Ancient oolitic environments consists of a typical suite of microfacies among which predominate high-encrgy oolitic calcarenites. These well-sorted carbonate sands, generated through the action of shallow agitated waters saturated in calcium carbonate, are either deposited in situ or transported to a variable extent and redeposited. In both instances they are essentially devoid of a micrite matrix and characterized by a variable amount of interstitial cavity-filling sparite. These particular rocks may became important oil reservoir either by
preservation of primary intergranular porosity or through secondary porosity following differential diagenetic solution involving the intersititial cement or the oolites themselves. In fact the association of primary and secondary porosity in these rocks is known and increases their economical interest. lt is, therefore, important to understand the environmental conditions which generated these bodies of oolitic calcarenite, their geometry, their space and time distribution, and to establish comparisons with modern analogs.

The present paper deals with the environmental interpretation of a classical oolitic sequence, the Ste. Genevieve Limestone (Mississipian ),producer of 26% of the oil in the Illinois Basin, called the McClosky oil "sand" and whose porosity appears mainly due to diffrential solution affecting ancient systems of oolite bars.

For the reconstruction of the oolitic environment 0f` the Ste. Genevieve Limestone, about 800 thin sections, collected from four localities in Southern Illinois and Eastern Missouri, were studied by detailed petrographic investigation which involved extensive use of computer techniques.

Eight major microfacies were recognized based on the values of measured parameters, combined with rnegascopic and petrographic textural variations. In order of decreasing relative depth, the microfacies are: calcilutite and calcisiltite (microfacies 1), grain-supported pelletoidal calcarcnite with calcisiltite matrix (microfacies 2), grain-supported biocalcarenite with calcisiltite matrix (microfacies 3), grain-supported biocalcarenite with clear calcite cement (microfacies 4), grain-supported pelletoidal calcarenite with clear calcite cement (microfacies
5), grain-supported oolitic calcarenite with clear calcite cement (microjacies 6), grain-supported oolitic calcarenite with calcisiltite matrix (microfacies 7), (and pure quartz sandstone with clear calcite cement (mlcrofacies 8). Relative energy levels were assigned to each microfacies calcilutite and calcisiltite (microfacies 1) being the lowest, followed by pelletoidal, bioclastic, and oolitic calcarenltes with calcisiltite matrix <microfacies 2, 3 and 7); by pelletoidal, bioclastic calcarenites with clear calcite cement (microfacies 4 and 5) and pure quartz sandstone with clear calcite cement (microfacies 8); the highest energy being
represented by the oolitic calcarenites with clear caleite cement (microfacies 6) which constitute 52% of the investigated samples.