Abstract: Development of Waulsortian Mounds and Hydrocarbon-Bearing Flanking Facies in the Middle Mississippian of the Illinois Basin

Zakaria Lasemi, Janis D. Treworgy, Rodney D. Norby

The Valmeyeran (lower Visean) Ullin Limestone of the Illinois basin contains numerous Waulsortian-type mud mound complexes. Mounds were initiated after a sea level rise that accompanied cessation of Borden Siltstone deposition. These mounds developed below storm-wave base in a deeper water outer ramp to basinal setting. Mound facies grade basinward into the dark-colored, argillaceous, spiculitic and cherty lime mudstone of the Fort Payne Formation.

The mound facies of the Ullin generally coalesce laterally and vertically into complex carbonate bodies that range from 50 to 500 ft wide and 20 to 60 ft thick in outcrop and thicken basinward. Mound facies include a massive mudstone and wackestone core flanked by (1) a bryozoan-echinoderm bafflestone buildup facies that shows little evidence of current reworking, and (2) a well-bedded, transported, bryozoan-echinoderm wackestone and packstone/grainstone. The bafflestone facies was the potential source for skeletal sands deposited as packstone/grainstone debris aprons in intermound areas and as storm-generated sand waves in a middle ramp setting. These skeletal sand deposits include fragmented and poorly sorted particles and are characterized by low-angle inclined bedding, hummocky cr ss-stratification, graded bedding, and escape burrows, all indicating rapid deposition.

Waulsortian-type mound facies similar to those in the Ullin Limestone are prolific hydrocarbon reservoirs in several basins in North America. In the Illinois basin, the Ullin is an oil producer, but its potential as a reservoir has been largely overlooked. Because of excellent preservation of intra- and interparticle porosity, the bafflestone buildup facies has a high potential for reservoir development, especially where bryozoan dominated. High porosity is also characteristic of debris aprons and storm-generated sand waves in the Ullin. Porosity, permeability, and reservoir quality of these carbonate sand bodies is highly variable and may be dependent on the relative abundance of echinoderm fragments, which are susceptible to overgrowth cementation.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994