What's Oil Doing on the Spindletop Dome? (continued)
...back
to page one of Spindletop
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.
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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?)
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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.
Incredibly, it was later discovered that if the Lucas well had been drilled just 15 meters (50 feet) to the south, it would not have struck oil.
