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SAXENA, RAM S., GeoConsultants International Inc., Kenner, LA

ABSTRACT: Gas Anomaly on Seismic-Seismic Interpretation Utilizing Geologic-Seismic Models Resulted in an Onshore U.S. Discovery

In an onshore south Louisiana field, a significant gas well was drilled in an area originally mapped as a "low," by reinterpreting the seismic data with the help of depositional concepts and a seismic-geologic model.

The gas-bearing sand is interpreted to be of reworked deltaic sand origin. Such sands are produced by the reworking of older abandoned deltaic complexes. The overall gas-bearing sand depositional sequence is comprised of, from base to top, (1) an organic-rich "prodelta shale" (velocity 7200 ft/sec), overlain by (2) a "distributary mouth bar sandstone," 350 ft thick in its middle parts, poorly sorted sandstones with abundant finer grained matrix (velocity 10,500 ft per sec.), a characteristic lensoid shape with a concave upward base and a flat top, overlain by (3) the "reworked deltaic sandstones," clean, well sorted, quartzose sandstones with excellent porosity and permeability, occasionally calcic, 40 to 60 ft thick, elongated along depositional strike (velocity 11,500 ft/sec), overl in by (4) "transgressive marine shales." The transgressive marine shales are deposited during the destructive phase of the delta system, contain abundant marine organisms, are very calcic, and have higher velocities (compared to prodelta shales) ranging between 7500 to 8000 ft/sec.

The velocities increase going upwards in the section, a reflection of deposition in increasingly deeper marine waters.

The seismic criteria that formed the basis for the original "synclinal" interpretation of the gas zone, once depositionally explained, became the key criteria for the recognition and mapping of the gas anomaly on seismic data.

The middle part of the reworked sand which was gas-bearing had a lower velocity, 6670 ft/sec (approximately 58% lower), than the corresponding water-bearing sand which had a velocity of 11,500 ft./sec. The gas sand velocities were lower than that of the overlying transgressive marine shales (7500 ft/sec). Thus, at this interface a weak negative (trough) reflection was generated. Laterally, going away from the middle part of the reworked sand along depositional strike both towards the east and the west, in areas where the reworked sand was water-bearing, there existed an excellent contrast of velocities between the overlying shales (7500 ft/sec) and the water-bearing reworked sand (11,500 ft/sec), resulting in the development of a strong "positive" amplitude at this interface. This lat ral change (complete reversal of amplitude), "strong positive" on the sides where the reworked sand was water-bearina to "weak negative" in the middle where the reworked sand was gas-bearing, was originally explained by faults on the two sides and a low in the middle part. The concave base of the distributary mouth bar sand body underlying the gas-bearing reworked sand showed up as a strong negative because of the contrast of higher velocity distributary mouth bar sands (10,500 ft/sec) overlying the lower velocity prodelta shale (7200 ft/sec). This concave upward lensoid shape, not being recognized as a depositional features further accentuated the erroneous synclinal interpretation of the gas-bearing zone.

A seismic model was developed for the reworked deltaic sand sequence. The model clearly showed that the polarity reversal in the reworked sand was because of gas and the strong, concave upward, negative event was the base of the distributary mouth bar sand body. The model provided excellent comparison for the recognition of various components of the reworked sand sequence on the seismic data.

Reworked sands are ubiquitous in the geologic record. An understanding of the geologic model is a must for the recognition of these sequences on seismic data.

AAPG Search and Discovery Article #90988©1993 AAPG/SVG International Congress and Exhibition, Caracas, Venezuela, March 14-17, 1993.