Print this pageSandstone 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