Identification of pore facies for evaluating the reservoir quality: A case study of the Hartha Formation in X Oilfield, Mesopotamian Basin, Iraq

Abstract

Characterization of pore systems is a key to better understand the heterogeneity of reservoir quality. However, the complicated pore types and pore facies of carbonates in the Hartha Formation of X Oilfield remain poorly understood. In this study, we recognized pore types and explored the impact of depositional environment and diagenetic alterations on pore facies and reservoir heterogeneity. Six types and nine subtypes of pore facies displaying various physical properties and fossils are classified based on the proportion of depositional pores, fabric‐ and non‐fabric‐selective pores. The distribution of pore facies are controlled by the interaction between depositional environment and diagenesis: (i) the depositional environment has an impact on the primary pore types mainly through different lithology and fossils. Micro‐interparticle pores and intraparticle pores of foraminifera are dominated in lagoon, middle ramp and outer ramp, while interparticle pores between grains are dominated in rudist shoal. Rocks deposited in medium‐ to low‐energy shoal contain interparticle pores between grains, micro‐interparticle pores and intraparticle pores of foraminifera. (ii) Major diagenetic events that modify pore types include micritization, fabric‐selective dissolution, cementation, compaction and non‐fabric‐selective dissolution. Micritization facilitates the development of micro‐interparticle pores, while compaction can cause the loss of interparticle pores between grains and micro‐interparticle pores. Cementation destroys not only interparticle pores between grains but also intraparticle pores of foraminifera. Fabric‐ and non‐fabric‐selective dissolution are the main causes for increasing the volume of pores. (iii) The intensity of the diagenetic event is linked to rock composition, rock fabric and palaeotopography that are controlled by depositional environment. Thus, the distribution of pore facies related to the environment is established, which can provide insights for areas lacking cores and adjacent fields with similar settings.