Regional Variations of Porosity and Cement: St. Peter and Mount Simon Sandstones in Illinois Basin

Abstract

Petrographic study of over 230 thin sections of cuttings, cores, and outcrop samples from the Ordovician St. Peter and Cambrian Mount Simon Sandstones of the craton-center Illinois basin yielded a series of maps that show a fairly regular distribution of primary and secondary porosity and cements. Both primary and secondary porosity are present in the St. Peter Sandstone. Primary porosity is dominant from outcrop to 4,000 ft (1,219 m), whereas secondary porosity is dominant at depths greater than 4,000 ft. The overall porosity decline with burial is best described by an exponential equation: ^phgr = 30.8 exp (-0.00032 d), where ^phgr equals porosity and d is depth in feet. However, two linear equations provide the best fit of primary (above 4,000 ft, 1,219 m) and secondary porosity (below 4,000 ft). The chronology of porosity development appears to be closely related to the models proposed by P. J. C. Nagtegaal and includes early cementation, leaching of cement and framework, stabilized framework, and framework collapse. Cements in the St. Peter are depth-dependent. They define distinct regions, are mappable, and include calcite, dolomite, silica overgrowths, chert, and chalcedony. Virtually everywhere in the Mount Simon Sandstone, porosity is secondary and results from dissolution of authigenic and replacement cements, framework grains, and fractures. In the subsurface, porosity reaches a maximum of 18% at 5,000 ft (1,524 m) and drops rather sharply to 8% below 5,000 ft. As with the St. Peter Sandstone, the overall decline is best described by an exponential equation of the form: ^phgr = 31.08 exp (-0.00026 d). Cements in the Mount Simon include quartz and feldspar overgrowths, hematite and kaolinite, carbonate, chlorite, and microquartz (chert). Basinwide maps show that they have a fairly regular distribution pattern in the basin. We suggest that in the future, basinwide maps of cement and porosity types will be abundant. Such maps should enhance understanding of diagenesis, especially where map pattern can be related to fluid and burial history. Maps of cements and porosity types can also aid the calibration of wireline logs, help find diagenetic traps, and help the design of drilling fluid systems and well-completion procedures. The routine use of cuttings rather than cores will hasten the advent of such maps.