Abstract

Bacterial degradation of reservoired crude oil may have been a fairly common phenomenon throughout geological history. The resulting asphalt, if located in a subsiding basin, will undergo diagenetic alteration as greater depths of burial and corresponding higher temperatures are reached. Knowledge about the outcome of such a process may have considerable bearing on exploration strategy.

In order to get the required information, asphalt samples from the Aquitaine Basin have been subjected to artificial diagenesis involving their exposure, in sealed glass tubes, to a temperature of 300°C for increasing lengths of time. Periodical analyses disclose:

1. An initial evolution featuring asphalts similar to shows that are actually encountered at increasing depths up to 3048 m (10,000 ft.);

2. an optimum evolution characterized by the formation of asphaltic oils chemically and physically resembling very immature (and frequently unproducible) oils;

3. an ultimate evolution resulting in abundant gas production.

In addition, after a first six weeks period, the whole process is accompanied by the gradual formation of insoluble, coal-like pyrobitumen which, eventually, accounts for over 50 per cent of the non-gaseous organic matter.

It was suspected that the state of maturity of the crude oil before bacterial degradation occurred might have an influence on the eventual diagenetic behavior. Accordingly, a similar series of experiments was carried out on samples from a Swiss asphalt which seems to have had a higher initial maturity. The major difference between this and the previous series turned out to be the sudden precipitation of the whole of the pyrobitumen after six weeks, the phenomenon being accompanied by abundant gasification. It is tentatively proposed that the described sequence of pooling of moderately mature oils, their bacterial degradation and renewed diagenetic alteration might explain the existence of certain sedimentary basins containing abundant asphalt and gas but very little producible oil.