MORSE, DAVID G.
Illinois State Geological Survey, Champaign, IL

Abstract: Mississippian Tar Springs Sandstone, A Fluvial-Deltaic, Cratonic Reservoir Target in the Illinois Basin

The Mississippian Tar Springs Sandstone, a cratonic fluvial-deltaic sequence, was deposited from north to south as one cycle in a rhythmic carbonate/siliciclastic package that is typical of the Chesterian in the Illinois Basin. Formation and net sand isopach maps and detailed cross-sections indicate a regional architecture consisting of stacked distributary channels which form sand bodies up to 125 feet thick, 1 to 2 miles wide and up to 100 miles long. The channel bases cut downward into older Tar Springs, delta fringe fine sands and shales, and the underlying Glen Dean Limestone. Above the stacked sandstones are intertidal to supratidal, interbedded fine sands and silts, and then shales with local coaly stringers, which are capped by the thin, widespread, shallow marine, Vienna Limestone.

The Tar Springs forms stratigraphic trap reservoirs, such as the Inman Field in southeastern Illinois, where thick sands are mature and composed of fine to medium quartz grains with less than 5% feldspar, and with traces of chert grains, zircon, tourmaline, and opaques. Quartz overgrowths are common, but rarely fill pores. Later diagenetic cements in intragranular pores consist of kaolinite with small amounts of illite and magnesium-rich chlorite and small irregular patches of sparry ankerite. Typical reservoir porosities range from 16 to 19% and permeabilities from 60 to 700 md. Delta fringe sediments have little reservoir potential and act as impermeable lateral and vertical seals, aiding in stratigraphic trapping. Knowledge of Tar Springs sand body architecture will improve the search for new fields.

AAPG Search and Discovery Article #90928©1999 AAPG Annual Convention, San Antonio, Texas