Hydrocarbon Reservoirs for Total Dummies

simplified petroleum reservoir model

Sometimes.... no, make that a lot of the time... we run across internet freelancers who get an answer more or less right, then blow any possible credibility by saying inane, stupid, and utterly clueless crap in an effort to expand on their answer. This frequently happens when the writers in question have been forced to pad the simple answer to meet some arbitrary minimum word count. At the former eHow.com (and the many niches into which Leaf Group has now stuffed its content), that was a common problem – so common, in fact, that the AN staffers have a tag for "minimum word count" on this blog. Well, here's another one for the tag: meet Sheila Johnson and her Sciencing.com post, "Oil & Gas Are Found in What Kind of Rocks?"

From the wording, it's pretty clear that this was a homework question for an earth science class. The expected answer would be "sedimentary rocks," although a more accurate answer would be "porous rocks, including sedimentary rock and occasionally, volcanic rock." Johnson managed to spit out the simple answer, although our geology types think she oversimplified:

"The types of rocks that contain oil and natural gas are all sedimentary rocks, rocks formed when grains and mineral particles deposited by running water fuse together."

The geologists tell us that her answer includes several mistakes:

1. As mentioned above, some oil reservoirs are porous or fractured igneous rocks (google "Railroad Valley, Nevada").
2. Not all sedimentary rocks are formed of "grains and mineral particles deposited by running water."
3. The process of lithification – turning sediment into sedimentary rock – is not generally considered "fusing." Compression, dewatering, and cementation; yes. Fusion, no.

Johnson, however, had plenty more to say and even though she cited half a dozen different fairly reputable references, we're almost certain that some of what she said was not to be found in any of them. Some examples:

• "Shale... is produced when layers of carbon-rich mud are compressed until they harden into rock that retains those layers. " – Sorry, Sheila, but "carbon-rich" isn't part of the definition of shale.
• "Other types of especially porous rocks often form above shale beds, trapping the low-density carbon compounds that may rise through the mud that becomes shale in their spaces. " – We aren't certain why Johnson thinks these rocks "form above shale beds," not to mention her rather bizarre description of petroleum migration.

• "Carbonates, however, are formed largely from remains of marine life, particularly shells and bones, combined with other minerals." – Did she really say "bones"? Oh, and much of the volume of limestone is comprised of micrite, mud-sized particles that result from biodegredation, comminution, and direct chemical precipitation. In other words, not "shells and bones."
• "Heating the kerogen contained in shale, for example, produces gas and liquid oil that flows readily to the surface..." – No one has ever managed to carry this out on a scale approaching economical (if at all).
• "...hydraulic fracturing applies high-pressure liquid streams to sedimentary rocks to exploit their fissures, allowing oil and gas to move freely." – No, Sheila, hydraulic fracturing doesn't "exploit... fissures," it creates fissures. 

All told, around half of the "facts" delivered by Johnson were questionable at best and, at worst, utter bushwa. For her powerful example of the stupidification of the internet, we do hereby name Sheila Johnson the Dumbass of the Day, hydrocarbon generation and exploration division. Go, and sin no more.

SI - OIL