Controls of depositional facies and diagenetic processes on reservoir quality of the Santonian carbonate sequences (Ilam Formation) in the Abadan Plain, Iran

Abstract

Ilam Formation (Santonian) is a member of the Bangestan Group, as an important oil reservoir of SW Iran. Various geological and reservoir quality aspects of this formation were discussed in the Zagros area. However, depositional–diagenetic controls on pore types of this formation are not defined, especially in the Abadan Plain. In this study, an integrated sedimentological and petrophysical evaluation has been carried out in six subsurface sections of this formation throughout the Abadan Plain. Facies analysis indicates deposition in shallow- to deep marine settings of a ramp-like carbonate platform. It includes lagoon, shoal, middle ramp, outer ramp and basinal settings. Various diagenetic alterations have been recorded including carbonate cementation, dissolution, mechanical to chemical compaction and fracturing. They indicate marine to meteoric and shallow- to deep burial diagenetic realms for the Ilam Fm. Controls of depositional and diagenetic processes on reservoir quality and pore types have been evaluated by integrating well logs and porosity–permeability data with petrographic evidence in sequence stratigraphic framework. Accordingly, a higher-order and three lower-order sequences have been defined and correlated across the study area. Various pore types of the reservoir are differentiated and classified into five classes: interparticle, intra-particle/skeletal, vuggy/moldic, fractures and micropores. Accordingly, two reservoir intervals are defined in the Ilam formation that include the following: (1) microporous mud-dominated reservoir in TST of higher-order sequence, in the lower Ilam, and (2) macroporous grain-supported facies in RST of higher-order sequence, in upper part of the Ilam formation.