--> Middle Mississippian Carbonate Reservoirs Facies in the Illinois Basin: Outcrop Analogues

Eastern Section Meeting

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Middle Mississippian Carbonate Reservoirs Facies in the Illinois Basin: Outcrop Analogues


The middle Mississippian carbonates (primarily limestone) contain significant petroleum reservoirs in the Illinois Basin and are being revisited as targets for unconventional drilling. This presentation will discuss a comparative study of reservoir facies in outcrops and the subsurface in an attempt to provide a better understanding of facies distribution, reservoir architecture, and factors that control porosity and permeability.

Deposition of the middle Mississippian carbonates began with accumulation of up to 200m of siliceous and spiculitic limestone of the Fort Payne Formation. The basin was connected to the deep, open ocean through a bathymetric depression, which was centered over the failed Precambrian-Early Cambrian Reelfoot Rift that extended from the Ouachita Trough in central Arkansas to southern Illinois. Upwelling of cool nutrient- and silica-rich deep oceanic water into the Illinois Basin may have resulted in the proliferation of crinoids and bryozoans during deposition of the Fort Payne and the overlying Ullin (Warsaw) to the south and Burlington-Keokuk to the west. The subsequent change from crinoidal-bryozoan carbonates to more typical oolitic and peritidal carbonates (the Salem, St. Louis, and Ste. Genevieve) was the result of decreased subsidence and gradual shallowing of the basin.

The middle Mississippian carbonates are each unique in their facies character and reservoir architecture. Whereas fracture porosity characterizes the Fort Payne, bryozoan-dominated buildups and storm-generated, bryozoan-dominated sand waves form the primary reservoir facies in the overlying Ullin (Warsaw). Partly oolitic, cyclic, skeletal grainstone facies characterize the Salem reservoir rocks, whereas the primary targets for exploration in the St. Louis are mainly peritidal, dolostone beds. Well-developed ooid shoals and, in some cases, the associated microcrystalline dolomite form the prolific reservoir facies of the Ste. Genevieve limestone. The preservation of porosity and permeability in these carbonates appears to be primarily controlled by the relative abundance of crinoidal fragments, which are susceptible to rapid cementation by syntaxial calcite. The best reservoirs are generally developed within bryozoan-dominated carbonates of the Ullin (Warsaw) and in the skeletal grainstones with coated grains and well-developed oolitic grainstone in the overlying Salem and Ste. Genevieve limestones.