Introduction
Deep geological storage is the linchpin of permanent CO₂ removal. While mineralization locks carbon into stone, you still need a secure subsurface environment to keep it out of the atmosphere during reaction and beyond. This article maps the four leading reservoir types—basalt, ultramafic, saline aquifers, and depleted fields—then unpacks capacity estimates, integrity risks, and selection criteria crucial for gigaton-scale deployment.
1. Types of Geological Reservoirs
1.1 Basalt Formations
Basalt’s high magnesium and calcium content makes it uniquely reactive. Projects like Carbfix (Iceland) inject CO₂-charged water into basalt flows—95% mineralization within two years. Basalt exists on every continent; pore space and fractures in cooled lava provide natural injection pathways.
1.2 Ultramafic Rocks
Ultramafic rocks (peridotite, serpentinite) boast even higher Mg-silicate percentages. The Oman Drilling Project has demonstrated in-situ carbonation rates of up to 0.3% per year—orders of magnitude faster than silicate weathering. Globally, ultramafic outcrops cover hundreds of thousands of square kilometers.
1.3 Saline Aquifers
Saline formations—porous sandstone or carbonates saturated with brine— offer the largest theoretical capacity. Their depths (800–3,000 m) keep CO₂ in supercritical form, maximizing density. The North Sea, Gulf Coast, and China’s Songliao Basin are prime examples.
1.4 Depleted Oil & Gas Fields
Repurposing mature hydrocarbon reservoirs leverages existing wells, infrastructure, and regulatory frameworks. Projects like Norway’s Sleipner and Canada’s Quest have cumulatively stored tens of megatons. However, legacy wells must be rigorously remediated to prevent leakage.
2. Estimating Capacity & Ensuring Integrity
Global capacity estimates for deep storage exceed 10,000 Gt CO₂— enough to sequester current annual emissions for centuries. But theoretical capacity is moot without integrity: sealing caprocks, fault mapping, and continuous monitoring are non-negotiable for permanence.
3. Site Selection Criteria
- Depth & Pressure: 800–3,000 m to keep CO₂ supercritical.
- Porosity & Permeability: High pore volume for injection; low-permeability caprock for seal.
- Geochemical Suitability: Mineralogy must support carbonate formation.
- Infrastructure Access: Proximity to capture source, pipelines, and monitoring networks.
- Regulatory & Community Fit: Clear land rights, public acceptance, and robust oversight.
4. Pioneering Case Studies
4.1 Carbfix (Iceland)
Since 2014, Carbfix has injected over 75,000 t of CO₂ into basalt, demonstrating rapid mineralization and zero detected surface leaks.
4.2 Illinois Basin (USA)
The Midwest Geological Sequestration Consortium’s saline trial injected 1 Mt CO₂ into brine sands, validating large-scale injectivity and tracer-based monitoring techniques.
5. Socio-Environmental Considerations
Deep storage isn’t just geology: you need community trust, fair benefit sharing, and transparent data. Indigenous rights, groundwater protection, and long-term stewardship funds must be baked into every project.
Conclusion
Choosing the right underground vault is as critical as capturing the CO₂ itself. By matching reservoir type to regional geology, prioritizing integrity, and engaging stakeholders early, we can unlock the global capacity needed for a permanent carbon future. Next up: our endgame blueprint—how to drive costs to $10–$50/ton while scaling these sites worldwide.