Geologists rethink Gulf of Mexico basins

Oct. 29, 2003
The Gulf of Mexico basin continues to challenge the conceptual thinking of geologists as the industry moves into deeper water, deeper sediments on the shelf, and into new areas off Mexico. Some of that new thinking was evident at the recent Gulf Coast Association of Geological Societies (GCAGS) annual convention in Baton Rouge, Louisiana.

The Gulf of Mexico basin continues to challenge the conceptual thinking of geologists as the industry moves into deeper water, deeper sediments on the shelf, and into new areas off Mexico. Some of that new thinking was evident at the recent Gulf Coast Association of Geological Societies (GCAGS) annual convention in Baton Rouge, Louisiana.

Speakers noted the dominance of vertical processes in fluid movement throughout the northern Gulf of Mexico due to salt movement and faulting. This is evidenced by the abundance of oil and gas seeps at the seabed and the presence of "gas chimneys" on seismic images that make imaging difficult beneath many structures on standard seismic data. Longer offset data and new processing techniques are helping overcome this issue.
The dominance of vertical fluid processes means that fluid pressure relationships control the movement of oil and gas. Evidence was presented that shows the development of "sweet spots" between 0.5-0.7 pressure gradients. This sweet spot is common on the shoreward side of shale diapirs formed near the base of normal listric faults. Ponded sediments on the downward side of the faults help create the shale diapirs that then concentrate reservoir sands. These in combination with the fluid pressure regime form a trap for hydrocarbons.

The upward movement of hydrocarbon fluids through the sediments has revealed a potential environmental hazard that the industry notes. Concentrated saline solutions expressed at the seabed are creating barite mounds and tubes, which occur across the GoM in deeper waters. These are similar to carbonate mounds with accompanying chemo-synthetic communities higher on the shelf. The fluids that form the barite tubes carry high levels of radium that could become an environmental hazard for future oil operations at the seabed.

On a structural geology note, the Bureau of Economic Geology (Texas) presented a study from the Laguna Madre-Tuxpan Shelf offshore Tampico, Mexico. This north-south trending shelf contains a major fault system, which shows rotation around a point in the southern part of the basin. This sets up very different faulting and structural styles across the basin and has implications for the structural history of the southern GoM. A new interpretation on the opening of the GoM will be presented next year.

10/29/03