The presence of clay minerals of any type, amount or distribution pattern in hydrocarbon wells causes numerous problems in the formation evaluation. In this research, 15 core samples from Asmari Formation in the Maroun Oilfield were selected in order to study the type and distribution pattern of clay minerals by laboratory investigations. The XRD and microscopic results showed the studied samples consisted mainly of quartz (14.7-72.2%) and carbonate minerals (3.0-65.4%) as main constituent phases; while the plagioclase (0.0-6.7%) and clay minerals (3.3–44.5%) were identified as the main accessory minerals. In some samples, sulfide and ferrous minerals (0.0-2%) were also identified. The calculated percentage of illite in mixed-layer smectite/illite showed the diagenesis has occurred at different depths of reservoir. The SEM/EDX analysis performed on various types of clay minerals showed that kaolinite size varied from 0.7 to 6.5 μm in studied samples. In addition, Illite size ranged in our studied samples from 0.4 to 3.6 μm. Our results indicate that the clays in the Asmari Formation occur in three main patterns as dispersed, pore-bridging and pore-filling with the variation of the total amount of clay minerals, min 3.3% and max 44.5% across the length of Asmari Formation.