PETROGRAPHY AND DIAGENESIS OF UPPER CRETACEOUS CORE SAMPLES IN MERSA MATRUH WELL NO.1, North WESTERN DESERT EGYPT. IBRAHIM, M.M. HASSANEIN Egyptian Petroleum Research Institute, Nasr City, Cairo, Egypt. The present study includes petrographic studies of some core samples representing different facies of Upper Cretaceous sediments. (Lower Cenomanian - Upper Turonian) from Mersa Matruh well no.1, North Western Desert. The main sandstone microfacies recorded are quartz arenites and quartz wackes. While the recorded carbonate microfacies include biomicrite, oobiosparite, sandy dolostone, dolomicrite and chalky micrite. The sandstone and carbonate rocks were subjected to several diagenetic processes; these are: compaction and pressure solution, cementation, recrystallization, replacement and dolomitization. Other diagenetic features recorded by the SEM, include quartz overgrowths and compaction of clayey sandstone producing tight and more compact aggregates. On the other hand, secondary porosity was created in some rock samples of the Cenomanian sandstones and limestones giving rise to an effective porosity and good reservoir quality.