Varieties of Chlorites and Illites and Porosity in Mississippian Sandstone Reservoirs in the Illinois Basin

Abstract

Shallow marine, Mississippian, siliclastics in the Illinois basin, although predominantly quartz, contain other minerals that directly influence the porosity and permeability of these reservoir rocks. These sandstones contain more chlorite and kaolinite, relative to illite, than the authors have observed for shales from other Chesterian and Valmeyeran strata. Clay mineral suites in reservoirs appear to be diagenetic. The Aux Vases Sandstone contains illite, illite/smectite, and chlorite; kaolinite is absent. The Cypress Sandstone contains illite, illite/smectite, chlorite, and kaolinite. Chlorite in the Aux Vases Sandstone varies from moderately Fe-rich to Mg-rich, whereas the chlorite in the Cypress Sandstone is uniformly Fe-rich. As the percentage of clay minerals in these rocks decreases, the proportion of chlorite to other clay minerals increases. In some chlorites, the width of the 003 and 005 peaks at half-height is greater than that of the 002 and 004 peaks. This suggests an interlayering of a 7{angstrom} mineral, probably berthierine- or serpentine-like. SEM photos show chlorite coating quartz grains. In some samples there are quartz overgrowths in spite of the presence of a coating of chlorite; in others, chlorite interlayered with the 7{angstrom} phase seems to have interfered with or suppressed overgrowths. Correspondingly, there is a correlation betweenmore » the 7{angstrom} phase/chlorite and porosity. Therefore, identification of the type of chlorite in a potential reservoir may be an indicator of porosity, as well as a guide for selecting completion and stimulation treatments.« less