Limits to the sealing capacity of rock salt: A case study of the infra-Cambrian Ara Salt from the South Oman salt basin

Abstract

In the South Oman salt basin (SOSB), diapirs of infra-Cambrian Ara Salt enclose isolated, commonly overpressured carbonate reservoirs. Hydrocarbon-impregnated black rock salt shows that it has repeatedly lost and then regained its sealing capacity. The black staining is caused by intragranular microcracks and grain boundaries filled with solid bitumen formed by the alteration of oil. The same samples show evidence for crystal plastic deformation and dynamic recrystallization. Subgrain-size piezometry indicates a maximum differential paleostress of less than 2 MPa. Under such low shear stress, laboratory-calibrated dilatancy criteria indicate that oil can only enter the rock salt at near-zero effective stresses, where fluid pressures are very close to lithostatic. In our model, the oil pressure in the carbonate reservoirs increases until it is equal to the fluid pressure in the low but interconnected porosity of the Ara Salt plus the capillary entry pressure. When this condition is met, oil is expelled into the rock salt, which dilates and increases its permeability by many orders of magnitude. Sealing capacity is lost, and fluid flow will continue until the fluid pressure drops below the minimal principal stress, at which point rock salt will reseal to maintain the fluid pressure at lithostatic values.