Subsurface biomass carbon storage projects are a carbon-removal project type within the engineered & geological segment of the carbon market. They involve biomass processed and stored in shallow subsurface environments designed for long-term sequestration. In practice, biomass carbon is isolated from decomposition in an approved storage environment. Common project configurations include Subsurface mine storage or similar approved shallow subsurface biomass storage.

Shallow subsurface concepts depend heavily on site stability and on preventing oxygen and moisture conditions that would allow decomposition. This segment is generally aimed at more durable carbon storage and is often attractive to buyers seeking high permanence, but lifecycle emissions, measurement precision, and storage durability remain the decisive quality questions. For buyers and program designers, the most important diligence questions are: Will the storage environment remain dry, oxygen-limited, and stable? How are physical containment and monitoring handled?