ASK-A-GEOLOGIST by Jeff Wynn

Citizen Scientists, Oil & Gas… and “Windshield Time”

– We’re all natural scientists

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 may 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. And you don’t need a PhD to be a scientist: famous examples include Isaac Newton, Michael Faraday, and James Clerk Maxwell.

I participated in an expedition that crossed the Empty Quarter desert in Saudi Arabia years ago. Our team had two formally designated scientists (we both 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 issue of *Scientific American). Our expedition companions first started asking questions, then offering what turned out to be critically important 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. All of them (hopefully including myself) were natural scientists.

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 intrusive bodies of carbon-rich 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 called Khanneshin 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 called Cerro Impacto. It’s a ~10 km circular feature — but is not an impact structure). These things often have unusual levels of Thorium, Uranium, and Rare Earth Elements in them, often in concentric halos. There are also diamond-bearing 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 plant and animal occupants during ages that reach back as far as life has 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 above often caused the basin to sag and get deeper in the middle.

As an example of how fast this sediment accumulation can happen, I was visiting an ancient mine site in the western Arabian Peninsula called an-Najadi (see a coming chapter “The Dustbowl, King Solomon, and Country-Western Music”). An-Najadi was one of 862 small ancient mines (yes, the US Geological Survey mapped and numbered them) that provided King Solomon with his gold about 3,500 years ago. In that 3,500 years, dust and sand blowing east across the Red Sea from the Sahara has 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 just to access the original mine-shaft. And this accumulation was on flat ground! When surrounded by eroding mountains, a basin’s sediments can build up much faster than this from weathering.

With increasing weight overlying these carbon-bearing sediments, both pressure and heat increase. Natural temperatures at the bottom of a 12,000-ft/4,000-meter diamond mine in South Africa reach about 60 degrees C (140 degrees F). Miners can work the rock faces there only with conditioned air pushed down to them.

Eventually the increasing depth of burial provides enough heat and pressure to “cook” the organic sediments — oil & gas geologists call this process “maturation.” When thus converted to a liquid these now relatively less dense, carbon-rich fluids tend to migrate upward, following weak zones in the sediments overlying them (see figure below). They will do this until they either escape (the Gulf of Mexico and parts of California are full of natural “seeps” like the La Brea Tar Pits) or they get to a blockage that traps them. The figure shows examples of an anticline trap, a fault trap, a stratigraphic trap, and a salt trap from sedimentary salt squeezed up by overlying rock pressure. The salt originated from a dried-up ancient sea. These kinds of natural traps are what the oil companies are looking for when they use sophisticated seismic prospecting, magnetics, gravimetry, and imaging systems.

Oil and gas formation, migration, and entrapment. The greater the depth and temperature, the greater the maturation of hydrocarbons (Image: Creative Commons, Steven Earle, Physical Geology, https://opentextbc.ca/geology).