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Global warming and petroleum geology

October 27, 2007

Although I am primarily, and fundamentally, a sedimentary geologist, I do have some experience applying geologic principles for oil companies. I am, by no means, however, an experienced prospector or seasoned petroleum geologist. I’m saying this up front to try and head off any comments about my limited experience in the industry. This post is about a perspective within the context of my own scientific training, not a statement intended to represent a (mythical) monolithic view people crave. Also, I think it’s important to remember the distinction between petroleum geology and the petroleum industry.



Over the past several years I’ve seen comments from some petroleum geologists that have a tone similar to the following:

Geologists study the history of the Earth and realize climate has changed often in the past due to natural causes. The Earth’s climate naturally varies constantly, in both directions, at varying rates and on many scales. In recent decades global temperatures have risen. Yet, our planet has been far warmer and cooler than today many times in the geologic past, including the past 10,000 years.

Actually, that is an excerpt from the American Association of Petroleum Geologist’s (AAPG) 2007 statement on climate change. I’m sure many of you have either read a comment similar to that or heard someone say something like that at some point. AAPG’s statement is better than it used to be thanks to its members. Here’s what the AAPG President had to say about the issue in March 2007:

Members have threatened to not renew their memberships if … AAPG does not alter its position on global climate change. And I have been told of members who already have resigned in previous years because of our current global climate change position.

And, although the current statement is better, it still includes a tone that follows this logic:

Because past climate changed naturally, modern climate change is natural.

— AND/OR —

Because Earth survived past climate changes, there’s little reason to worry about modern climate change.

Either way, the rationalization is that nothing needs to be done. The AAPG statement itself does not explicitly say these things, but I hear them often from some in petroleum geology. These lines of reasoning are nonsensical and do not deserve yet another deconstruction on a blog. The “official” position of the powers-that-be at AAPG is also not the subject of this post. What I do want to discuss is a different perspective on climate change that is grounded firmly and very specifically in petroleum geology.



At this point I’d like to briefly review some petroleum geology basics. Like most natural systems, the origin, generation, migration, and accumulation of hydrocarbons is an extraordinarily complex affair. The transformation of organic matter (mostly tiny marine life, not dinosaurs) into some type of hydrocarbon requires elevated temperatures and pressures. If long-term net deposition of sediment buries sufficient volumes of organic matter to the “oil kitchen”, about 60-120° C, there is a chance the organic matter will transform into hydrocarbons (it is obviously more complicated than this, but I want to keep this brief…find out more). Liquid or gas hydrocarbons are mobile and will migrate upwards through permeable layers of the stratigraphic column (read a recent post about this at Olelog here). Most of the oil naturally formed on Earth has escaped from the subsurface over very long time periods. However, if a seal rock (i.e., impermeable) and a “trap” exist, large volumes of oil can accumulate in relatively porous sedimentary rocks. These large accumulations are what we have discovered and from which we extracted oil.

How long have subsurface accumulations of oil been around? Well, it really depends in which sedimentary basin you are looking. Different basins have different ages of petroleum source rocks and different ages of maturation/migration. Here’s a quote directly from a 1999 study by Klemme and Ulmishek, hereafter referred to as K&U [1]:

More than 90% of original recoverable oil and gas reserves in the world has been generated from source rocks of six stratigraphic intervals–(1) Silurian (generated 9% of the world’s reserves), (2) Upper Devonian-Tournaisian (8% of reserves), (3) Pennsylvanian-Lower Permian (8% of reserves), (4) Upper Jurassic (25% of reserves), (5) middle Cretaceous (29% of reserves), and (6) Oligocene-Miocene (12.5% of reserves)

But this is the age of the source rock, not the age of the oil. A organic-rich source rock can sit around for some time before actually generating hydrocarbons. The plot below, also from K&U, shows geologic time on the x-axis and petroleum reserves (as a percentage) on the y-axis (click on it for a slightly less fuzzy version).


Note the lower dashed line…this is the age of maturation of the world’s oil. Here’s what K&U say about this (italics are mine):

The cumulative maturation line demonstrates the generally young age of most of the world’s discovered oil and gas. Almost 70% of the world’s oil and gas reserves was generated since the Coniacian (~87 million yrs ago), and nearly 50% of the reserves was generated and trapped since the Oligocene (~30 million years ago).

In a rigorous sense, it’s invalid to try and determine a single number for the time it has taken for the Earth’s hydrocarbons to mature. For the sake of this post, however, I’m going to do it anyway. Let’s be very conservative and say that the oil extracted thus far took, on average, about 20 million years to mature and accumulate.


The Earth’s carbon cycle has various sources and sinks that interact over various time scales that collectively circulate carbon among the spheres (litho, bio, hydro, atmo, etc.). Storage and transfer of CO2 among the components drives the system towards an equilibrium. As we know, however, a state of true equilibrium is never attained. The system is dynamic, it is in a state of constant adjustment, always responding to various perturbations, both external and internal (e.g., feedbacks) to the system.

In only 150 years we’ve released CO2 that took 20 million years to accumulate. Put another way, the duration of release is a mere 0.000008% of the duration of accumulation. The evidence for “excess” CO2 input to the system is the measured increase in CO2 concentration in the atmosphere (i.e., not all of it is being cycled back into sinks). In other words, from the geological perspective, the current rate of CO2 concentration in the atmosphere is extremely rapid. Does this imbalance in cycling of Earth materials significantly perturb the system? My petroleum geological perspective leads me to answer “yes indeed”.

One might argue that the Earth has experienced rapid increases of greenhouse gases in the atmosphere in the geologic past and survived just fine. The confusion of a planet’s survival with that of effects on habitability notwithstanding let’s consider the best known example a rapid global warming from the geologic past – the Paleocene-Eocene Thermal Maximum (PETM). Although this occurred 55 million years ago, records indicate the global warming occurred in less than 10,000 years [2, 3, 4]. The temporal resolution of archives going back that far is inherently lower thus making it very possible the event occurred over an even shorter time period. In other words, the system was significantly perturbed. Included among the effects of this perturbation was an increase in global temperatures by ~5° C [5], oceanic acidification [6], and a mass extinction of benthic foraminifera [7]. Although the exact cause(s) of the CO2 release are debated (e.g., 8 and 9), the point for this post is that something happened and it had significant effects on the biosphere. Furthermore, it has been postulated that it took 100,000 years for the carbon cycling system to “recover” from the abrupt increase in CO2 (i.e., for it to be absorbed in the ocean) [10].



The whole point of this post is to present a perspective of global climate change that is rooted in petroleum geology that people may not hear about. Yes, of course the science of understanding petroleum systems is intimately intertwined with the business of production (oil companies) and consumption (you), but I would reiterate that they are also distinct. Petroleum geology is just that — it is geology. While some may cite the long-view afforded by a geological perspective as a reason to not be concerned about the effects of climate change, I think that it is quite the opposite. Perturbations to Earth’s carbon cycling system, and the significant impacts it can have on short-term habitability for components of the biosphere, is better appreciated with a geological perspective.


References cited above:
[1]: (note: this page condensed from their article published in the AAPG Bulletin, v. 75, 1991, p. 1809-1851; reprinted and adapted on the Search and Discovery website)
[2]: Zachos, J. C. et al., Science 302, 1151–1154 (2003)
[3]: Kennett, J. P. & Stott, L. D. , Nature 353, 225–229 (1991)
[4]: Tripati, A. & Elderfield, H., Science 308, 1894–1898 (2005)
[5]: A. Sluijs et al., Nature 441, 610 (2006)
[6]: J. C. Zachos et al., Science 308, (2005)
[7]: S. L. Wing et al., Eds., Causes and Consequences of Globally Warm Climates in the Early Paleogene, Geological Society of America special paper 369 (2003)
[8]: Dickens, G. R. , O’Neil, J. R. , Rea, D. K. & Owen, R. M. Dissociation of oceanic methane hydrate as a cause of the carbon isotope excursion at the end of the Paleocene. Paleoceanography 10, 965–971 (1995)
[9]: M. Pagani et al., Science 314, (2005)
Hydrocarbon molecule image from here


13 Comments leave one →
  1. Karen permalink
    October 28, 2007 7:44 am

    Nice summary. If you’re interested in a wider audience, the San Jose State Geology Club would be delighted to have a presentation (Mondays, 12:30pm, ~45 minutes max; parking and restaurant lunch provided to the speaker).

    Now, get back to working on that thesis!

  2. October 28, 2007 9:15 am

    Well said!

  3. October 28, 2007 9:23 am

    Karen…thanks…if you mean giving a talk about the subject of this post, I’m not sure my opinion on this issue would be appropriate for a formal talk. Plus, to fill 45 min I would have to expand it significantly (which would be fun, but I simply don’t have the time right now).

    But, if your group would be interested in a talk about the specific projects I have going on (get an idea here), I would entertain that. Although, I might have to wait until after the defense :)

    You have my email…feel free to contact me.

  4. October 29, 2007 11:18 pm

    Of course, those of us who are industrial hard rock geologists look at petroleum as a mere precursor for the formation of MVT and Zambian style base metal mineralization.

    The great thing about that perspective is that we get to see how much of the world’s past oil reserves have leaked out and aren’t around any more.

    Here’s a question, though. How to predicted rates of GHG-induced sea level rise compare with the eustatic component of transgression seen in various petroleum-bearing strata?

  5. October 30, 2007 12:23 am

    “How do predicted rates of GHG-induced sea level rise compare with the eustatic component of transgression seen in various petroleum-bearing strata?”

    Getting at such short-term rates for ancient strata is pretty much a pipe dream. We just don’t have the temporal resolution to evaluate transgression/regression rates at the millennial (much less centennial) scale back too far.

    But, if we want to look at the last glacial-to-interglacial transition (end of the Pleistocene) we can get some idea… let’s see…during the last deglaciation the fastest rate of sea-level rise was about 75 m over 6,000 years, which would equate to about 1.25 m per 100 years. I’d have to look at the various projections for sea-level rise, I forget, but I think some of them are getting close to that.

    My figures above were done very quick and dirty…I very easily could have made an error. I’ll try and do it more rigorous tomorrow.

  6. October 30, 2007 8:44 am

    lab lemming…did my previous comment address what you were asking exactly?

    also…since i’m not using Blogger anymore, I can’t comment on your blog…it only allows Google accounts to comment…just so you know

  7. November 12, 2007 12:53 pm

    Brian, this post prepared me to give an impromptu rant today in my intro earth systems science class. (I was just starting to talk about the atmosphere, and what makes it up. I had the students list gases that they knew were in the atmosphere, and of course they listed CO2 along with nitrogen and oxygen. And then I talked about how much of the atmosphere was nitrogen and how much was oxygen, and how little was CO2. And then I asked why we cared about such a small component… and they said nothing. I was horrified. So I spontaneously explained the greenhouse effect, out of my normal sequence. But I also talked about how quickly we were changing CO2, and part of my understanding of the scale had to do with this post of yours.)

    So thank you!

  8. November 12, 2007 11:21 pm

    Kim…thanks…I’m glad you liked the post. As I say above, this is more about my perspective than it is a hard-core analysis. Nevertheless, it’s a perspective I rarely see or hear outside of my own geoscience circles. Those who like to talk about how much the Earth has warmed and cooled naturally over geologic time, and somehow make the leap that because of that then we have nothing to worry about, need to hear this perspective. Can the Earth handle this much CO2? Possibly…I don’t know.

  9. joyce permalink
    December 12, 2009 12:58 am

    Thank you Thank you Thank you

    With all due respect, it does not take a geologist to figure that out. What we have done to the earth in 150 years is dispicable. Now here we are and there is all the biggest money in the world at that summit, what do you think they are really trying to accomplish. Remember these are very swred affluent bottom line type of people. Do you really think they believe after destroying the earth with could fix it by taxing people, no like I said they know what they are doing and they are just making themselfs look really foolish if they think we believe them. What really is warming globally is how they managed to put this together, it has taken years, look at all the changes that our financial structructures have gone through and all the deregulation of the past 25 years and the new biology that they introduced about that time too. I could go on and on. I am just encouraging everyone to look for themselfs it is out there on the net and what you will find will make you ill. Be prepared.
    Peace and Blessings

  10. September 5, 2010 10:24 am

    After listening to these arguments for most of my 72 yrears, I am still unconvinced that we understand petroleum generation and migration. There are just too many things that cannot be explained. For instance, small fault blocks containing oil swept from the samp down dip source often have radically different compositions. Geologic history is full of abandoned hypotheses. As for global warming, the causes of historical changes must still operate so climate change is a given. Why are we wasting intellectual and financial resources on CO2 when we should be building dams and levees and moving populations. Academia is good a science but poor at solutions.

  11. November 10, 2010 10:29 pm

    1. Most of the worlds co2 is sequestered in limestone not oil.
    2. It is unlikely that we have taken even a fraction of 1% of the worlds oil reserves not to mention coal.
    3. Co2 absorbs IR in a very narrow band which is nearly saturated.
    4. There appears to be nothing unusual about the current warming trend.

    Click to access globlwrmw99.pdf

  12. January 23, 2013 4:33 am

    “Global warming and petroleum geology Clastic Detritus”
    ended up being a terrific read and also I actually was quite glad to come
    across it. I appreciate it-Nan


  1. Sea-Floor Sunday #1: Golden Gate sandwaves « Clastic Detritus

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