WAPLES, DOUGLAS W., and HIROMI KAMATA, Japan National Oil Company, Chiba, Japan
Functions predicting porosity reduction normally are dependent only on burial depth for a given rock type. However, this purely empirical approach lumps together the effects of several different porosity-reducing processes, some of which may depend on additional factors. Much of the porosity reduction in older rocks in particular is due to time- and temperature-dependent diagenetic reactions.
We have developed an empirical equation to take into account the porosity-reducing effects of seven different processes in limestone, shale, and sandstone: repacking of grains, crushing of microfossils, ductile grain flow, pressure solution, welding following pressure solution, clay diagenesis, and cementation by minerals precipitated from moving fluids. Porosity reduction in limestone uses five of these processes, shales use three, and sandstones use six.
We have determined preliminary default values for the various constants in these equations for each lithology. The user need only specify the lithologic composition of the rock (including fraction of lithic + clay components for sandstones) and the relative availability of calcium carbonate and silica in pore fluids for cementation.
Our porosity-reduction equation can easily be incorporated into existing numerical basin models. This method should provide more accurate estimates of true porosities (and hence permeabilities) in a wide variety of basins than the current depth-dependent functions, which require local calibration.
Our model only attempts to account for the regular progression of porosity reduction as an aid in basin modeling and is not designed to predict anomalous events or to account for natural data scatter within a single lithologic unit. Since reservoir characteristics are often strongly influenced by anomalous or local events, we do not recommend the use of our model to predict production characteristics of specific reservoirs.
AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)