The Berriasian-Valanginian Fahliyan Formation forms one of the giant reservoirs in the subsurface of the Abadan plain, onshore Iran. A detailed petrographical analysis of the available cores and thin sections revealed that the different diagenetic parameters influenced the reservoir quality of the Fahliyan Formation in this field. The Fahliyan Formation has been influenced by three diagenetic environments, including marine, meteoric and shallow and deep burial environments. The main diagenetic parameters identified in the field under study are dissolution, fracturing, cementation, compaction and dolomitization. Among all, dissolution is the main diagenetic feature improving porosity and reservoir quality. This feature formed as a result of meteoric diagenesis during subearially exposure of the Fahliyan sediments. Fracturing and dolomitization also locally have positive effects on reservoir quality, while compaction, cementation and dolomitization (as cement) have destructive effect on reservoir characteristics. Late stage diagenetic cements such as sparry calcite cement and with lower amount saddle dolomite are the most important and also widespread types of cement decreasing reservoir quality. Based on new genetic classification of porosity, porosity in the Fahliyan Formation are hybrids of three depositional, diagenetic and fracturing, but diagenetic porosity is the most important types of porosity and so Fahliyan reservoir is a type of diagenetic reservoir. Based on this study using petrophysical and petrographical data, the Fahliyan reservoir is not a homogeneous reservoir, so it was divided into eight reservoir zones with different specifications.