ABSTRACT: The Role of Diagenesis on the Formation of Reservoir Heterogeneity in Two Middle Mississippian Sandstone Reservoirs in the Illinois Basin

SEYLER, BEVERLY, and D. SCOTT BEATY, Illinois State Geological Survey, Champaign, IL

Two highly productive Middle Mississippian siliciclastic units, the Aux Vases and Cypress formations, currently at 1,000 to 3,000 ft subsea and separated vertically by approximately 150 ft, have responded differently to similar burial conditions and diagenetic processes. Diagenetic alterations have caused reservoir heterogeneities and affected the responses of reservoir rocks to drilling, completion, and recovery treatments. Middle- and late-stage diagenetic alterations have enhanced porosity and permeability in both reservoirs. These alterations were controlled by initial sandstone composition, which is a function of both provenance and depositional facies. Studies utilizing sequence stratigraphy suggest that most Aux Vases reservoirs were deposited in shallow marine, mixed silicicla tic-carbonate tidal bars and channels. Cypress reservoirs were apparently deposited in shallow marine environments more dominated by deltaic activity and

consequently contain less carbonate than the Aux Vases.

In reservoir intervals, the Aux Vases sandstone is a friable unit, loosely cemented by authigenic clays, that typically possess porosities greater than 22% and permeabilities greater than 250 md in the best reservoir sandstones. Cypress sandstones contain few grains with clay rims. They therefore tend to be more highly cemented with quartz-overgrowths and commonly are less porous and permeable than Aux Vases sandstone reservoirs. Porosity-enhancing diagenetic alternations in both these reservoirs include dissolution of original carbonate cement, dissolution of feldspars, and dissolution of quartz overgrowths. Other diagenetic events which occlude porosity include late-stage carbonate cement and quartz overgrowths and the formation of authigenic clay. Together, these diagenetic alterat ons affect the development and distribution of porosity and permeability and are responsible for varying pore geometries and mineralogies within these two reservoirs.

AAPG Search and Discovery Article #91013©1992 AAPG Eastern Section Meeting, Champaign, Illinois, September 20-22, 1992 (2009)