The horizons of primary interest in the M A 668 Field are of middle Miocene age. They are present in the field as fan complexes which prograded from west to east. They were subsequently folded into a shallow dome with four-way dips and cut by two generations of orthogonally-oriented faults, the first down-to-the-west and the second down-to-the- north. Efficient development of the field required that a 3-D seismic survey be conducted in advance of development drilling so the prospective fault blocks could be properly mapped. Exploration drilling had also revealed the strata at the prospective horizon(s) to be complex, and we hoped that the seismic program would also help with mapping stratigraphic variations within the reservoir.

Seismic-stratigraphic interpretation was conducted on an interactive color terminal which facilitated the selection of optimum display scales, colors, and orientations. Facies analyses relied on identification of such features as top-and-bottom- lap, foresets, top truncations, and hummocky reflectors.

Well log analyses of facies utilized SP and gamma curve shapes: fining upward (increasing gamma and SP), coarsening upward (decreasing gamma and SP), and more blocky and rounded shapes were all identified and compared to record sections printed or displayed at the same scale as the well logs.

Top-lap seismic sequences were found to correlate with fining-upward log character, bottom-lap seismic sequences were found to correlate with coarsening-upward log character, and log forms with shapes intermediate to these were found to correlate with foreset sequences. Hummocky reflectors were found in the core of the fan complex.

Time-slice displays and displays flattened on events were also helpful in defining the limits of selected fans, and these could be correlated to limits implied from the log interpretation.

Subsurface facies analyses are often done using either well logs or seismic-stratigraphic techniques. There are pitfalls inherent in depending too much on either, but a high-quality data set is not always available for cross-testing alternate hypotheses. Our analyses suggest specifically that electric log curve shapes may be interpreted with greater confidence in the future than has generally been the case.

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