Wednesday, May 16, 2007

Salt Dome Lore

Salty Tales about the Earth

Since I became deeply involved in exploring the relation of buried oil and the salt water enveloping it, I investigated quite a number of Salt Domes- both at the surface expression and inside salt mines. Some of these are one kilometer in the earth, and I have descended into two of them, to make measurements concerning the environment there and to investigate whether there might be a cosmic ray-free zone for use with radiation instruments.
I found no low radiation environment in the domed salt, because of the presence of KCl, which is also a salt, but I did collect a number of events which are lively enough to pass on. One of these is a model of the dynamics which incur in these natural anomalies:
On the Gulf Coast of Texas and Louisiana, there are a number of these kilometer diameter shafts of almost pure salt, the most interesting of which was the Avery Island dome. This dome reaches to the surface by creating a hill of sediments, below which there exists the salt- both NaCl and CaSO4 and some exotic compounds, such as sulphur, white gypsum (compared to the anhydrite in the dome proper) and stinky H2S below bedded limestone Caprock.
In the center of this surface hill is a fresh water lake, and the whole island is surrounded by brackish or sea water from the Gulf of Mexico. The hill is only some 50- 100 feet above sea level, but nevertheless is sufficiently elevated such that the salt water does not contaminate the rain-filled lake. In the swamps of Louisiana these hills can be easily seen protruding above the monotonous swamp vegetation. Visualize this small island covered by southern woods (oak and ash) contrasted to the mangrove and salt-tolerating vegetation off the hill. The island is fairly round, since the salt is pushing it upward on the order of a centimeter yearly, and the erosion of the uplifted sediments is not able to keep up with the uplift. The fresh-water lake is formed due to dissolution of some of the underlying salt as subsurface waters flow downward due to the tendency of NaCl to attract water (hygroscopic attraction)- where some sinking of the hilltop occurs.
Where this dissolution occurs, other compounds flow into the gap, such as oil and gas, and there is a chemical rearrangement of some of the previous salt compounds- which are now ionic solutions in the downward percolating meteoric waters. If one were to make a slice of the earth as you went into the earth (or drilled out the rock with a tool able to return the samples, called a “core”, you would see the following as you went deeper:
a. a Caprock of a few feet thickness would occur at the kilometer or less depth, of limestone (in otherwise sand and shale rock);
b. Below the limestone is a vuggy or hole-containing section, where stinky gases and sulphur occur.
c. Gypsum occurs below this, which is CaSO4.H2O (hydrated anhydrite);
d. Oil will occur in pores in the rock, along with salt water and fractured sandstone; and finally
e. A dome of salt, containing both halite and anhydrite, will increase in purity and percentage with depth until 100% salt occurs. In this zone, sufficiently deep so that water does not enter the diggings, there will be a zone which is mined for rock salt.
On Avery Island there are other interesting features, which are internationally known. First there is the McIlhenny Tabasco works, where Tabasco peppers are grown, salt is taken from the mine, tomatoes are raised and Tabasco Sauce is bottled and shipped. Everything is the color of Tabasco red- similar to the tomato sauce- even the owner’s sailboat with Tabasco-colored sails.
Then there is the salt works, where rooms the size of houses are excavated in the deep rock salt, and where pure salt us taken out by elevator. Workers are able to descend into the 3000 foot shaft, as have been complete trucks and front-end loaders, dissembled and reassembled in the half mile working rooms. The air is completely dry there, since the salt absorbs all moisture, and salt dust is everywhere.
Nearby, working on the edge of the island are the various petroleum facilities, some producing, some drilling for new oil, and some abandoned. Now you have the complete picture- there is a surface factory for Tabasco, there is a picturesque fresh-water lake, the oil people are drilling away and producing black gold in the distance, and there is the hidden salt works chipping away on the solid salt walls down below.
To complete this romantic scene, a fisherman has floated his boat out to the lake where he, with luck, can catch bass or freshwater catfish. All is serene for the moment. But the local newspaper developed headlines later for the events which were about to transpire:
Picture the angler, who is sitting in his pirogue, eagerly awaiting the sluggish fish. Then off in the distance with hardly a ripple on the lake ensuing, the rig has encountered a fault in the earth, which will take some of the heavy weighted drilling fluid and allow it to descend into the earth along what is usually a 45 degree angle toward the solid salt below. The mine crew is chipping away for the daily output of rock salt which is elevated to the ground surface and which has produced additional mine space. Oil is being produced nearby, thereby leaving a void in the earth which tends to cause the earth to sag, and finally the Tabasco plant is ginning away, producing America’s favorite barbeque sauce (especially for
Texans who are depleting the oil).
When the extending fault and fracture takes the drilling fluid, the fluid falls rapidly into the earth, eventually finding its way into the mine space to start filling it. This whole procedure opens a fault all the way to the bottom of the lake and water begins to rush downward into the mine also. The mining crew takes notice and escapes by the elevator, rushing to the Tabasco works to give the alarm.
The fisherman notices nothing at first, since the downward flow is gradual. But eventually, the flow increases and a whirlpool develops. Faster and faster the water sinks into the bowels of the earth, and this focuses the attention of the angler on the falling water level. His boat moves rapidly toward the gigantic whirlpool in the now-draining lake. Rowing frantically, since he has not the time to engage the outboard engine, he grimaces and gestures wildly: “Get out, get out, “he screeches” escape with your lives- the world is coming to an end!”
From the files and memoirs of Harold L. Overton (Jan 20. 2007)