Organic Geochemistry of the Late Triassic Nayband Formation at the Parvadeh area, Tabas, East-Central Iran

The Upper Triassic Nayband Formation is widely exposed in the Tabas subzone, East Central Iran. This formation is 1410 meters thick, mainly comprises sandstone, sandy limestone, marl, coalificated shale (Coal seam) and dark shale in Parvadeh area. This study tries to reconstruct the hydrocarbon production potentiality, organic geochemistry, kerogen type, thermal maturity and recognition of depositional condition of the Nayband Formation. Totally 86 samples (coal seams and dark shales) were collected. These samples were analyzed by Rock-Eval type VI. Geochemical analysis results revealed that TOC content is more than 4 Wt% in the coalificated shales and in the dark shales is less than 4 Wt%. Parameters such as S1, S2, S1 S2 suggest poor hydrocarbon production. Other parameters such as HI suggest poor hydrocarbon production potential (coalificated shales) to gas prone source rock (dark shales). Results also revealed that the organic matter mainly contain type IV kerogen in the coalificated shales and predominately comprises kerogen types III and IV in the dark shales. Vitrinite reflectance of the coalificated shales (0.84-1.12 %) indicate that the coalificated shales are at the mature stage (enrichment of oxidized organic matter indicates poor hydrocarbon production). Organic facies of this rock unit indicates that these sediments predominately comprises organic facies types D and CD which implying highly oxic to oxic conditions during the deposition. Based on HI and TOC parameters, major of these sediments were deposited during relative sea-level fall.