Porosity and permeability of karst carbonate rocks along an unconformity outcrop: A case study from the Upper Dammam Formation exposure in Kuwait, Arabian Gulf.

Diagenetic changes are a concern in carbonate petroleum reservoir management. One of the challenges is to determine whether the pore systems are related to diagenesis and/or depositions, and how associated mechanisms affect reservoir quality. The purpose of this study was to assess variations in porosity and permeability due to diagenetic processes in the paleokarst zone of the Middle Eocene Upper Dammam Formation in Kuwait. This zone is 14 m thick and exposed in a quarry in southern Kuwait. The exposed section is divided into three lithological units, described here from the bottom to top: chalky dolostone, karst dolostone, and karst carapace. These rocks have been affected by several diagenetic processes, including dolomitisation, dissolution, cementation, replacement, recrystallisation, and fracturing. Significant variations have been observed in porosity and permeability, both vertically and horizontally. In the chalky dolostone layer, the highest porosity and permeability were measured at 53 % and 6000 mD, respectively. The maximum porosity and permeability in the karst dolostone layer were 44.23 % and 1140 mD, respectively. The karst carapace had the lowest porosity (30 %) and permeability (100 mD). Majority of the porosities were of isolated mouldic or vuggy forms. Pores formed in the rock framework were vuggy, fractured, and more connected. They were formed at later stages by meteoric water dissolution, acidic gases produced during the thermal maturation of kerogen in the petroleum source rocks, or a combination of both processes. The results of this study may be applicable to analogous subsurface carbonate reservoir rocks.