Depositional and diagenetic controls on anomalously high porosity within a deeply buried CO₂ storage reservoir—The Cambrian Mt. Simon Sandstone, Illinois Basin, USA

Current diagenetic models predict that sandstone porosity and reservoir quality decrease with burial depth and formation age. However, the opposite trend is true for the Cambrian Mt. Simon Sandstone. The lowermost section of the Mt. Simon is highly porous—it has an average log-derived effective porosity of 17.0%. To understand the possible reason(s) for this anomaly, the amount of porosity reduction due to compaction porosity loss (COPL) and cementation porosity loss (CEPL) was analyzed. The COPL and CEPL were coupled with sedimentary characteristics in order to ascertain which factors controlled porosity and reservoir quality in the Mt. Simon. The underlying Argenta sedimentary unit disconformably underlies the Mt. Simon and is included for diagenetic and depositional comparison. Similar to expected trends from other deep basin reservoirs, the COPL generally increases with depth in the Mt. Simon. The CEPL is significantly lower in the lower half of the Mt. Simon compared to the upper half of the Mt. Simon, indicating that higher porosity is attributed to having a lower volume of intergranular cement. Early episodes of diagenesis within the arkosic lower half of the Mt. Simon, such as clay infiltration and feldspar alteration, produced diagenetic grain-coating clays, which inhibited the development of quartz cements. Thus, these grain coatings assisted in porosity preservation within the Lower Mt. Simon by reducing CEPL. The upper half of the Mt. Simon is more heavily cemented as a result of changes in the depositional setting and thus changes in the lithology. Later, largely post-compaction feldspar dissolution further increased porosity in the Lower Mt. Simon Sandstone.