The conventional explanation for minibasin subsidence is that it is driven by gravity— that minibasins exist because their fill is dense enough to sink into the underlying evaporites, expelling salt into the adjacent salt highs. This explanation is valid if the average density of the sediments is greater than the density of the salt, but it cannot account for subsidence of thin, less dense clastic sequences into salt. Seismic data show that many minibasins started sinking into salt when their siliciclastic fill was much thinner than the 1.5- to 2-km thickness necessary for compaction to invert the density contrast. For such minibasins, some mechanism other than gravity must be involved.

We investigated mechanisms of minibasin subsidence using a 3,600-km2 prestack depth-migrated 3D seismic dataset near the Sigsbee Scarp, northern Gulf of Mexico. This dataset covers 27 minibasins of varying size and thickness. These data indicate that minibasin initiation was synchronous with shortening, as indicated by the presence of thrust faults in the deeper parts of many minibasins (Figure 1). A compressional origin of minibasins is also consistent with finite-element models showing that laterally shortened minibasins will subside even if their fill is less dense than the salt.

The sedimentary fill of compressional minibasins can be divided into three stages (Figure 1): (1) prethrusting, which is typically shaleprone and may predate the existence of a basin, (2) synthrusting, in which sands are deposited in synclinal subbasins between thrusts, and (3) postthrusting, in which sand bodies may extend across the entire minibasin. Understanding minibasin evolution can therefore improve prediction of reservoir continuity in suprasalt plays.

Figure 1. Thrust faults affecting the deep section in many minibasins indicate that these basins formed in compression. Reservoir distribution within the minibasin depends on whether the sands were deposited prethrusting, synthrusting, or postthrusting. Data © Veritas Marine Surveys, Houston, Texas.

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The specific cause of shortening that led to minibasin formation is currently unknown. The orientation of thrust structures is highly variable. Their pattern suggests that shortening was partitioned by flow boundaries defined at shallow levels within and above the salt sheet. If so, suprasalt processes may have been an important control.

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