Better Resolution Or Coincidence?

Many attempts have been made throughout the history of modern seismic to image thin beds (<1/4 of dominant wavelength) by extracting higher frequencies from seismic. In addition to simply imaging zones below normal resolution, two of the more common goals to aid in reservoir development are:

• To define pinch-outs of producing zones.
• To resolve internal bed geometries.

Techniques to enhance seismic frequencies are critical to achieve optimum thin bed resolution.

The most common post-stack method is spectral whitening or boosting the amplitudes of all frequencies within a certain bandpass to the same level. The problem with this method is that it does not discriminate noise from signal. Noise is boosted along with the subsurface signal and, depending on the signal-to-noise ratio (SNR), whitening may fail to extract the very information we hope to resolve.

Other techniques such as coherence cube technology and seismic inversion also can help define some of the thin bed properties we seek through a different approach but can still be limited by the inherent bandwidth of standard seismic.

This column focuses on application of a method that attempts to separate the signal from the noise while enhancing only the high frequency "earth signal" -- a technique that helped identify new well locations in thinly bedded reservoirs that would not have otherwise been drilled.

More importantly, it helped to nearly quadruple daily production rates and add significant new reserves to a 27-year-old Gulf of Mexico field.

The example used here comes from South Marsh Island Block 128 Field (Figure 1). The discovery well for this prolific field was drilled in June 1974. The field is a stratigraphically complex, salt cored NW-SE trending anticline bounded on the west by a large down-to-the-west fault.

Reservoir age ranges from Angulogerina B (Early Pliestocene) to Lenticulina 1 (Late Pliocene) at depths of 4,500 to 9,000 feet subsea. Paleobathymetry ranges from inner neritic at the shallower levels to upper bathyl in the deeper zones, with all reservoirs being normally pressured.

The field has seven exploratory wells and 93 development wells, including sidetracks, drilled from four offshore platforms. In January 2000, cumulative production was 115 MMBO and 203 BCF, and average daily production rates were 3,500 BO and 4 MMCF.

Structural interpretation there had been difficult from the outset with various interpreters producing different structural pictures (the lack of seismically mapped faulting was the variable in the interpretations). Even with the acquisition of proprietary, first generation 3-D seismic in 1989, the uncertainties persisted.