Open Access

Jared Freiburg

• Anthropogenic greenhouse gases such as carbon dioxide (C02) must be mitigated and reduced to preserve a stable climate for future generations. One promising technology is carbon capture and storage (CCS) in geologic formations, which is currently being deployed in numerous pilot projects across the United States. One of these is the Illinois Basin–Decatur Project that has successfully stored 1 million metric tons of C02 in the Mt. Simon storage complex. The Mt. Simon Sandstone reservoir has been largely unexplored due to a previous lack of economic interest. Oil-bearing formations in the Illinois Basin are in younger successions and formation waters in the Mt. Simon are highly saline but with low levels of critical elements (i.e. lithium, magnesium). In the Illinois Basin, a limited number of drill holes penetrate the Mt. Simon formation with an even smaller number of core samples in these deep strata. This has left the earliest Paleozoic rocks in the Illinois Basin poorly understood. The stratigraphic test well at the IBDP revealed the lowest most section of the Mt. Simon to be a thick highly porous and permeable sandstone. With a near to complete lack of other wells penetrating this lower Mt. Simon unit, major questions arose such as 1) what is the origin of this deep porous sandstone; 2) what controls the distribution of this sandstone and where can more of it be found; 3) what controls porosity at this depth when overlying sandstones have largely poor reservoir properties; and 4) is it suitable for geologic carbon storage (i.e. are there high quality seals that provide secure storage and prevent vertical migration)? This research examines the origin and diagenesis of the Mt. Simon storage complex by first resolving the age of the underlying Precambrian basement and investigating basement structures associated with sediment accommodation (chapter ii). Basement geochronology and a comprehensive investigation of the Mt. Simon provenance (chapter iii) suggests a largely local sediment supply depositing into a rift basin. Detrital zircon geochronology of the lower Mt. Simon yields a dominant Mesoproterozoic proximal source as confirmed in regional basement samples yielding Eastern Granite-Rhyolite, Southern Granite-Rhyolite, and Mazatzal Province rocks. A small peak of Early Cambrian zircons (527 to 541 Ma) in the lower Mt. Simon is indicative of rift volcanics as confirmed by the geochronology of a basalt sample recovered in a deep stratigraphic test hole along the rift axis in west-central Indiana. Failed rifting pre-dated the formation of the Illinois Basin with the earliest Paleozoic sediments deposited in a northward trending Cambrian aulacogen. Locally sourced arkose in the lower Mt. Simon is considered to present an anomalously high porosity that was preserved throughout its diagenetic history. Petrographic characterization shows the lower Mt. Simon contains abundant diagenetic grain coatings of illite that prevented pervasive nucleation of authigenic quartz found in the other overlying Mt. Simon units (chapter iv). These clay coating are considered the most significant feature that controlled porosity preservation in the Mt. Simon storage complex. Geochronology of these illite coatings reveals two major events of illitization both of which correspond with structural deformation and igneous activity in and around the basin in response to regional orogenic events (chapter v). The early illitization event (mostly Carboniferous) was associated with smectite illitization and potassium feldspar dissolution, which caused significant secondary porosity. The later illitization event (Triassic) is identified in non-reservoir units of the Mt. Simon where pore occluding kaolinite was partially illitized. Lastly, high-resolution pore space characterization of thick pervasive shale formations overlying the Mt. Simon indicates the Eau Claire shale, directly overlying the Mt. Simon, provides the best seal to the Mt. Simon reservoir completing the Mt. Simon storage complex (chapter vi).