Oil Reservoir Depth for Dummies (HSW Week 1)

Depth of petroleum generation

It's been a while since we featured a site for a week, but since we figured out that much of HowStuffWorks.com is actually written by freelancers with journalism degrees, we've been itching to look at them more closely. So, welcome to HowStuffWorks week...

You've certainly heard someone say to a friend or coworker, "It's on your right. No, your other right!" Even well into adulthood, people still sometimes get simple things like right and left confused. It's no wonder, then, that content-farm freelancers who've picked topics with which they have only a vague familiarity (if any) are going to make some mistakes – sometimes pretty serious mistakes. With that in mind, we submit for your consideration a HowStuffWorks post by Alice Truong, allegedly written to address the question, "How far underground are oil deposits?"

Truong may be a veteran freelancer, but we think she blew it on this one. Instead of actually answering the question, Truong instead took a two-pronged approach. The first was to cite a few historical statistics for average well depth, which required some 83 words of text. Her second approach was to write almost twice as much (159 words) to pull in eyeballs about pollution and oil spills, shoehorning in a reference to the Deepwater Horizon disaster. Feh on Alice.

The real answer has nothing to do with historical trends in average depth of wildcat wells. Such statistics are controlled in part by technological advances and in part by the fact that humans will always find the easy, i.e., shallow reservoirs first. The real answer has to do with the origins of hydrocarbons and how reservoirs are formed. Here's what we mean:

Point 1: The raw materials that become oil must be heated to turn into crude oil, which requires a minimum temperature of about 80°C. The process is called maturation. The depth at which maturation occurs is controlled by the local geothermal gradient, which typically ranges from 10° to 40° C per kilometer. Hydrocarbon maturation continues as the temperature increases: at temperatures above about 100° C, crude oil begins to "cook" to natural gas. Little liquid hydrocarbon remains if the temperature exceeds 150° C. In other words, the maximum possible depth at which oil "deposits" can exist is in the range of 5 to 9 km, or 16,000 to 30,000 feet (depending on the geothermal gradient).

Point 2: Hydrocarbons do not stay at depth, however, because they are relatively light and buoyant. As a result, oil migrates surfaceward until it is trapped and can no longer rise. Some petroleum makes it all the way to the surface, appearing in oil seeps. Those are usually gooey, tar-like deposits because the light hydrocarbons have been washed out by groundwater. A good example is the La Brea Tar Pits in southern California. In other words, "oil deposits" can, theoretically, be found anywhere from about 30,000 feet deep to the surface; although few reservoirs are deeper than about 20,000 feet. That means that Truong's "answer" is both off-topic and uninformed. When it comes to the depth of oil reservoirs, this is not "How Stuff Works." Instead, it's typical freelance Dumbass of the Day work.

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