Porosity and Permeability Study of a Karst Carbonate Rocks along Unconformity Outcrop: A Case Study from Upper Dammam Formation Exposure in Kuwait, Arabian Gulf

Diagenetic changes by karstification and/ or dolomitization has been one of the target studies in petroleum carbonate reservoirs. This has been applied in many carbonate reservoirs in the Arabian Gulf Basin such as the Khuff... [ view full abstract ]

Diagenetic changes by karstification and/ or dolomitization has been one of the target studies in petroleum carbonate reservoirs. This has been applied in many carbonate reservoirs in the Arabian Gulf Basin such as the Khuff Formation in Saudi Arabia and Sarvak Formation in Iran. The purpose of this study is to assess the porosity and permeability variation as a result of the various diagenetic processes on the palaeokarst zone of the Middle Eocene Upper Dammam Formation in Kuwait. This zone is well exposed within rock cut cliffs in a quarry located at Al Ahmadi area, south of Kuwait. The exposed rocks in the quarry represent a palaeokarst zone, which ranges in thickness between 2.0 and 14.0 m. The exposed part of Dammam rock is overlain by clastic rocks of Kuwait Group.
181 Representative rock samples were collected from the exposed section and divided into 15 stratigraphic profiles. The rocks were studied using rock slabs, thin sections, as well as, electron microscope. Geochemical analyses on powdered rock samples were carried out using X-Ray Diffraction, X-Ray Fluorescence and isotopic analyses. 150 plug samples were studied for their porosity and permeability. Microscopic examinations for porosity were carried out for 122 thin section using image analyses.
The exposed top part of the Upper Dammam Formation can be divided into five lithological units arranged from bottom to top: Chalky Dolostone, Karst Zone, Karst Carapace, Weathered Zone and Residual Clastics. The rock has been affected by several stages of diagenetic processes; dolimtization, dissolution, cementation, replacement, recrystallization, and fracturing.
Porosity and permeability results vary in the above lithological units. The variation is vertical and horizontal along each lithological unit. The highest porosity and permeability measured are those in the Chalky Dolostone and Karst Zone. In the Chalky Dolostone, the porosity may reach as high as 53%, while highest permeability value is 6000 mD. The Karst Zone has higher porosity (78%) but lower permeability (1140 mD). The other three zones have lower porosities (30%) but very low permeability (average 50 mD).
Most of the pores are isolated of moldic and vuggy types. Pores created on the rock frame work are vuggy and fractures and are more connected. They were created at later stages as a result of meteoric water and/ or acidic gases produced during thermal maturation of the kerogen in the deep subsurface petroleum source rocks.