Abstract: Porosity and Reservoir Quality Prediction in the St. Peter Sandstone, Illinois Basin

Alan P. Byrnes, Michael D. Wilson

The St. Peter Sandstone has been used as a test case to examine empirical porosity prediction models and the quantitative role of clay coats on porosity preservation in older quartzose sands. This analysis expands on previously reported work which was limited to depths >6,000 ft by insufficient population variance. Core porosities (0-35%), quantitative petrographic data (30 variables), thermal maturity (TTI, -2.5 to 2.1), and maximum burial depth (MBD, 0-10,200 ft) data were obtained for 150 samples from 17 wells distributed across the basin. Multivariate regression analysis, balanced for colinearity between TTI and MBD, provided a basin-wide porosity prediction equation:

[EQUATION]

where ^phgr = porosity (%), Coat = degree of grain coating by clays (zero to 1), Cc = carbonate cement (% whole rock), and Mtx = matrix (% whole rock). The equation illustrates that clay coats, where relatively complete, can preserve up to 10% porosity be prevention of quartz overgrowth development. This equation represents a subset of a previously reported generalized quartzose sandstone equation developed for several U.S. basins. Some terms, such as overpressuring and age, are not incorporated because they are absent or constant for the formation. a somewhat high standard error of prediction (+/-2.5; 1 std. dev.) is primarily due to insufficient variance in sorting, which could not be appropriately corrected be weighting, and to imprecision in the Coat term.

Predictive equations an measurements for other petrophysical variables were also determined:

[EQUATION]

where K_m,i = insitu Klinkenberg permeability (md, m = measured), ^phgr_m = porosity, measured (%); Clay = Authigenic clay + Mtx (% whole rock); Size = Grain size (^phgr); and S_wi = irreducible water saturation (%); m^* = Archie cementation exponent.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994