Microporosity evolution in Iranian reservoirs, Dalan and Dariyan formations, the central Persian Gulf

Abstract

Tight limestones with high amounts of microporosity will be the main targets of future hydrocarbon exploration and production in Middle East. In this paper, microporosity evolution in two Iranian reservoirs, Dalan and Dariyan formations, is investigated. These two formations represent the main hydrocarbon reservoirs in most of the Iranian hydrocarbon fields. Aptian Dariyan Formation is mainly composed of mud-dominated samples deposited in a ramp environment. The upper part of Permian Dalan Formation has the same facies distribution while the lower part is composed mainly of grain-dominated samples. Mud-dominated facies of Dariyan Formation have high porosity with moderate to low permeability. Micrites are rounded with punctic to serrate contacts. In contrast, mud-dominated facies of Dalan Formation have low porosity and permeability. SEM studies revealed that micrite texture has a major effect on final reservoir properties of these two formations. MICP and SEM studies showed that in grain-dominated samples of Dalan Formation, both small and large pore size throats are present. Mud-dominated samples of this formation are composed of micrites with compact-anhedral shape and coalescent contacts. These results combined with diagenetic studies confirmed the role of meteoric dissolution in microporosity development in both formations. In humid conditions of the Aptian time, micrites have been dissolved and a large amounts of microporosity have been developed. In contrast, in dry conditions of Late Permian, the original fabric of the micrites preserved until the late stages of burial. Rounded micrites are present in Dalan Formation where meteoric dissolution had a major effect on porosity development in this formation.