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A Comparison of Reservoir Heterogeneity and Recovery Efficiency of Two Offshore Bar Complexes in an Intracratonic Basin

WHITAKER, STEPHEN T., and HANNES E. LEETARU, Illinois State Geological Survey, Champaign, IL

The U.S. Department of Energy is developing a nation-wide reservoir classification system to update and upgrade its TORIS database. This new information will allow targeting of key reservoir types for focused research on improved and/or enhanced recovery. A comparison of two shallow marine siliciclastic reservoirs from the intracratonic Illinois basin offers an example of the level to which reservoir types need to be differentiated in order to predict optimal recovery methods. These predictions should provide the focus for any reservoir classification system.

The lateral homogeneity of offshore bars within the Mississippian Cypress Formation at Bartelso field is a major factor contributing to a relatively high primary and secondary recovery efficiency. Vertical permeability barriers do not pose a significant problem to recovery efficiency with a 10 ac well spacing. On the other hand, the lateral heterogeneity of a partially dissected offshore bar complex within the Mississippian Aux Vases Formation at King field is the major factor adversely affecting the efficiency of primary and secondary recovery techniques there.

Although both these fields produce from sandstones that were deposited in shallow marine environments, a classification system must further differentiate between the two in order to stress the different exploitation procedures required to optimize recovery. Significant quantities of remaining mobile reserves may be recovered in King field-type Aux Vases reservoirs by programs involving infill drillingand selective waterflooding. Infill drilling would not be effective in recovering additional reserves from the offshore bar sands of the Cypress Formation where compartments have been effectively drained by typical well spacing and waterflooding. Instead, tertiary methods will need to be applied. Reservoir studies from a variety of basins will be needed before it can be determined to what levels reservoir classification needs to be subdivided to enable prediction of optimal recovery methods.


AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)