ABSTRACT: Characteristics of Aux Vases and Cypress Reservoirs in Illinois

WHITAKER, STEPHEN T., Illinois State Geological Survey, Champaign, IL

The Cypress and Aux Vases Formations (Mississippian) contain the most significant clastic petroleum reservoirs within the Illinois basin. Reservoir characteristics within these formations vary laterally and vertically due to different environments of deposition and the degree of post-depositional alterations.

Sequence stratigraphy can be used to demonstrate that the Aux Vases Formation may be subdivided into two sequences. The lower sequence represents a progradational wedge advancing toward what is now the southeastern part of the basin. This sequence contains sandstones that thin toward the southeast and grade into limestones and shales. It is in these mixed siliciclastic-carbonate intervals in which isolated hydrocarbon-bearing reservoirs are formed. The upper sequence is transgressional and contains thin sandstones, shales, and limestones. Reservoirs in this sequence are extremely variable in quality and size.

The Cypress Formation also can be subdivided into two sequences with the lower of the two representing a progradational wedge of sand advancing southward into the basin. Sandstones within this lower sequence may be as much as 100 ft thick, and tend to be relatively homogeneous. Stratigraphic traps are rare. The upper

sequence contains shales and discontinuous transgressive sandstones that commonly form offshore bars. Stratigraphic entrapment of oil in this sequence is common. Reservoir quality of the Cypress sandstones is relatively consistent.

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