Source and diagenesis of Middle Jurassic marine mudstones, Kopet - Dagh Basin, NE Iran

Middle Jurassic fluvio - deltaic and turbiditic mudstones of the Kashafrud Formation are important hydrocarbon sources in the gas - rich Kopet - Dagh Basin, northeastern Iran. Clay mineral assemblages are important for the interpretation of sediment provenance and for understanding burial diagenetic cementation in sandstones. The clay mineral assemblages in fluvio - deltaic and turbiditic mudrocks in two areas, Saleh - Abad and Senjedak, were investigated by X - ray diffraction analysis. A total of 28 bulk samples (half of which were taken from Saleh - Abad and the other half from Senjedak) were analysed and from 11 of these samples the < 2 ?m fraction was separated and analysed. The clay minerals identified are mostly kaolinite, illite, muscovite, chlorite, mixed - layer illite - chlorite, and very rarely smectite. Clay mineral assemblages result partly from variable det rital supply and partly from burial diagenesis. Detrital muscovite is more abundant at higher stratigraphic levels. Detrital smectite was rare; the lack of detectable illite - smectite mixed layer clays indicates that low smectite abundance is not the result of illitization. Vitrinite reflectance suggests burial to depths of only 2 – 3 km. Burial diagenesis resulted in more chlorite and more ordered mixed - layer chlorite - illite at greater burial depths. The source area for the basin was a rapidly rising area th at included illitic shales, overlying crystalline basement of granite, and low - grade metamorphic rocks, which supplied muscovite and chlorite