Diagenetic history of the Upper Jurassic sediments in easternmost part of the Kopet-Dagh Basin, NE Iran

The Kopet-Dagh Basin is located in northeastern Iran and southwestern Turkmenistan. It is believed that this basin may provide a significant petroleum reserves in the 21th century. The Upper Jurassic Mozduran Formation (Oxfordian-Kimmeridgian) is the most important gas-bearing reservoir in this basin and is mainly composed of limestone, dolomite and several calcareous shale and gypsum interbeds. Eight measured stratigraphic sections and petrography studies of 800 thin sections showed that the carbonate sediments of this interval have been deposited in a platform of ramp type including open marine, shoal, lagoon and tidal flat environments. The petrographic analysis also indicates that the Mozduran Formation carbonates have undergone a complex diagenetic history which includes compaction, cementation, micritization, dissolution, silicification, dolomitization, neomorphism and fracturing. Carbon and oxygen isotopes as well as trace elemental (Mn, Na, Fe, Sr) analysis of 30 samples are also used for interpretation of diagenetic processes. δ18O and δ13C values in Mozduran Formation limestones in the study area range between -1.50 to -11.86 PDB and 5.56 to -5.04 PDB, respectively. These variations are interpreted to reflect meteoric and burial diagenetic processes. Variations in trace elements concentration (Fe and Mn increased while Na decreased) also indicate the meteoric flushing. The paragenetic sequence indicates that the primary porosity decreased during early stages of diagenesis, while the secondary porosity was subsequently created and improved the reservoir quality of the carbonate rocks of the Mozduran Formation.