Kunle Dare, Mojisola A.¹ 
(1) University of Missouri-Rolla, Rolla, MO

ABSTRACT: Relationship Between Reservoir Properties of the Galesville Sandstone and Diagenesis of Eau Claire Shale, Cambrian of Illinois Basin

Reservoir properties of sandstones are influenced by clay and organic matter diagenesis in adjacent shales. Fluids rich in organic acids and complexers expelled into overlying sandstones dissolve cement and framework grains, organic complexers prevent re-precipitation, often enhancing reservoir properties. Other by-products of clay reactions may precipitate quartz and chlorite, which would occlude porosity and diminish reservoir properties. However, in sequences with thin shales and slow sedimentation, porosity enhancement often occurs. 
Petrographic studies of the Cambrian Galesville Sandstone of the Illinois basin reveal abundant secondary porosity from dissolution of feldspars, as well as patchy chlorite cement in the six feet adjacent to the underlying Eau Claire shale. 
X-Ray diffraction studies of the Eau Claire Shale show a clay mineralogy dominated by 1M illite with minor kaolinite and K-feldspar content decreasing with depth. Illite crystallinity indices and intensity ratios obtained indicate well crystallized illite with less than 10% smectite. Porosity in the Galesville was preserved by early, limited quartz cementation which formed a rigid, stabilized framework of quartz overgrowths that resisted compaction. Possible sources of silica include early feldspar dissolution, probably influenced by shale derived organic acids, and aqueous silica from clay mineral diagenesis. 
The presence of well crystallized illite of diagenetic origin (1M) with very few smectite layers indicates advanced clay diagenesis in the Eau Claire. The alteration of feldspars to kaolinite in the Eau Claire is consistent with feldspar dissolution in the Galesville and probably resulted from the action of organic acids produced during diagenesis.

AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.