. In October 1981, a mini-3-D seismic survey covering 2.5 mi2 (6.2 km2) was acquired over this field in order to facilitate development drilling by delineating the field's reservoirs and obtaining a more accurate image of the subsurface structure.
A multiline system, consisting of 240 geophone groups distributed on 8 lines, was used. The energy source was shothole dynamite using 5 lbs (2.3 kg) charges at 250 ft (46 m). The shotpoints were arranged in a cross pattern with extra shotpoints included to provide necessary control on the weathered zone. The average subsurface coverage was 600%, with CDP bins 165 ft (50 m) square. Prior to the actual shooting, a computer simulation of the resulting fold was performed to verify the field geometry. The entire survey was recorded in one day with no movement of the geophones, thus minimizing costs.
The data volume was processed in preserved amplitude through 3-D migration and 1-D inversion. The subsurface image was substantially improved by the 3-D migration process. The advantages of this enhanced focusing ability are particularly important when attempting to delineate the lateral extent of reservoirs and detect lithologic variations.
The Medicine Lake field is located on a structural high, although there are stratigraphic implications for several of the producing zones. The interpretation of the data therefore focused on both structural and stratigraphic features.
The Medicine Lake structure is prominently displayed on the Winnipeg event, showing a closure in excess of 180 ft (55 m). Several reflectors near the base of the Red River interval terminated against the Winnipeg event, indicating that this structure was a high in Red River time. Discontinuities in the Cambrian and Precambrian reflectors suggest that the Medicine Lake structure is a result of basement faulting.
The objective of the stratigraphic interpretation was to outline zones of possible porosity, particularly in the Madison and Red River intervals. The horizontal and vertical inverted sections were particularly useful for ascertaining the location and lateral extent of those anomalous zones. The results correlate well with known production, and should aid in the location of future development wells.
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