Tuesday, 26 August 2008

Abiogenic oil

This is the first post of what I hope will become a renaissance for this blog. I've tended to neglect it, partly because I've been working hard, and partly because when I do get time to do some blogging, it's much more fun to go moron-baiting over at hawk/handsaw. The intention is to publish something here once a week in future.

This post is about abiogenic oil, a collection of hypotheses by which oil could be created in the mantle, rather than in sedimentary basins at relatively shallow depth. A recent(ish) Hedberg Conference of the American Association of Petroleum Geologists (AAPG) addressed this issue, and the results were written up in the AAPG Bulletin earlier this year.

First of all, why is this important? For two reasons. If significant quantities of oil are produced through inorganic chemical processes in the mantle, it is possible that there is far, far more oil available for production than is conventionally thought. The much-dreaded peak oil could be put off for a considerable period of time. Secondly, current exploration is based on the biogenic origin of hydrocarbons; that hydrocarbons are created during the burial of organic material. This hypothesis makes predictions about where hydrocarbons should be found. If a significant amount of oil really is abiogenic in origin, then we need to urgently revisit how we explore for oil and gas resources.

How could abiogenic processes produce hydrocarbons? There are two possibilities: firstly, degassing of the mantle, followed by polymerisation the low-molecular-weight compounds released. Secondly, the serpentinisation of ultramafic rocks combined with a Fischer-Tropsch reaction, in which carbon monoxide and hydrogen are combined, using a catalyst, to form hydrocarbons. What both these hypotheses have in common is that hydrocarbons are produced deep in the crust or in the mantle, and must migrate considerable vertical distances to reach the relatively shallow reservoirs in sedimentary basins where hydrocarbons are normally found.

Unfortunately, the evidence for large accumulations of abiogenic hydrocarbons seems to be somewhat thin. No-one disputes that non-commercial deposits of abiogenic hydrocarbons do exist, but commercial quantities seem to be elusive. The problem is really one of Occam's Razor; where there are suggestions that hydrocarbon accumulations might be abiogenic, they can also be explained within the biogenic paradigm. For example, hydrocarbons have been found in fractured basement rocks such as granite. At first glance, this seems to be contrary to the biogenic model. On closer analysis, though, these occurences can be explained coventionally. The hydrocarbon source rock is subjected to heat and pressure, generating hydrocarbons, which then migrate via fractures. If the horizon they are generated in is overpressured, they can be forced downward. Alternatively, uplifted areas of granitic basement might have source rocks adjacent to them across faults, for example.

Another problem for the abiogenic oil hypothesis is the presence of age-restricted biomarkers. These are organic compounds that are only produced from certain types of organism. For example, a compound called oleanane is derived only from angiosperms (flowering plants). It is therefore restricted to hydrocarbons derived from Late Cretaceous or younger source rocks. Not only that, but the radiation of angiosperms through time should mean that the proportion of oleanane increases as the source rock becomes younger. This is exactly what is observed in real hydrocarbon provinces. This is consistent with the biogenic origin of oil, and not with the abiogenic origin of oil.

A third problem is the existence of oil shales. These can be explained by the biogenic paradigm: they are the strata that contain the organic material that gets buried, matures into hydrocarbons, and migrates into reservoir units of high porosity and permeability. But the shales themselves are of very low porosity and permeability. So how could fluids migrate from deep in the crust or mantle and accumulate in them?

Does all this mean that there is no such thing as commercial abiogenic oil? Not necessarily. If you wanted to find abiogenic oil in exploitable quantities, you would have to look in areas where crustal-scale fault or fracture systems could allow fluids generated deep in the Earth to migrate towards the surface, perhaps in areas of basement where there are no sedimentary basins. No-one has been looking in those places, because the biogenic model works perfectly well. It would be hugely risky to go looking for hydrocarbons using the abiogenic theory as your guide, but it could ultimately be worth it. Especially if oil prices rise much further...