What's Oil Doing on the Spindletop Dome?

150 million years ago, this region along the Gulf Coast of Mexico looked very different than what we see today. It was part of a shallow sea that would occasionally dry up. This salt water, when dry, left behind its salt. Lots of it. Over time, great thicknesses (hundreds of meters thick) of salt accumulated.

Later, when the Gulf of Mexico was similar to what we see today, thousands of meters of sediments piled on top. These sediments have accumulated over the recent geologic past. The closer to the coast you are, the "newer" the rocks. The term "rock" is used very liberally here - if you live in Houston, Texas or New Orleans, Louisiana, it is unlikely you have seen any natural rocks along the ground in your hometown (not counting the skyscrapers or concrete). This is because all of the sediments here are so new, and have not been buried. Thus, they have not had the chance to turn into rocks - they are still clays, silts and sands. The further inland you go, the older the rocks get in this region. Similarly, the deeper you go, the older the sediments. After a few thousand meters down, the sediments do start to lithify (turn to rock).

Salt is a funny kind of rock. If you put lots of pressure on it, much as the thousands of meters of sediments overlying the Gulf Coast salts have done, it begins to move. It actually begins to move up towards the surface, because its density is less than the surrounding rocks. Salt under pressure (and heat) moves very much like the material in a lava lamp, gurgling and bubbling upward, breaking through and deforming the rocks above. This is how a salt dome forms. As we know, broken rocks can act as very effective traps for oil and natural gas. Salt also acts as a trap itself (see a diagram of how this happens.)

Yet another method of trapping oil occurs at the top of the salt dome in a zone called the cap-rock. Nearly all of the salt domes in along the Gulf Coast have a disk-like cap-rock, composed of minerals such as gypsum, anhydrite, limestone, sulphur and dolomite, over part or all of their surface. This cap-rock generally results from ground water interacting with the surface of the salt, which causes the mineral to change. Because calcite and dolomite dissolve when exposed to water (forming caves), there are often caverous expanses that form in this zone. The discovery well drilled at Spindletop found this salt dome's cap rock. Because this zone has miniature caves filled with oil, it comes out of the ground very easily. When this zone is penetrated by the drill, oil comes out of the ground fast and furious.

What drew Patillo Higgins to Big Hill (what locals call Spindletop) was just that - a big hill. Not big by most standards, but big enough along the flat coastal plain of Texas that it rises high above the landscape. As the salt thousands of meters deep moved upward through the overlying rocks and sediments, it pushed everything above it upwards. Higgins suspected that oil was moving towards the surface, creating this giant "bubble". He was only partly right - there was large amounts of oil there, but it wasn't the oil that created the hill - it was the salt.

This is what Big Hill looks like - it rises up about 28 feet (8.5 meters) above sea level. (Need help reading this contour map?)

It just so happens that there is a large amount of source rock deep beneath the sediments of the Gulf of Mexico - so much so, that geologists seldom worry about oil being there. It's just a matter of the oil being trapped. At Spindletop, and many other salt dome structures in the region, oil is not only trapped in the cap rock, but also on the sides of the salt domes, and within fault traps created by the fractures as the salt plows through the rocks above. (click here to learn about these types of traps)

The Lucas Well drilled through unlithified (loose) clays and sands, and struck the cap rock at a depth of 270 meters (880 feet). At 310 meters (1,020 feet), the drill cracked through to the cavernous section of the cap rock, and they struck oil.

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