DIAGENESIS OF NON-MARINE PETROLEUM RESERVOIRS: THE NEOCOMIAN (LOWER CRETACEOUS) SHURIJEH FORMATION, KOPET-DAGH BASIN, NE IRAN

Abstract

Detailed petrologic, sedimentologic and geohistory analyses have been carried out on the<br><br>Neocomian (Lower Cretaceous) Shurijeh Formation in the eastern part of the Kopet-Dagh Basin, NE<br><br>Iran. The results of these analyses show that the diagenetic history of Shurijeh sandstones was<br><br>affected by their depositional settings, and the subsequent subsidence of these units through meteoric<br><br>and compactional hydrologic regimes in this part of the Kopet-Dagh Basin. These rocks consist<br><br>mostly of sub-litharenitic redbeds deposited during a regressive phase of sedimentation dominated by<br><br>rapid siliciclastic sediment supply. The lower and middle portions of the interval studied were<br><br>deposited in low-sinuosity braided fluvial systems, and the upper portion was deposited in highsinuosity<br><br>meandering systems.<br><br>The porous and permeable sand-rich units were partially filled by infiltrated clays, and early<br><br>calcite, anhydrite and silica cements as they subsided through the meteoric regime. In the<br><br>compactional regime, feldspars altered to clays, dolomite replaced calcite and penetrated authigenic<br><br>silica, and a late-stage of calcite cementation and replacement took place. These diagenetic events<br><br>were followed by partial dissolution of early and late-stage calcite cements that created secondary<br><br>porosity in the sandstone units. During the late Miocene—early Pliocene (approximately 10 million<br><br>years), the Late Alpine Orogeny resulted in the formation of structural traps in the Shurijeh<br><br>Formation of the Sarakhs area. This was followed by migration of gas into the porous units of this<br><br>formation, accounting for the Khangiran and Gonbadli gasfields.<br><br>Relationships revealed in this study can be used to develop diagenetic models that will help<br><br>predict diagenetic styles in dominantly non-marine sandstones in other basins similar to the Kopet-<br><br>Dagh of Iran