Porosity and pore and throat size distributions in carbonate-rich salt caprock of halite diapirs: effect of deformation and geochemical processes

Abstract

Microscopic porosity of salt caprock on two salt diapirs, Karmostaj and Siah Taq in southern Iran, was studied on a range of samples that represent the major lithological types of salt caprock in the area: the gypsum matrix-supported and clast-supported dissolution breccia, the dolomite and limestone stringers of the Hormuz Formation and gypsum mylonites. Mercury intrusion porosimetry alongside the microstructural analysis with digital image analysis of resin-saturated thin-sections was employed and revealed a large variation in porosity of 2–35 % and in the median throat size of 0.1–50 μm. The highest porosity values are associated with vuggy dolomites and limestones interspersed by a dense network of gypsum veins and gypsum matrix-supported breccia with low degree of deformation. In contrast, low porosity is linked with strongly deformed gypsum matrix-supported breccias and dark micritic carbonate stringers that were not interspersed with the gypsum. Large variation in porosity and pore throat size and pore shape is attributed to 1) metasomatization of carbonate to gypsum due to reaction of H₂SO₄ with carbonate after oxidation of H₂S or elemental sulfur in caprock and subsequent dissolution of gypsum in metasomatized carbonates creating the abundant vuggy porosity and 2) deformation of the caprock that was responsible for recrystallization and mechanical closure of pores in the frontal, downslope margins of both reactivated diapirs. While the deformed and gypsum-rich parts of the diapir caprocks (∼30–60 % of caprock on the diapirs) are associated with estimated permeabilities of ∼10⁻¹⁶ - 10⁻¹⁵ m², caprocks in central parts of reactivated diapirs with less deformed caprock (undeformed gypsum- and clast-supported breccia) display higher permeabilities ranging between ∼10⁻¹⁵ m² and 10⁻¹³ m².