Petrographic investigation into the development of secondary porosity in sandstones: A case study of the Cambrian Mount Simon and Galesville Sandstones, Illinois Basin

Petrographic studies of the Mount Simon and Galesville sandstones (Cambrian, Illinois basin) reveal the presence of secondary porosity that developed during mesodiagenesis. The Mount Simon Sandstone contains secondary porosity by hematite cement dissolution and minor framework grain (feldspar and quartz) dissolution. Early hematite cement preserved potential porosity during compaction. The Galesville Sandstone contains 1--8% secondary porosity by framework grain (mainly feldspar with minor rock fragment and quartz) dissolution. This porosity was preserved in a mechanically stable, compaction resistant framework of quartz grains; this phenomenon is named "The Buckyball Effect" in this study.; In the Mount Simon Sandstone, lamination is defined by the presence or lack of hematite cement. Hematite rich laminae have high porosity, contain rounded quartz grains rimmed by hematite, and hematite patches within pore space; whereas non-hematite cemented laminae have low porosity and consist of sutured grains with intergranular detrital (2M1) illite. The laminations resulted from periodic synsedimentary hematite cementation. Upon burial, non-hematite cemented laminae became sutured by pressure solution. Hematite cemented laminae preserved intergranular volume by resisting compaction. Subsequent dissolution of hematite cement by basinal fluids resulted in new porosity. Sutured zones present permeability barriers (0.1--2.7 md horizontal and 2.9--4.0 md vertical permeability) compared to non-sutured zones (4.3--12.7 md horizontal and 10.2--12.7 md vertical permeability). This study is the first report of this type of suturing and its effect on reservoir properties in the Mount Simon Sandstone.; X-Ray diffraction studies of Eau Claire Formation shales reveal that the < 2mum fraction is dominated by well crystallized diagenetic 1M illite, with illite crystallinity values of 0.43--0.54 Delta2theta and intensity ratios of 1.18--1.25. Particulate organic matter recovered is dark brown to black in color and corresponds to Type II kerogen. These parameters indicate advanced diagenesis and thermal maturity for the Eau Claire Formation. The Galesville Sandstone contains a high percentage of pores recognizable as being secondary in origin, chlorite cementation, and tangential quartz overgrowths (up to 13 vol. %), in close proximity to its contact with the underlying Eau Claire Formation. 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 its mechanical strength and resisted compaction, thus preserving secondary porosity. This is the first report of a stabilized framework for the Galesville Sandstone.; The results of this study underscore the significance of primary depositional characteristics, stratigraphy (proximity to shales) and early cementation before significant compaction as factors in the development and preservation of secondary porosity. This reiterates the importance of understanding syn- and post-depositional controls on the occurrence and distribution of cements in porosity prediction and reservoir characterization.