It is likely that ice sheets contributed significantly to natural fracturing and hence gas production in low permeability Devonian shale. The large ice sheet of the last ice age caused considerable deflection of the lithosphere. Predicted horizontal tensional stresses south of the glacial limit resulting from this crustal flexing exceed the tensile strength of Devonian shale. Glacially induced fracturing is therefore expected in these regions which correspond with regions of historical gas production. The north-south variation in observed stress ratios is also explained by the glacial model. Lineaments and faults will perturb locally the glacially induced stresses and, when coupled with the glacial model, offer an exploration rationale.

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