ABSTRACT: Porosity, Permeability, and Capillary Pressure Trends in the St. Peter and Mt. Simon Sandstones, Illinois Basin.

BYRNES, ALAN P., GeoCore, Loveland, CO, and MICHAEL D. WILSON, Consultant, Golden, CO

Multiple regression analysis was used to develop predictive equations for porosity, permeability, and capillary pressure for St. Peter and Mt. Simon sandstones, Illinois basin. Equations were developed by regressing the measured petrophysical data against pressure and time-temperature variables (maximum burial depth and time-temperature index, or TTI), and over 30 compositional variables. Maximum burial depth and TTIs were obtained from burial/thermal history analysis of wells across the basin.Compositional data were obtained from quantitative petrographic (point count) analysis. A porosity prediction equation providing maximum accuracy using the minimum number of easily measured variables is

Porosityin situ = 0.00097 X MBD + 8.9 X Clay coat - 0.14 X Cc + 10.8

where porosity is in porosity percent, clay coat is a measure of the

completeness of coating of framework grains by clays (zero to 1), MBD is the maximum burial depth (feet), and Cc is intergranular carbonate cement content (percent total rock). The standard error of prediction of this equation is (plus/minus) 1.7 porosity percent and R2 = 0.83. The significant role that clay coats play in preventing or delaying quartz overgrowth development is evident. A term for thermal maturity was not incorporated because of the high colinearity between TTI and MBD. This equation represents a subset of a more generalized quartzose sandstone equation developed for sandstones in seven basins:

Porosityin situ = 3.1 X log10TTI - 0.00037 X MBD - 17.7 X log10Sort + 10.8 X Clay coat - 5.3 X log10Age + 21.9

where sort = standard deviation of the size (phi units), age (M.Y.), and Cc < 10%. Predictive equations developed for permeability and capillary pressure included terms for porosity, matrix content and size. These equations may change slightly as additional samples are included in the database.

AAPG Search and Discovery Article #91013©1992 AAPG Eastern Section Meeting, Champaign, Illinois, September 20-22, 1992 (2009)