Subsurface Recognition of Oolitic Facies in Carbonate Sequence; Exploration and Development Applications: Ste. Genevieve Formation (Mississippian), Illinois Basin

William F. Bandy

The oolitic grainstone facies of the Ste. Genevieve Limestone is a widespread and highly productive reservoir in the Illinois basin. However, exploration and development of these oolitic facies are hampered by the inability to recognize the reservoir on logs. In many areas, the only log data available are old wireline electric logs. Comparison of cores with log response in northern Lawrence field, Lawrence County, Illinois, indicates a subjective but predictable relationship between log signature and carbonate lithology. Two productive lithologies, dolomite and oolitic grainstone, display well-developed SP curves. However, resistivity response is greatest in dense limestone, less well developed in oolitic grainstone, and poorly developed in dolomites. On gamma-ray logs, oolitic facies can be differentiated from dolomites by their lower radioactivity. Oolitic sands are most easily recognized on porosity logs, where their average porosity is 13.7%, only half the average porosity of dolomit s. In a new well, the best information for subsequent offset and development of an oolitic reservoir is provided by porosity and dipmeter logs.

During the Valmeyeran and Chesterian, the eastern Illinois basin was a gently sloping ramp that dipped southwest. Oolitic sands likely formed on high-energy shoals, such as topographic highs or structures. Hence, successful exploration for oolitic reservoirs includes recognition of oolitic grainstones on available electric logs, and delineation of paleostructures. Further development or waterflooding of existing oolitic reservoirs can be accomplished by constructing facies maps from existing electric logs.

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