Characterization of rudist-dominated units as potential reservoirs in the middle Cretaceous Sarvak Formation, SW Iran

Abstract

The Sarvak reservoirs are characterized by thick rudist-dominated intervals in the south and southwest Iran. During the middle Cretaceous, rudistid communities were widely developed on the shallow parts of the Sarvak platform, mainly in the central Zagros and eastern Persian Gulf. Regional distribution and subsequent diagenesis of the rudist-dominated facies were strongly controlled by a number of paleostructures (i.e., fault-related paleohighs and salt domes) in the area. Facies analysis in 16 Sarvak reservoirs reveals the rudistid units can be classified into three main facies groups, developing thick depositional cycles in the Sarvak Formation. Integrated petrographic and geochemical data shows the rudist-dominated facies were subjected to fresh water diagenesis caused by a considerable drop in relative sea level after the deposition. Subsequently, where they were subaerially exposed over the crests and flanks of the paleohighs, the initial porosity of the rudist facies was enhanced by extensive meteoric dissolution (types A and B). Farther away from the paleohighs, same facies have poor reservoir quality, because the porosity was reduced by meteoric and shallow to deep burial cements, and mechanical/chemical compaction (type C). Despite their deep burial depth, significant amounts of porosity (>10 %) are still preserved in the rudist-dominated facies of the Sarvak Formation, especially below disconformity surfaces. Results of this study reveal the controls of early diagenesis on later diagenetic modification and porosity evolution in the Sarvak Formation, as the second important oil-producing reservoir of Iran.