Pore-type characterization of the Shurijeh formation in the Kopet-Dagh Basin, Northeastern Iran, using the integration of petrographic data, well logs, and production data

The Early Cretaceous (Neocomian-Barremian) Formation of the Shurijeh is located in the northeast of Iran, east of the Caspian Sea and consists of predominantly red detrital clastic and carbonate facies extending into the Kopet-Dagh Basin. The main purposes of this paper are determination and classification of pore types, analysis of static and dynamic behavior of each pore type class. Integration of the results from petrographic studies, well logs, image logs, and production data is used as the main approach for such purposes. Based on petrographic studies, nine petrofacies including sandy conglomerate, quartz-arenite, sub-litharenite, sub-arkose, quartz wacke, claystone/shale, nodular anhydrite, sandy dolomudstone, and dolo-ooid grainstone, were identified. The main diagenetic processes include compaction, cementation, bioturbation, primary dolomitization, and anhydritization. Furthermore, the pore types of the Shurijeh Formation were investigated by using an integrated study of thin-sections, velocity deviation log, results from interpretation of image logs, mud loss, and production data. The results allowed identifying negative, zero, and positive deviations based on the created synthetic velocity log. The study defines three deviation classes (positive, zero, and negative) and links them to reservoir quality. These classifications are supported by image log data and core observations. The results indicate that the reservoir quality of the Shurijeh Formation was affected by the sedimentary and diagenetic processes. Detecting and understanding the pore types as one of the crucial attributes of the porous media based on geophysical logs is an essential step in reservoir properties mapping. The results of this study provide a reasonable finding regarding the very poor contributions of some porous gas-saturated zones in the production due to the insufficient pore connectivity and isolated porosities. The results indicate a good correlation between gas production rate with mud loss data, image log derivative fractures, pore types, and the Velocity Deviation Log (VDL).