carbon sequester in or under the clay?

At an impasse whether a pore water wood chip matrix is better sequestered inside a clay layer or under a clay layer. Many pro-glacial (ice age meltwater) lake geo-formations are alternating thin layers of silt and clay. They aren't to deep and tens of metres thick. The risks is the pore water would squeeze out of the formation or that it isn't thick enough to store much. It is shallow and easy to drill too, and plentiful. There are other clay layers too, often from older pro-glacial lakes. There are a few really thick layers of clay in some places just about granite. Aren't as many as per the proglacial formations. And there is a risk they would be exposed to air; ruining the sequester (air de-actives the pore-water enzymes). Deep to drill to; but much more stable formation.