The “Buckyball Effect”: Framework Grain Stabilization and Secondary Porosity Preservation in the Cambrian Galesville Sandstone, Illinois Basin

KunleDare, Mojisola¹ (1) University of Missouri-Rolla, presently ConocoPhillips Nigeria, Houston, TX

Petrographic studies of the Galesville sandstone reveal the presence of secondary porosity by framework grain dissolution that developed during mesodiagenesis. A high percentage of the pores recognizable as being secondary in origin, chlorite cementation, and tangential quartz overgrowths occur in close proximity to its contact with the underlying Eau Claire Formation.

X-Ray diffraction studies of Eau Claire Formation shales reveal that the < 2µm fraction is dominated by well crystallized diagenetic 1M illite, with illite crystallinity values of 0.43-0.54 Δ2Θ and intensity ratios of 1.18-1.25. Particulate organic matter (Type II kerogen) recovered is dark brown to black in color. These parameters indicate advanced diagenesis and thermal maturity. Thus fluids derived from clay and organic matter diagenesis in the shales were probably available to the Galesville Sandstone. The action of these organic acid and aqueous silica-rich fluids probably resulted in framework grain dissolution, chlorite and quartz cementation in the Galesville Sandstone.

The tangential quartz overgrowths form a framework around pores created by framework grain dissolution; this stabilized the framework, increased mechanical strength and resisted compaction, thus preserving secondary porosity. This phenomenon of porosity preservation in a mechanically stable, compaction resistant framework of quartz grains is named ″The Buckyball Effect″ in this study. The results of this study underscore the importance of primary depositional characteristics, stratigraphy (proximity to shales) and early cementation as factors in the development and preservation of secondary porosity; and the importance of understanding controls on the occurrence and distribution of cements in porosity prediction and reservoir characterization.