Experimental Evaluation of the Physical Properties of Basalt Flows and Diabase Sills of the South Georgia Rift Basin: Potential Implications for CO₂ Sequestration

The South Georgia Rift (SGR) basin is buried beneath the Cretaceous Coastal Plain sediments in parts of southern South Carolina, Southeast Georgia, Western Florida and Southern Alabama. The focus of our study, funded by the Department of Energy (DOE), is on the clastic (sandstone) reservoir units in southern South Carolina that have been identified as deep saline formations suitable for subsurface CO₂ storage. A key attraction to ongoing efforts aimed at evaluating prospectivity for long term CO₂ sequestration is the presence of basalt flows and diabase sills that could potentially serve as caprocks to the underlying deep saline formations. Unlike shale-capped CO₂ reservoirs, very little is known about the ability of these unconventional mafic igneous deposits to act as viable seals for CO₂ storage. This is due to the lack of physical data on properties of these rocks that are important for the evaluation of seal integrity. To date, our petrophysical and geological understanding of these rocks has come primarily from limited well data and examination of core samples.

Therefore, with this study, we designed and carried out a series of physics-based laboratory experiments on selected samples considered strategic to the goals of the overall project. The basalt samples come from the Clubhouse Crossroads wells 2 and 3 (CC-2 and CC-3) and Dorchester (DOR 211) well in South Carolina. Due to the non-availability of diabase sill samples from our study area, we used samples from the Palisade Sills in the Newark series that are considered analogues of the SGR basin diabase sills. Our goals for the lab analysis were to understand and quantify the internal composition and microstructure as well as the petrophysical (porosity and permeability) and acoustic (P-wave and S-wave velocities) properties of these samples, in addition to making a clear distinction between what is basalt flow and what is diabase sill.

With the exception of CC-2 at 2382 ft (with a porosity of 14.3 % and permeability of 0.045 mD), preliminary results show that the tested samples from CC-2 and CC-3 and DOR-211 returned near zero porosity and permeability. These values as well as analysis of micro-xray computed tomographic images suggest that these rocks are suitable as CO₂ reservoir seals. Results from experimental analysis of the Palisade sill will be also discussed.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California