Inorganic control on original carbonate mineralogy and creation of gas reservoir of the Upper Jurassic carbonates in the Kopet-Dagh Basin, NE, Iran

The Upper Jurassic Mozduran Formation (Oxfordian–Tithonian) is the main petroleum reservoir in the Kopet-Dagh Basin, northeast Iran, which consists predominantly of carbonate rocks with subordinate evaporites and siliciclastics deposited in platform to deep marine settings of a subtropical sea. Detailed field surveys, petrographic investigations, facies and wire line log analyses were carried out at eight surface sections and four wells in the Kopet-Dagh Basin. Integration of petrographic and isotopic data suggests primary low-Mg calcite mineralogy of Oxfordian and Tithonian ooids. On the other hand, in the wells, Kimmeridgian ooids and cements are dominantly aragonitic and high-Mg calcite mineralogy (HMC). Marine cements with isopachous, fibrous and isopachous bladed fabrics indicate original aragonite and HMC mineralogy, respectively. The domination of aragonite mineralogy could be related to increased hypersalinity, evaporite precipitation and consequently an increase in Mg/Ca ratio, which resulted in formation of aragonite in preference to calcite. Preserved ooids with radial and concentric cortices in shallow-water settings that are nearby siliciclastic source, together with aragonitic and HMC ooids accompanied by evaporites in the drilled fields, suggest original mineralogy was probably controlled inorganically following local salinity variations. This study suggests that Kimmeridgian pay zones are mainly controlled by depositional facies, aragonitic and HMC mineralogy, and diagenetic processes such as dolomitization and dissolution.