Clastic Detritus

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.

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CLIMATE CHANGE COMMENTS FROM PETROLEUM GEOLOGISTS

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.

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GENERATION AND ACCUMULATION OF OIL IN THE EARTH

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.

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PERTURBING THE SYSTEM
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].

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A GEOLOGICAL PERSPECTIVE ON CLIMATE CHANGE

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.

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References cited above:

[1]: http://www.searchanddiscovery.net/documents/Animator/klemme2.htm (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)

[10]: http://www.physorg.com/news4491.html

Hydrocarbon molecule image from here

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