The basal Silurian Alexandrian carbonates, sandwiched between the Ordovician-Silurian and Silurian-Devonian unconformities, have produced more than 3 million bbl of oil from the Buckhorn East and Brooklyn fields on the western margin of Illinois. These fields are shallow (450-670 ft), remote from the most-productive portion of the Illinois basin, and potential exists for additional production from this unconformity play. Three lithofacies were recognized in Alexandrian carbonates in detailed core and wireline studies that included thin-section, x-ray, and SEM analyses. These carbonates are regressive sequences composed of shallow-shelf, restricted-subtidal, and intertidal facies. The shallow-shelf facies is primarily biosparite composed of crinoid and brachiopod debris; t is facies is dolomitic in places. The restricted-subtidal facies consists of both bioturbated and undisturbed gray micrites containing stylolites, green chloritic clay, and glauconite; sparry clacite fills vertical cracks. The intertidal facies is tan, mottled, vuggy dolomite composed of algal mats, anhydrite-filled birdseye structures, burrows, and scattered marine fossils.

Petroleum reservoirs apparently developed as a result of dolomitization in intertidal and shallow-shelf carbonates. Vuggy porosity, a possible result of karst leaching, is found in pay zones within the dolomitic intertidal facies. Evidence of emergence and subaerial weathering include the presence of vadose silt, anhydrite, and nontectonic breccia.

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