Analogue modeling: keep it simple, stupid

Geology is difficult to study. In my chosen field of basin analysis, it's very rare that you have complete information on which to base an interpretation. You can only directly observe the final configuration of the basin, and have to use inference and educated guesswork to understand how the basin evolved through time. You usually have incomplete exposure, and even where exposure is 100% it rarely gives good 3D coverage. If you're using subsurface data, you can only observe objects that are large enough to be resolved in the data (for seismic data, faults that displace strata less than 20 m are typically not resolved).

So, to look at how a whole basin evolves through time, geologists build analogue models. At the simple level, you put some sand in a box, extend the box, and look what happens to the sand. This sounds simple, but actually scaling these models correctly is quite complicated. Still, geologists have produced sandbox models that look remarkably like rift basins using these kind of methods.

A model is essentially a simplification of the system you're trying to study, in order to make it tractable. So you start off with simple models, and gradually add more and more complexity as you try to understand the complexity of the real system. In basin research, analogue modeling hit the buffers a few years ago as geologists tried to incorporate reactivated faults into their models. Generally, once a fault forms it remains a relatively weak point in the crust for a very long time. This tends to mean that when you apply tectonic forces to the crust, old faults will become active again. In analogue models, this was usually addressed through cuts in a plate at the base of the model. But, as was pointed out by C.K. Morley [1], it is the sand itself that should contain the pre-existing faults, if the model is to accurately mimic the natural system. This is hard to do with sand, because it has no tensile strength. So what next?

The answer is provided by Bellahsen and Daniel [2], and it's remarkably simple. They created 'faults' in the sand by introducing a piece of card into the sand layer, then removing it. This created a zone of disturbed sand that had a lower coefficient of internal friction than the undisturbed sand. The authors went on to create models that look remarkably like real basins that contain reactivated faults. Sometimes apparently complex problems have simple solutions.

[1] Morley, C.K. 1999. How successful are analogue models in addressing the influence of pre-existing fabrics on rift structure? Journal of Structural Geology, 21, 1267-1274.

[2] Bellahsen, N. and Daniel, J.M. 2005. Fault reactivation control on normal fault growth: an experimental study. Journal of Structural Geology, 27, 769-780

Plateau oil?

How much oil is left? Are we looking at a looming supply crunch? This is one of the biggest questions in geopolitics now, and it's partly a question of petroleum geology. The problem is that arriving at an estimate of how much oil might be left to produce is extremely difficult, because there are so many factors to take into account, and most of those factors are subject to large uncertainties. Improvements in technology mean more oil is recoverable. Changes in the price of oil affect which fields it is economical to produce from. The fact that supply is partly controlled by the OPEC cartel, itself subject to political and economical considerations, is an important factor. Various countries have been less than truthful about what reserves they have. Advances in producing from unconventional sources (e.g. shale oil, gas hydrates) will increase potential reserves, but not without environmental cost. Replacements such as biofuels may be able to take up some of the demand (again, subject to political factors).

A recent Hedberg conference of the American Association of Petroleum Geologists (AAPG) tried to address this issue. The AAPG reported the results at their conference in sunny Long Beach, CA, where I happened to be lugging an enormous poster around. You can find a summary of the results here. Three types of additions to future reserves were addressed: improvements in recovery from existing fields, new discoveries, and new unconventional resources. The conference didn't look at gas-to-oil or coal-to-oil conversions, or oil substitutes such as biofuels.

It makes for uneasy reading. While the conference concluded that peak oil was not 'imminent', it was not far away either. Oil production was predicted to reach a plateau sometime between 2020 and 2040, lasting for 25 to 30 years. Production will then inexorably fall. Because consumption is now so high, even this would require a mammoth effort from the oil industry. And a supply crunch is likely even earlier, as demand continues to increase and the rate of increasing production falls off before the plateau.

This is good news for petroleum geologists, who are not going to be out of work (although we are probably going to have work in more politically unstable and inaccessible areas), but it isn't particularly good news for anyone else. I don't see any evidence for western governments having a plan B that is going to be of much help before 2020.

I think this is interesting, because the 'peak oil' hypothesis is often dismissed by oil industry insiders (but generally not by petroleum geologists). Here's a large group of experts coming to the conclusion that peak oil (or at least plateau oil) is real.

An experiment

As a long-standing fan of Ben Goldacre's Bad Science website, I was interested to read Dr. Goldacre's recent pieces of advice to old media. I reproduce point 5 below:

"Employ more editors, and fewer journalists. The lesson of this site is that journalists are not good at mediating the knowledge and understanding of experts. Instead, give us unmediated expertise. Do not write about the expert: get the expert to write for you, and get an editor to make it read better, if you need to. Editors are the unsung heroes of print media, not journalists. Understand this: unless you’re a voice, a Charlie Brooker, we don’t actually care if you think you can write “like a professional”. Online we can go straight to people who actually know about stuff, and they can usually write just fine. Give them to us, or we will enjoy them without you."

'I can do that', I thought. After all, I know something about the structure of geological basins, and something about petroleum geology. It will have the added benefit of making me keep up with current research. So this is my attempt at giving the world my 'unmediated expertise', for what it's worth. Watch this space.