Sunday, March 2, 2008

Trace Fossils and Hydrocarbons

The utility of trace fossils in exploration geology is fairly obvious, such as constraining depositional environments or identifying chronostratigraphically important surfaces. Their impact on the production side of things, however, tends to be viewed as a purely negative effect; the critters are churning up all the lovely sandbeds, bringing mud into the system, and generally screwing-up all the nicely sorted sands. A relatively recent paper (Pemberton and Gingras 2005) shows how this assumption may be incorrect.

The authors point out that the view of bioturbation as a simple sediment-churning activity is a gross oversimplification. The nature of the activity defines what the trace morphology and characteristics will be. For instance, some critters may excavate extensive dwelling burrows, penetrating meters into the sediment; depending on the density and interconnectedness of the animals and their burrows, some of these dwelling structures may be quite extensive.

As an example, the authors point to the Jurassic Arab-D, Ghawar Field, Saudi Arabia, which is the largest oil field on earth, with estimated reserves of 75 and 83 billion bbl. The reservoir interval had been interpreted previously as being dominated by fracture porosity. In this paper, however, Pemberton and Gingras (2005) reinterpret the unit as an expression of the Glossifungites ichnofacies, developed on a regional ravinement surface. The traces were made in a micritic substrate, and have a sucrosic dolomite infill (interpreted to be detrital in origin). The differences in porosity and permeability between these two contrasting lithologies is striking, resulting in what the authors term a “biogenic plumbing system”.

This paper has some very nice figures in it; I suggest you print it in color, if it’s available, just to take advantage of some of the nice core photos. It also has some artwork quality images, painted by T.D.A. Saunders. The figure above is Fig. 6 of Pemberton and Gingras (2005), found on page 1500, and shows the conceptual model for the development of the Ghawar field plumbing.

The paper goes on to identify five separate kinds of biogenically-enhanced permeability, and presents several case studies in producing reservoirs where these different processes seem to play an important role. These processes range across a variety of depositional environments and facies, and include both clastic and carbonate systems.

The impact that these burrow systems have on the reservoir potential of some of these units is staggering, as is the potential for vastly under- or over-estimating the potential reserves by not taking these networks into account. All in all, it’s a pretty nifty paper, and one I would strongly recommend for the petroleum-focused among us.

Pemberton, S.G., and Gingras, M.K., 2005, Classification and characterizations of biogenically
enhanced permeability: AAPG Bulletin, v. 89, n. 11, p. 1493-1517.


John Burgess said...

Very nice piece!

Jeannette said...

A brief analysis of your 'Arbitrary Nomenclatural Scheme' shows the following: 4 posts about trace fossils, 3 posts about the geoscience community, and 2 posts each about policies, fluvial facies, and geomorphology.

Perhaps you should consider running for an elected position in the geoscience community on a platform of 'equal time for ichnofacies'...?

Great trace fossil posts. I'm amazed at the amount of information that can be gleaned from the traces these little critters leave behind.

Eric said...

It would never work; first of all, my trace fossils stuff is REALLY about sedimentology/stratigraphy. I'd be like McCain trying to placate all the rabid Reaganites or fire-and-brimstone bible-folk.

Most trace people can't muster up the energy to get concerned about flow hydrodynamics, and would rather talk about Trilobite leg morphology or something arcane like that (ha!).

Of course, IF I did have a position, it would have to be "anything but deep-sea fans". I think lots of people could get behind a movement like that.