Reservoir Geology and Description for Supplemental Recovery, Tar Springs Deltaic Reservoirs, Benton Field, Illinois

Robert M. Sneider

The Tar Springs sandstone along the southwestern flank of the Illinois basin is part of a major deltaic system formed by deposition from a large river (Mississippi River scale) into a slowly subsiding intracratonic basin. Benton, the largest field in the delta system, has produced for more than 45 years by primary depletion and waterflooding. Parts of the field have been sites for enhanced oil recovery projects by steam and chemical (sulphonate) flooding. Sand body, internal shale barriers, and pore space geometry and continuity directly result from deltaic deposition and have significantly influenced primary and supplemental recovery.

The Tar Springs at Benton is composed of a major distributary channel (up to 80 ft thick) that has deposited and later cut through four delta bar sands (each up to 20 ft thick), separated from one another by very thick, vertically impermeable prodelta and/or marine siltstone and shale. The upward-fining distributary channel sands comprise about 25% of the reservoir pore space. The four upward-coarsening delta bar sands comprise the remaining 75%.

Permeability contrasts and shale layers, which are depositionally controlled, provided the basis to subdivide the reservoir into separate zones. The major distributary channel produced during primary recovery by water drive and had less oil to be recovered by supplemental recovery. The deltaic bar sands produced by depletion drive during primary recovery and are the major target for waterflood and supplemental recovery.

AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.