Mississippian Biohermal Buildups in the Illinois Basin and Adjacent Regions and Their Hydrocarbon Potential
LASEMI ZAKARIA and RODNEY D. NORBY
Illinois State Geological Survey, Champaign, IL,
JOHN E. UTGAARD
Department of Geology, Southern Illinois University, Carbondale, IL
Mississippian biohermal buildups occur as (1) small mud mounds in the upper Kinderhookian Chouteau and Compton Formations, (2) biodetrital calcisiltite mounds in the Osagean-lower Meramecian Burlington, Fort Payne and Ullin Formations, (3) biohermal to biostromal bryozoan frame thickets in the lower Meramecian Ullin Limestone, and (4) microbial mud mounds and bryozoan-serpulid-solenoporoid (red algae) bioherms in the lower and upper Chesterian Ste. Genevieve and Kinkaid Limestones. Among these, the only volumetrically important buildups are fenestrate bryozoan frame thickets that are up to 60 m thick. These widely distributed mud-free buildups primarily developed during deposition of the lower Ullin and generally occupied the same niche as Waulsortian mounds. The Kinderhookian-early Meramecian buildups in the Illinois Basin and adjacent regions coexist with crinoidal-bryozoan grainstone facies interpreted to have been deposited in a relatively deep, sub-photic environment. Post-early Meramecian buildups formed in a shallow marine setting and were terminated by rapidly prograding oolitic grainstone facies.
Hydrocarbon production from buildups similar to those we have described here has not yet been proven, but the potential exists. The buildups formed by the bryozoan frame thickets and the other large mounds also create local structures over which porosity zones in the overlying units may be draped, forming hydrocarbon reservoirs. The best such reservoir facies occur in compartmentalized, grainstone sand shoals. The sand shoals consist of crinoidal-bryozoan grainstones in the Osagean-early Meramecian units (e.g., Ullin) and oolitic grainstones in the post-early Meramecian units (e.g., Salem and Ste. Genevieve). Porosity and permeability development in the Osagean-early Meramecian shoal facies is primarily controlled by the relative abundance of echinoderm fragments, which are susceptible to rapid cementation by syntaxial calcite.