Sandstone vs. carbonate petroleum reservoirs: A global perspective on porosity-depth and porosity-permeability relationships: Discussion*

The recent article by Ehrenberg and Nadeau (2005) presents a useful compilation of porosity, permeability, lithology, geothermal gradients, and preuplift depths for the Alberta basin in Canada and other petroleum reservoirs around the world. The authors state that “the relative paucity of low-porosity (0–8%) siliciclastic reservoirs at all depths compared with carbonates may reflect the most common occurrence of fractures in carbonates and the effectiveness of these fractures for facilitating economic flow rates in low-porosity rock” (Ehrenberg and Nadeau, 2005, p. 435). One purpose of this discussion is to present data for 5915 reservoirs that tend to support the Ehrenberg and Nadeau statement. They conclude that their results “can serve as a general exploration guide for the probability of encountering the desired porosity and permeability at a specified depth in a frontier area” (Ehrenberg and Nadeau, 2005, p. 443). A second purpose of this discussion is to show a possible extension of their data for calculating pore-throat apertures ( r p35) and estimating the probability of encountering the desired oil rate in a given area. Figure 1 shows a crossplot of average porosity versus weighted mean oil recovery for 5915 pools in Alberta (adapted from Petroleum Society of the Canadian Institute of Mining, Metallurgy and Petroleum Engineers (CIM), 2004, p. 282). The recoveries apply to conventional crude oil. A wide variation in porosities and recoveries exists. Included in this plot are carbonates and clastics. The dashed line labeled “Max.,” drawn following the …