Porosity in shale, siltstone, and other unconventional reservoirs is a major factor in estimating hydrocarbon resources in place. Accurately quantifying the pore space is important because the vast sizes of continuous unconventional plays mean that even a relatively small bias will greatly affect resource estimation.
A common way to calculate porosity is to use density logs. This process uses specified grain and fluid densities. The grain density is often set at certain assumed values based on the broad lithology of the reservoir, such as sandstone or limestone.
The Petroleum Systems and Earth Resources Team of the Alberta Geological Survey has found that when evaluating shale and siltstone reservoirs, the grain density must be calibrated for each unit of interest. Our preferred method for determining grain density is XRD mineralogy.