THE MISSISSIPPIAN CARPER SANDSTONE OF THE ILLINOIS BASIN: REINTERPRETATION OF SEDIMENT GRAVITY FLOWS AND IMPLICATIONS FOR RESERVOIR QUALITY

The Carper Sandstone is a Mississippian (Osagean) oil reservoir in the Illinois Basin that is being evaluated for CO₂ enhanced oil recovery and storage potential. The Carper Sandstone occurs within the Borden Siltstone, the western extension of the Price-Pocono deltaic complex and was initially interpreted as turbidite deposits. This interpretation was developed in the 1960s based on subsurface mapping, using well logs of lobe-shaped bodies and identification of graded marine sediments in core. The Carper is herein re-examined using modern process-based paradigms of sediment gravity flow deposition. End members of sediment gravity flows include turbidites, debrites, and hybrid event beds (HEB; co-genetic turbidite and debrite beds). A new facies model for the Carper Sandstone highlights both the internal architecture of the deposits and its implications for reservoir quality.

Five cores (83.3 m) underwent petrographic and facies analysis. Five facies were classified: massive sandstone (Sm), parallel laminated sandstone (Sp), muddy sandstone (Su), laminated mudstone (Ml), and fossiliferous mudstone (Mf). Facies associations were identified to quantify the scale and abundance of the sequences and evaluate these against existing facies models. Twenty turbidites (61 cm mean thickness) were identified, 80% of which exhibit a Sm base to Ml cap transition. In addition, 60 HEB (47 cm mean thickness) were observed, commonly with Su-Mf and Sm-Su-Mf sequences; ~57% contained Sm. The high mud:sand ratio of Su predicts poor reservoir quality, exhibiting low porosity and permeability. In contrast, Sm exhibits the highest porosity and permeability of all identified facies. Turbidites contain more Sm than HEB (80% to 57%) and are superb reservoirs for extraction and storage. Identifying HEB and debrite facies within previously interpreted turbidites is crucial to understanding reservoir potential in distal, fine-grained sediment gravity flow deposits in basins around the world.