Microporosity evolution of naturally deformed caprock on salt diapirs in southern Iran

Caprock on the top of salt diapirs represents the accumulated solid residuum left behind after the salt dissolution by meteoric fluids or deeper cognate fluids from surrounding rock formations. It usually consists of the sulphate matrix-supported sedimentary breccia with clasts representing the various rock types incorporated in the original evaporite sequence that were dismembered and transported towards the surface within the rock salt. Since salt diapirs are important targets of hydrocarbon repositories, salt caprock physical properties, namely the permeability, are critical for safety evaluations of such facilities. Caprock itself can become a hydrocarbon reservoir. 

In this study, we present a microporosity and microstructural analysis of a series of samples from the Karmostaj and Siah Taq diapirs, located 20 km south of Lar city in Southern Iran. Both diapir exposures resemble salt glaciers and contain caprock deformed to a different degree above the rock salt outcrops. Mercury intrusion porosimetry was employed on 13 samples, each represented by 2-3 specimens. The selected samples represent an unaltered micritic dark dolomite, vuggy dark dolomite, undeformed and deformed sulphate matrix-supported breccia (floatbreccia), clast-supported breccia (packbreccia) and gypsum mylonite enclosing the vuggy carbonate clasts. 

Microstructural study identified reactions of carbonate replacement by gypsum in the vuggy carbonates, typified with vuggy or channel-like porosity. Clast-supported breccia is characterized by fractures and interparticle voids. The porosity of matrix-supported breccia (floatbreccia) is defined by inter-particle voids, foliation parallel high-porosity bands and gypsum foliation parallel fracture porosity, prominent in the highly deformed samples. The porosity values are the lowest in the micritic dolomite (2.4 - 4.2 %) with unimodal throat size (TSD) distribution of the pores at 0.1 μm. In contrast to the unaltered dolomite, the vuggy dolomite features very high porosity from 28 to 33 % connected by throats with a large span of sizes 6 to 60 μm and their average median throat size (MTS) is 23 μm. The breccia types have intermediate porosities between 15 and 23 % and differ markedly in the position of the dominant peak of their TSDs: 0.7 - 2.8 μm for the packbreccia and 20 μm for the floatbreccia. The free porosity is high (4 to 6 %) in both the types of packbreccias and low (1 to 2 %) in the floatbreccia. The gypsum mylonite features low porosity between 6 and 10 % and very low-size TSD between 6 and 40 μm.

We present a model of the microporosity evolution on the basis of caprock structure reconstruction and comparison of the porosity values and microstructures of the samples. The model proposes an important role of sulphate-rich fluids that dissolve the solid rafts of dark carbonate blocks that become partly replaced by gypsum. Consequently, deformation of caprock culminating at the lower edges of the salt glaciers is responsible for collapse of interclastic pores and development of fracture cleavage along the deformation fabrics of dynamically recrystallized gypsum. Sulphate rich fluids percolating through the caprock also promote local overpressure-driven fracturing along ductile shear zones.