<DrumBeat: September 27, 2006 The Oil Drum DrumBeat: September 28, 2006>

257 comments on Burning Buried Sunshine

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joule on September 27, 2006 - 9:54am Permalink | Subthread | Comments top
I still have a little trouble fully understanding the exact mechanism by while the lipid fraction of plant and animal life gets converted into low-molecular weight petroleum constituents.  Most simple textbook explanations portray it as a form of breakdown or cracking and leave it at that.

However,  I wonder if the conditions are really sufficient for cracking. Take a typical petroleum deposit a mile down. While the pressure is high (say roughly 3,500 psi), the temperature is probably less than 200 degrees F.  So here's the question: don't you need a much higher temperature for any cracking to take place?

For example,  if you take say soy bean oil, put it in a pressurized vessel at 3,500 psi and raise the temperature to 200 degrees F, I doubt that you're going to see much happen. And I'm not sure that time is the answer, because if a chemical reaction is not thermodynamically favored in the first place,  the supposed reactants can sit there forever and nothing will happen. (I think this is one of the arguments put forth by some of the abiotic crowd, but I haven't the expertise in petroleum chemistry to judge the merits of such arguments.)

Or is the answer that these deposits were at one time much deeper and that the product of the cracking having migrated upward to their present level?

To put the question  another way, is there some minimum pressure/temperature regime below which no petroleum will form regardless of what time period is allowed?

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odograph on September 27, 2006 - 10:02am Permalink | Subthread | Parent | Comments top
Modern production needs a much higher rate of conversion.  For something that has time (millions of years of time), a statistically unlikely reaction, which nonetheless reaches a lower energy state may be sufficient.
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memmel on September 27, 2006 - 11:44am Permalink | Subthread | Parent | Comments top

Its the shale source rock that is baked not the resiovor where you find the oil. The temperature need not be that high I guess the lower limit might be surprising say 200-500 C and I think the role of microbes in the formation of oil is under estimated.

This link claims 600 C

http://www.hubbertpeak.com/shale/

But this is on the surface and for rapid formation. As I said
I think you can get formation at much lower temperatures once you consider microbes and millions of years.

I'm guessing your looking for a uplift event on a sedimentry plain or dried ocean to provide the energy and the oil will flow into the sandstone or carbonate basin and be trapped under salt domes.
Its intresting that the uplift is probably the cause of both the conditions for capture and the source of heat for oil formation.

There are a few places on earth were you have heat and shale
that may have significant oil deposits that we have not looked into. Its along subduction zones and along the deep sea trenches where the sea floor is spreading. Also there are a number of mountain ranges in the deep ocean which might have oil nearby. I'd call this ultra desperate oil.

Methane hydrates are found in abundance along the subduction zones for example and the formation is biological. This is why I think the contributions of bugs to making the oil is underestimated. In any case if we are ever crazy enough to start drilling into these areas it probably makes more sense to go after the methane then any oil thats forming. Call it biotic deep oil. There is a huge amount of reduced carbon locked away in the deep ocean silt.
Here is a drill record note carbon content is in the percentage range.

http://www-odp.tamu.edu/publications/prelim/188_prel/188s1166.html

So in a sense the earth probably still has large quantities
of oil and methane left considering the source conditions of
silt and heat are common but very little of it is probably in the nice large pools we like to drill. I think that almost all dry holes hit traces of oil pretty much anywhere you drill. So there are huge quantities of dispersed small oil pockets.

There is so much focus on commercial deposits that a lot of people don't think to much about the extent of far more common non commercial oil pockets. Like any natural resource small quantities can be found wherever conditions are halfway reasonable but large deposits are rare.

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joule on September 27, 2006 - 1:04pm Permalink | Subthread | Parent | Comments top
memmel -

Thank you. That was a very nice explanation.

Having spent most of my career in the environmental field, I am far more familiar with anaerobic biological processes than petroleum geology and chemistry.  Given the ubiquity and incredible versatility of microorganisms, I would be very surprised if some sort of very slow biological processes had NOT been at work in the formation of oil.

However, all biological processes involve any number of oxidation/reduction reactions. So if indeed some of the final petroleum constituents are in a more reduced state than the starting bio lipid material, then it would follow that something down there must have been functioning as a reducing agent. I wonder what that something might have been?  One possibility that comes to mind is sulfur and its many compounds, which always seem to be found, in greater or lesser amounts, with oil.

I'm also glad that you pointed out that at a certain depth it is quite common to find traces of petroleum, but that commercially feasible deposits are quite rare. The highly dispersed oil may amount to a total many billions of barrels but it ain't gonna do no one no good never.

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oilmanbob on September 27, 2006 - 4:04pm Permalink | Subthread | Parent | Comments top
Memmel, the uplift around salt domes is provided by 1. the salt diapurs rising through sediments and by the sinking of sediments into the subsurface. Although salt domes are very prolific, they are a structure providing the trapping mechanism,fault traps providing many others plus also anticlines (hills under the earth) caused by unequal compaction of sediments during subsidence, and stratigraphic traps during lithology or permiability changes.
   I'm a landman, not a geologist, but I had a couple of courses a quarter of a century ago. A geologist like West Texas or a geochemist could provide you with a much better explanation. But, the article you linked to said that Shell Oil was heating oil shale to 600 degrees C. in order to cook kerogens in to oil that could be produced. Thats not a natural process, the real old fashioned grease out of rocks was originally produced at lower temperatures. There is a thermal window below which oil is not produced and above which oil is cracked to natural gas and condensate. There is very little oil more shallow than 1,000 ft or deeper than 10,000 ft.
  But you are definitely right about marginal, sub-economic amounts of oil being present in many rocks, particularly shales. I'm also fairly certain many microbes help in this process.
  I'm glad guys like you think about things and are curious. A huge amount of basic science was done and is stil done by gifted amatuers. Einstein was a postal clerk!
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memmel on September 28, 2006 - 2:13am Permalink | Subthread | Parent | Comments top

My understanding is salt domes are caused by salt deposits left from dried up shallow seas.

I don't know how common they are in the deep ocean if they do exist then the other place to look for larg pockets of oil would be near volcanic or undersea mount regions. Even in the deap sea. This would be caused by mangma intrusions baking the deep silt of the ocean floor. The conditions to recover it would be insane.  Large carbonate deposits exist to act as a resevior I just no nothing about salt domes existing past the continental shelf.

http://www.nps.gov/archive/brca/geodetect/Rocks%20&%20Minerals/sed%20extention.htm

And this suggest there are salt domes and deposits off the continetal shelf.

http://geology.about.com/od/regional_geology/a/aa042698whales.htm

And this is a really cool link about the gulf geology.

http://www.oceanexplorer.noaa.gov/explorations/06mexico/background/geology/geology.html

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257 comments on Burning Buried Sunshine

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