The role of cementation and compaction in porosity of Middle Jurassic sandstones, along the outcrop belt, northeast Iran



The siliciclastic Kashafrud Formation (Middle Jurassic) of the Kope-Dagh Basin rests unconformably on Triassic volcanogenic sedimentary rocks (Sina Formation), and nonconformably on older ultramabasic intrusive rocks in study area. This formation is disconformably overlain by marine carbonates of the Mozduran Formation (Oxfordian-Kimmeridgian). Based on field and laboratories studies, 3 facies association (12 lithofacies) have been identifies which were deposited in a fluvio-deltaic and turbities environments. The sandstones are mostly lithicarkose and feldespatic litharenite, rich in sedimentary and volcanic rock fragments. The main identified cements are Fe-oxide, carbonates (calcite, dolomite, siderite and ankerite), silica, clay minerals (kaolinte, illite, serecite and mostly cholorite), pyrite and rarely barite. Therefore cementation and compaction (mechanical and chemical) could be two factors to reduce the porosities in this formation. Based on petrological and geochemical studies, we interpreted the diagenetic history for the Kashafrud sandstones, which consists of early, deep burial and late stages.