Analysis of a Valmeyeran (Middle Mississippian) Aux Vases Sandstone Reservoir

Beverly Seyler

Four closely spaced cores of the Aux Vases Sandstone near the Zeigler field in Franklin County allow detailed reservoir analysis of a typical problematic producer in Illinois. These cores show large lateral and vertical facies variations that influence reservoir quality and oil recovery.

Reservoir analysis includes detailed core description, log analysis, cross-section construction, porosity and permeability measurements, petrographic analysis, x-ray diffraction, and scanning electron microscopy (SEM) with energy dispersive x-ray system analysis (EDS). These analyses were performed to determine reservoir geometry, environments of deposition, and to provide data to assist in design of drilling, completion, and recovery programs.

Sedimentary structures and facies observed in these cores indicate deposition in a shallow marine subtidal to tidal-flat environment. The reservoir facies is interpreted as a sand-filled tidal channel. Reservoir-quality sandstone typically has approximately 24% porosity, greater than 100 md permeability, is cross bedded, and is poorly cemented.

SEM/EDS, and x-ray diffraction analyses of reservoir samples show that the sand grains are coated with authigenic chlorite and illite clays. The clay coatings are so pervasive that they comprise the cementing agent loosely holding this sandstone together. Authigenic clays compose the bulk of surfaces exposed to and interacting with any drilling, completion, or recovery fluids contacting this reservoir; therefore, they must be considered in design of treatment programs.

AAPG Search and Discovery Article #91023©1989 AAPG Eastern Section, Sept. 10-13, 1989, Bloomington, Indiana.