Carbonate Reservoirs and Increasing Depth — It’s Not All Bad News!

Wignall, Brent ¹
(1)Shell International Exploration and Production, Royal Dutch Shell, Den Haag, Netherlands.

It is generally recognized that porosity in carbonate reservoirs can suffer ongoing degradation with increasing burial depth, however, carbonate burial diagenesis is rarely well behaved, much less well understood. Recent work with a core dataset from western Canada provides an example of a system where the global “rules” of reservoir destruction may be broken.

A dataset of core plug measurements from Paleozoic carbonates in the Western Canadian Sedimentary Basin allow the following observations: Firstly, lithology is a primary control on the rate of porosity loss, and datasets are best sorted for limestone, dolostone, and mixed lithologies. Lithology trends of porosity loss to a maximum burial depth in excess of 7km are very clear. Limestones rapidly decay to 2-3 p.u. over the first 3km of burial, and this seems to be consistent for most stratigraphic levels. Dolostones, on the other hand, preserve porosity to surprising depths, with reservoir quality rock surviving 7km of maximum burial. Secondly, the trends that emerge for one stratigraphic interval do not hold true for other intervals. This begs the question of whether “global” burial trends have relevance for explorers trying to anticipate porosity of prospects as they drill deeper and deeper in their basins. Thirdly, the dataset contains a wealth of permeability data, and the “quality” of the porosity can be assessed along regional trends. Surprisingly, despite loss of porosity the Paleozoic rocks have preserved their reservoir quality fairly well. Limestones can still maintain permeabilities in excess of 0.1mD at nearly terminal porosity loss, whereas the dolostones in some stratigraphic intervals maintain geometric permeabilities in excess of 1.0mD with no perceptible shift over 4km of progressive burial.

The implication is that for the WCSB, at least, the simplistic approach of taking initial porosity and assuming it will only decay with progressive burial and time is wrong. The most important rule which emerges for the exploration geologist is “Know thy basin!”. Attempting to apply inappropriate global rules to an exploration play can result in unduly pessimistic assessments, resulting in discarding a deepening trend that still has legs.

AAPG Search and Discovery Article #90135©2011 AAPG International Conference and Exhibition, Milan, Italy, 23-26 October 2011.