Geologic features and genesis of the barriers and intercalations in carbonates: A case study of the Cretaceous Mishrif Formation, West Qurna oil field, Iraq

The type, poroperm relationship, log response characteristics and identification criteria of the barriers and intercalations in the carbonates of the Cretaceous Mishrif Formation in the West Qurna oil field of Iraq are studied through comprehensive analysis of cores, thin sections and well logs. The genesis and distribution of the barriers and intercalations are analyzed from the perspectives of sequence stratigraphy, depositional facies and diagenesis. The barriers and intercalations can be classified into three types: grainstone, packstone and wackestone. The barriers and intercalations generally exist in restricted platform facies and evaporative platform facies, some in open platform. They are common in transgressive cycles and early regressive cycles, forming continuously, extensive barriers and intercalations near the sequence boundaries. Penecontemporaneous cementation, burial compaction, and burial cementation led to the decrease of the porosity, damaged the space of the reservoir pores and became the important factors for the genesis of the barriers and intercalations. In the epidiagenetic phase, a large number of CaCO3 precipitated in the phreatic water zone below free-water table, leading to the formation of packstone and grainstone barriers and intercalations in a large scale. Through comprehensive log analysis, log identification criteria and the distribution of the barriers and intercalations are determined. The barriers are mainly distributed in the sections of CRI and CRII, and the intercalations are concentrated in the section of mB1. For different types of barriers and intercalations, packstones are the most in quantity, followed by grainstones and wackestones.