Print this pageAAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Salt Welds in the Deepwater Gulf of Mexico – Uncertainties in the Amount of Remnant Salt: Seismic and Well Examples
(1) CNAEP, Chevron, Houston, TX.
Salt welds mark the disconformable surface resulting from the removal of salt from a stratigraphic sequence. Tertiary welds represent the removal of salt from a canopy (areally extensive) while secondary welds often represent the compressive deformation of a salt stock (areally limited). Whereas a true salt weld represents total removal of salt, in practice variable amounts of salt may remain along the weld surface.
Part of the uncertainty lies in the distinction between an true weld and a “seismic weld”, that is, where there is the inference of prior salt being reduced to zero or negligible thickness, but the practical limits of seismic resolution could allow for 50 meters of salt to be present but “absent” in the seismic response.
Formation of a salt weld typically results in a significant stratal disconformity; this is the primary means of identifying salt welds. Most typically, the strata underlying the disconformity will correspond to the regional canopy emplacement age although rubble zones can be present locally; the overlying material may vary in age depending on the amount of translation of supra-salt strata and the time of deflation, ranging from old, rafted blocks to young minibasins. In the limiting case, a young minibasin may form above a collapsing salt stock, ultimately juxtaposing very young strata on Jurassic.
The Ponza well (KC 774) encountered perhaps the most “typical” salt weld. Condensed Paleogene and Cretaceous section overlie Miocene section, with no salt encountered. The corresponding seismic clearly indicates a dramatically thinned weld in continuity with two significant salt canopies.
The Sumatra well (GB 941) encountered a significant age disconformity, with Cretaceous overlying Oligocene, interpreted as a salt weld with no salt apparent in the limited logging suite. While the Paleogene/Cretaceous section is interpreted as a canopy raft, the regional extent of the interpreted salt weld appears to be limited.
The Cortez Bank well (KC 244) drilled through approximately 260 feet of salt at the equivalent position of the base of adjacent salt canopy. This seismic weld bears similarities to the Sumatra well in having limited areal extent.