[[Claystone]]s, [[Halite|salt]], and sulfates ([[gypsum]] and [[anhydrite]]) make seals of this sort because they are ductile under most geological strain rates and confining pressures. Ductility is important; if [[Fracture|fracturing]] occurs, oil can leak through the [[fracture]]s without invading the matrix porosity. | [[Claystone]]s, [[Halite|salt]], and sulfates ([[gypsum]] and [[anhydrite]]) make seals of this sort because they are ductile under most geological strain rates and confining pressures. Ductility is important; if [[Fracture|fracturing]] occurs, oil can leak through the [[fracture]]s without invading the matrix porosity. |