Sandstone Porosity as Function of Thermal Maturity--an Approach to Porosity Comparisons and Prediction

James W. Schmoker, Donald L. Gautier

The porosity of sandstones typically decreases during burial, and porosity-reducing processes of burial diagenesis operate at all depths so long as porosity exists. Plots of porosity against depth describe sandstones at a point in time, but do not incorporate the idea that the section is in disequilibrium. In many cases, sandstone porosity can be more advantageously described as a function of thermal maturation (time-temperature exposure) than of depth. Specifically, empirical data indicate that general trends of sandstone porosity in the subsurface can be well represented by a power function of thermal maturity: ^phgr = A(M)^B, where ^phgr is porosity, M is a measure of time-temperature history such as vitrinite reflectance or Lopatin's index of thermal maturit , and A and B are constants for a given sandstone population but which vary between data sets.

Plots of porosity vs. thermal maturity take into account the overprint of time and temperature effects upon burial diagenesis and thus aid in direct comparisons of sandstones from diverse tectonic settings; differences in porosity trends on such plots are primarily related to petrologic properties. Logarithmic plots of porosity vs. a measure of thermal maturity also establish norms by which secondary porosity development and unusual porosity (and petrology) within a sandstone sequence can be recognized, and they offer a systematic rationale for the prediction of porosity ahead of drilling and at times in the geologic past.

AAPG Search and Discovery Article #91033©1988 AAPG Rocky Mountain Section, Bismarck, North Dakota, 21-24 August 1988