Geologic and Tectonic Setting

The Gulf of Mexico basin, south of the U.S. coastline along Texas and Louisiana, offers a challenging geologic and tectonic setting to the best of geoscientists and engineers. Far beneath the surface of the coastal areas, and extending far offshore, the underlying basement crust is part of the ancient Precambrian “continental” mass that is part of the North American tectonic plate. Farther to the south offshore, at the edge of the continental slope where the seafloor of the Gulf of Mexico begins to descend precipitously to depth, the underlying crust is composed of “oceanic” material, primarily basalt. These rock types are the foundation of the Earth’s crust in the region, upon which all else rests.

Chevron’s deep-water success began with its ability to capitalize on many decades of fundamental research into oceanography, bathymetry, and geophysics that has allowed researchers to gain some semblance of understanding of what lies beneath the waves, let alone the seafloor, of the Gulf of Mexico. Immense sums have been spent by both government and industry to map the ocean floor, to measure the force of gravity and chart gravitational anomalies, to measure the magnetic fields in the area, and to gauge myriad other physical parameters. This is just some of the cultural, social, and scientific foundation for Jack #2. Continued.

Jurassic Salt

There was another body of geological and geophysical knowledge that was also essential to Chevron’s success. That is that the continental and oceanic crust of the Gulf of Mexico is overlain by sedimentary rocks of Mesozoic age and younger. There may even be some rocks of even older Paleozoic age, but I am not aware that any drill bit has ever cut into them.

Toward the bottom of the known stratigraphic sequence is the Louann salt of Triassic-Jurassic age, about 225-150 million years old. In the deep waters of the Gulf, this is also called the Sigsbee salt. Due to much deformation in the crust of the Earth over millions of years, the salt beds have developed into what are called “salt basins.” These salt basins are key geologic controls over the structures in the sediments that have trapped and now hold oil and gas deposits. And it has taken quite a lot of research just to figure that out.

These salt beds are the remnant of an ancient series of shallow seas that were, from time to time over a 75-million-year period, isolated or cut off from the main circulation of the world’s oceans. How does this occur? Imagine, for example, that the Mediterranean Sea was shut off at the Straits of Gibraltar on the west and that the Bosporus Straits farther east were also closed. While you are at it, imagine that the Black Sea was isolated from main currents of ocean circulation.

Now imagine that over time and under millions of years of a hot, dry climate, evaporation exceeded the amount of water entering into the Mediterranean and Black Sea basins from precipitation and river flow. Thus, over time, the sea levels in both bodies of water would fall. The waters would become brackish from the dissolved salts (think of the Great Salt Lake or the Dead Sea), and eventually, the brackish water would begin to precipitate out salt crystals. These salts are mostly what you would expect: sodium chloride (called “halite” in mineralogical terms), as well as other minerals like gypsum, anhydrite, and other exotic chemical forms of dissolved minerals contained in seawater.