Preliminary Reservoir Characterization of a Chesterian Cypress Sandstone in Illinois Basin

Beverly Seyler

Reservoir characterization was undertaken, based on detailed analyses of a core in the middle Mississippian (Chesterian) Cypress Sandstone from an overlooked pay zone in the Parkersburg field, Edwards County, Illinois. The purpose of the study was to evaluate the producing interval in order to prescribe specific completion and recovery techniques for optimization of oil production. The Cypress pay at Parkersburg is variable, with both economic and marginal wells.

Evaluation techniques included log analyses, thin-section petrography, porosity and permeability analysis, x-ray diffraction, and Scanning Electron Microscopy (SEM) with Energy Dispersive System (EDS) analyses.

Authigenic chlorite and illite were found coating detrital quartz and feldspar grains, and partially filling spores in all samples. Permeability ranged from 0.1 to 24 md and porosity from 11.5 to 16%. Minimal hydrochloric acid treatment is necessary to avoid dissolving authigenic chlorite clays which could lead to the formation of an iron hydroxide gel, resulting in the obstruction of pores. The well was completed using light HCl treatment as a mud clean-out agent. Clay stabilizers were added. Production from this well has been at a steady 25-30 BOPD at less than 3,000 ft since treatment in November 1987.

An expanded program will focus on core-flow experiments treating reservoir samples to different drilling, completion, and recovery fluids. Changes in porosity and permeability under these conditions will be monitored to document treatments leading to optimal recovery under specific reservoir conditions.

AAPG Search and Discovery Article #91031©1988 AAPG Eastern Section, Charleston, West Virginia, 13-16 September 1988.