Eugene Island Block 77 field is a shallow (1,100 ft, 335 m) piercement salt dome with a low relief overhang which is productive from upper Miocene sands at depths between 13,300 and 15,800 ft (4,054 and 4,816 m, or between 3.260 and 3.690 sec). These hydrocarbon accumulations are trapped in steeply dipping beds (10 to 40° by dipmeter) between the salt mass and the rim syncline. Small radial faults (50 ft, 15.2 m, of throw) also affect reservoir limits, as is evidenced by varied gas/water contacts and reservoir production performance.

This combination of factors (piercement dome, salt overhang, deep objective, steeply dipping beds, tight rim syncline, small faults) makes Block 77 field difficult to map accurately with 2-D seismic. Unmigrated lines do not show the dip between the salt mass and rim syncline; migrated lines contain migration artifacts, making a salt interface interpretation ambiguous and recognition of small faults impossible. To overcome the problems inherent to the 2-D seismic method, a dense grid of data (trace spacing of 110 ft, 33.5 m, in both X and Y directions) was collected over this field and migrated in three dimensions. This placed events in their proper spatial relationship, thus enhancing both fault delineation and salt face interpretation. In previous interpretations (based on 2-D data), only one fault was mapped--a large down-to-the-north fault extending eastward from the northeast quadrant of the dome. The 3-D data show a more complicated combination of fault systems including other large parallel faults, some with compensating faults; buried down-to-the-south faults; and an extensive system of small radial faults.

Salt-face maps based on 2-D data were very inaccurate. Due to sideswipe problems, only lines shot radially across the dome were interpretable. This created large areas of no control. Using a 3-D grid eliminated the sideswipe problem and increased the amount of control on which to base the interpretation. The salt face, as seen on the seismic was tied to existing well control to make an accurate salt-face interpretation.

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