Petrography of calcretes and their effects on the hydrology of Kuwait group aquifer

The petrography of the calcretized sediments of Kuwait Group (Mio-Pleistocene) in the state of Kuwait was investigated using core and drill cuttings samples from water wells. Seven major-original sediment types, namely, muddy sand, gravelly muddy sand, granular sand, sand, sandy mud, muddy sandy gravel and mud, were reorganized following the classification scheme proposed by Folk (1974). Muddy sand was the most dominant type in the studied samples. The precipitation of the authigenic calcite within the pore spaces and its gradual replacement of the original framework minerals were responsible for the consolidation of these friable elastic sediments and their alteration to hard calcareous rocks. Six stages of progressive calcretization were identified, giving rise to incipient calcrete, nodular calcrete, massive calcrete, concretionary calcrete, and chalky calcrete with the increase in the intensity of calcretization. The variation in the height of the groundwater levels, variation in concentrations of calcium and carbonate in the infiltrated water, and climatic changes with time, have given rise to a verticle sequence of clastic sediments at different stages of calcretization. Calcretization has led to reduction in porosity and permeability of clastic sediments, adversely affecting the infiltration rates, and creation of potentiometric head differences across the calcretic layers that act as aquitards.