Friday, June 6, 2014

Windshield Time

Q: Thank you very much. Your answer was more than adequate. Not only did you answer my primary question but also preemptively answered some follow-up questions I may have come up with.
My only remaining question is how the extremely deep oil reservoirs they are finding were formed. I've read some of the oil is at depths that would seem to pre-date the Carboniferous age.
I'm a plumber but I love pondering things such as this during my frequent "windshield time ". I appreciate you taking the time to explain this to me and to do so in a way I can understand.
Thank you very much,
- Patrick D

A: You can call yourself a "Plumber" if you want, but you are clearly and instinctively a natural scientist. That's the only definition that would apply to someone who ponders the world around them to such a deep extent during "windshield time" as you call it. I was involved on an expedition that crossed the Empty Quarter desert in Saudi Arabia and had two formally-designated scientists (we had PhD's). However, most of the other 15 expedition members got deeply into what we were trying to map at the Wabar asteroid impact site (Gene Shoemaker and I published this in an article in the November 1998 Scientific American). Our expedition companions first started asking questions, then offering ideas - and as a scientific TEAM we did the partial crater excavations and the surface mapping of the site. There were 17 people on that science team.

To answer your other question, there was carbon on this planet from its original formation. Some is magmatic in origin - things like carbonate volcanoes, or crystallized carbonate magma more commonly called "carbonatites".  This is primordial carbon that is thought to come from the mid-to-upper mantle. There is a carbonatite in southwestern Afghanistan that stands out from the surrounding rocks both chemically and structurally like a big red flag. There is another, a real monster, in southern Venezuela (Cerro Impacto is ~10 km across, but is NOT an impact feature). These things often have unusual levels of Thorium and Uranium in them, often in concentric zones. There are also Kimberlite Pipes - these are generally but not always tubes that carry diamonds up to the surface from the upper Mantle.

However, most oil & gas deposits come from sedimentary deposition of swamps and their occupants during ages that reach back as far as life existed. The carbon in the vegetation and animal life was buried to increasingly greater depths by later sediments, sequestering it and getting it out of the atmosphere. This usually happened in large basins, and the accumulating weight of these sediments often caused the basin to bow and get deeper in the middle. As an example of how fast this accumulation can happen, I was visiting an ancient mine site in the western Arabian peninsula. This was one of ~862 small ancient mines that provided King Solomon with his gold about 3,500 years ago. In that 3,500 years, dust and sand blowing across the Red Sea from the Sahara have buried the original mine site in nearly 4.5 meters (14 feet) of loess, silt and dust that we now have to dig down through to access the original shaft. And this accumulation was on flat ground! When surrounded by eroding mountains, a basin’s sedimentation can build up much faster than this.

With increasing weight overlying organic-containing sediments, both pressure and heat rise. Natural temperatures at the bottom of a 12,000-ft diamond mine in South Africa are about 60 C (140 degrees F). Eventually you get enough heat and pressure to "cook" the organic sediments - oil geologists call this process "maturation" among other things. When converted to a liquid these relatively less dense, carbon-rich fluids (oil and gas and water) tend to migrate upward, following weak zones in the sediments overlying them. They will do this until they either escape (the Gulf of Mexico is full of natural "seeps") or they get to a blockage that traps them: for instance some sedimentary salt from a dried-up ancient sea. THIS kind of natural trap is what the oil companies are looking for using sophisticated seismic prospecting and imaging systems.  

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