Depositional Environments and Diagenesis of the Lower Cretaceous (Neocomian) Fahliyan Formation in Bibi-Hakimeh, Khyrabad, Kilurkarim and Sulabedar Fields in the Zagros Basin, Southwest Iran

Forooz Keyvani¹, Ezat Heydari², and Alireza Rostami^1
1 NIOC, Teran, Iran
² Jackson State University, Jackson, MS

Detailed lithologic and petrographic analyses of the Fahliyan Formation in the Bibi Hakimeh, Khyrabad, Kilur Karim and Sulabedar field show depositional and diagenetic history on the Lower Cretaceous(Neocomian) strata of a part of Dezful South in the Zagros region. Observations of core and cuttings samples indicate that lower part of the Fahliyan Formation consists of stromatoporoid boundstone and skeletal grainstones, indicating deposition in a shallow, high energy platform. These high energy facies alternate with wackesone and mudstones suggestive of deposition below wave base. The mudstone dominated lithofaices increases upward and eventually grade into organic-rich shales of the overlying Gadvan Formation, indicative of deepening of the depositional environments in response to a relative rise in sea-level.

The Fahliyan Formation has experienced a complex diagenetic history that encompasses the following four diagenetic realms. (1) Marine diagenesis resulted in precipitation of circumgranular fibrous cements, early dolomite, and early anhydrite. (2) Passive margin diagenesis occurred during slow and continuous burial of the unit from Early Cretaceous to Oligocene and led to physical and chemical compaction, and the formation of mosaic and syntaxial calcite cement. (3) Collision margin burial diagenesis occurred with Africa collided with Eurasia during Oligocene – Middle Miocene time. Deep burial by overriding thrust blocks, heating, and migration of fluids toward the platform caused organic maturation and hydrocarbon migration. Combined, these resulted in a complex set of diagenetic changes that included precipitation of late anhydrite, saddle dolomite, quartz, replacement of anhydrite by saddle dolomite, and pyrite. (4) The post-orogenic burial diagenesis has been active since the Middle Miocene and encompasses the continuation of diagenetic processes that began during the collision margin phase.

The reservoir porosity in the Fahliyan Formation is dominated by between crystal(BC)porosity resulted from recrytalisation and dolomitisation , and fractures which possibly enhanced by late dissolution.