Diagenesis, hydrocarbon migration and MVT mineralization in Cretaceous carbonates of west central Iran

摘要

A series of Mississippi Valley-type deposits occur within shallow marine dolomitized carbonate platform of Lower Cretaceous age along the Sanandaj Sirjan zone and the Zagros orogenic belt in west-central Iran. Evidences for presence of hydrocarbons in those deposits are provided by numerous indications including bituminous impregnation of carbonate rocks along stylolites. Hydrocarbon is also associated with saddle dolomite which occurs as matrix of breccia with sulphides. Saddle dolomites show ~(87)Sr/~(86)Sr ratios enriched in ~(87)Sr, which is explained by extensive interaction of fluids with underlying shale. Depleted δ~(13)C (ranging to -6.4 per thousand PDB) of saddle and fracture-filling dolomites indicate thermal decomposition of organic sulphur in petroleum. Similarity in the δ~(34)S data of sulphide minerals from the deposits at Irankuh and oil from the Lower Cretaceous reservoirs (-6.2+-1.4 per thousand) indicate that sulphur may have been derived from oil generated from the shale. Brines responsible for dolomitization and MVT mineralization were expelled from a basin aligned parallel to the Zagros fold belt and migrated along reactivated faults and permeable zones during the Late Cretaceous convergence of the Arabian and Iranian plates.