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PSThe “Buckyball Effect”: Previous HitFrameworkNext Hit Grain Stabilization and Secondary Porosity Preservation in the Cambrian Galesville Sandstone, Illinois Previous HitBasinNext Hit*

By

Mojisola A. KunleDare1

 

Search and Discovery Article #50043 (2007)

Posted June 12, 2007

 

*Adapted from poster presentation at AAPG Annual Convention, Long Beach, California, April 1-4, 2007.

 

1University of Missouri-Rolla; currently ConocoPhillips Nigeria ([email protected])

 

Abstract 

Petrographic studies of the Galesville Sandstone reveal the presence of secondary porosity by Previous HitframeworkNext Hit 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 Previous HitframeworkNext Hit grain dissolution, chlorite and quartz cementation in the Galesville Sandstone.  

The tangential quartz overgrowths form a Previous HitframeworkNext Hit around pores created by Previous HitframeworkNext Hit grain dissolution; this stabilized the Previous HitframeworkNext Hit, increased mechanical strength, and resisted compaction, thus preserving secondary porosity. This phenomenon of porosity preservation in a mechanically stable, compaction resistant Previous HitframeworkNext Hit 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.

 

Selected Figures 

Map showing the location of Shores #1 well, Section 22, T9N-R4W, Media field, Illinois (Henderson County location and township maps from http://www.outfitters.com/illinois/henderson).

Galesville Sandstone and Eau Claire Formation shale were sampled from cores from the Shores #1 well, section 22, T9N-R4W, Media field, Henderson County, IL. The well was drilled to a total depth of 2935 feet (895 meters) and cored between 2120 feet (646 meters) and 2759 feet (841 meters).

Galesville Sandstone at 2233 feet showing secondary porosity after Previous HitframeworkNext Hit grain dissolution. Diagnostic features: Oversize pores, embayed and corroded quartz grains (Q), dissolution remnants of feldspar (F) and grain-shaped pores (P).

Galesville Sandstone at 2232 feet showing patchy chlorite cement (CH), embayed quartz grains (Q), and quartz overgrowths (arrowed).

 

Conclusion 

  • The occurrence of patchy chlorite cement and tangential quartz overgrowths in the Galesville Sandstone close to its contact with the Eau Claire shale and their absence higher in the sequence point to the deposition of products of clay diagenesis within the sandstone.

  • The occurrence of secondary porosity in the Galesville Sandstone indicates the effect of shale-derived organic acids.

  • Tangential quartz overgrowths formed a rigid, stabilized Previous HitframeworkNext Hit that resisted compaction (“Buckyball Effect”), thus preserving secondary porosity in the Galesville Sandstone. 

  • The optimum volume of tangential quartz cementation of ≤ 10% suggests that the amount and disposition of quartz overgrowths is key to Previous HitframeworkTop stabilization.

 

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