Dirty vs. Clean Salt: 

Their Impact on the Subsalt Wilcox Deepwater Exploration Plays

Selim Shaker

Geopressure Analysis Services, Inc. (G.A.S.), 14027 Memorial Dr., Ste. 130, Houston, Texas  77079

ABSTRACT

This paper explores the assertion that risks in the subsalt Wilcox deepwater frontier at the Sigsbee Escarpment in the Gulf of Mexico exploration play can be attributed to the salt type.  Drilling through and below the salt can be costly and lead to the abandonment of the whole project.

As the salt mass toe creeps downdip near the mud line, it creates potential traps in the older underlying sediments as thrust folds and faults.  A clean salt mass is usually driven downdip by gravity/ buoyancy.  On the other hand, dirty salt is pushed downdip by the sediment influx, in addition to buoyancy.  Occasionally, sediments intrude into the salt body and are carried within as rafted blocks.  This causes a challenge in testing the subsalt traps in this deepwater frontier play.  Moreover, in both dirty and clean salt cases, plowing the older sediment underneath the salt creates subsalt gouges which represent a substantial drilling hazard.

The Jack prospect (Walker Ridge Block 759) is a part of the emerging Wilcox-equivalent salt toe belt.  The rafted sediment blocks and the subsalt gouge made this prospect very expensive to test.  The borehole experienced multiple losses of circulation and had to be sidetracked several times.  This was due to the narrow drilling window created by the pressure kicks released from the embedded sediments throughout the salt section.  On the contrary, St. Malo (Walker Ridge Block 678) is an adjacent prospect on the same trend, but was drilled through clean salt.  It was tested with a minimum of difficulties, compared to the Jack prospect.

Tracking the mechanism of the salt movements and their history as dirty or clean type can predict the risk of testing a prospect along this new exploration fairway.

Shaker, S., 2009, Dirty vs. clean salt:  Their impact on the subsalt Wilcox deepwater exploration plays:  Gulf Coast Association of Geological Societies Transactions, v. 59, p. 689-698.

AAPG Search and Discover Article #90093 © 2009 GCAGS 59th Annual Meeting, Shreveport, Louisiana