Kupal oil field is located at 50 km NE of Ahwaz and following follows the Zagros trend. Every unpermeable rock or bed that covers the reservoir is called cap-rock. Member 1 of the Gachsaran Formation as cap-rock of the Asmari reservoir, it has 6 key beds namely, A, B, C, D, E, F (top to base) that alternate with anhydrite and marl. GR well logs with stratigraphical column are designed by logplot 2003, applied for petrophysical evaluation of cap-rock. To get more information, the cut selected samples were run through SEM (Leo1455vp). The results of changed GR well logs of selected wells, thin sections petrography and SEM analysis, show that key beds lithologicaly are formed from A(anhydrite), B (bitumineous shale) and C, D, E, F (limestone). In a few wells, some clay was added to limestone and caused the API degree to increase. Spectrographic studies of sulphates between key beds show that anhydritization, cementation, compaction, recrystalization and replacing are the common diagenetic processes in cap-rock occurrence. Anhydrite is found more than the other rocks in the cap-rock. From NW to SE of the field, the cap-rock thickness is reduced.