Reservoir quality and micrite textures of microporous intervals in the Upper Cretaceous successions in the Zagros area, SW Iran

Abstract

Here the reservoir potential of microporous intervals is evaluated in the Upper Cretaceous successions in the Zagros area of SW Iran. The upper Sarvak and Ilam formations were studied in 10 fields and subdivided into three third-order sequences. Geochemical and petrographic data imply meteoric alteration of the Sarvak sequences in an open diagenetic system while denote burial diagenesis in a closed system for the Ilam sequence. Scanning electron microscopy of micrite led to the identification of six different micrite textural classes (MTC1–6) which occur predictably in the identified sequence stratigraphic framework and specific digenetic systems. Porous MTCs (1–2, porosity up to 32%) are characterized by dissolved and rounded micrite particles, and they are mostly found beneath the subaerial exposures in regressive systems tracts of the Sarvak Fm. Intermediate MTCs (3–4, porosity up to 20%) exhibit rhombic to subhedral micrite particles and occur in sporadic stratigraphic positions. Tight MTCs (5–6, porosity up to 6%) are fused and compacted micrite texture that occur far from the subaerial exposure in mud-dominated facies of the Sarvak Fm. Porous MTCs are very rare and Intermediate MTCs are the main micrite textures in the Ilam Fm. The distribution of MTCs is controlled by the stratigraphic distance from subaerial exposures and the dominant facies (mud vs. grain dominated) which limited the availability of meteoric water and feasibility of fluid circulation. Reservoir properties data show that mud-dominated intervals of the Sarvak and Ilam formations can be considered as microporous reservoirs with high (up to 32%) porosity and variable (<0.01 to >1mD) permeability.