Porosity Evolution and Microfacies Analysis of the Khurmala Formation (Paleocene-Early Eocene) from Selected Sections in Kurdistan Region, Northern Iraq Revealed by Cathodoluminescence Spectroscopy

Abstract

The microfacies analysis and porosity evolution of the Khurmala Formation (Paleocene-Early Eocene) were studied in the Gomaspan and Sheraswar sections in the High Folded Zone, northeastern Erbil City, Kurdistan Region (N-Iraq). Lithologically, the Khurmala Formation in the Gomaspan section is about 23.5 m comprised of thick to massive bedded brecciated grey dolomitic limestone in lower and middle parts and interbedding of medium to thick beds of grey dolomitic limestone and yellow calcareous shale in the upper part. The Sheraswar section comprises 18.5 m yellow marl, thick yellowish-grey dolomitic marly limestone in the lower part, and bedded grey limestone interbedded with thin blue marl, shale, and mudstone in the upper part. The petrographic and cathodoluminescence (CL) studies of 20 thin sections of the Khurmala carbonates show that most skeletal grains are shallow marine derivative faunas and non-skeletal grains represent intraclasts, peloids, ooids, and extraclasts (monocrystalline quartz). Five main microfacies were identified in the studied rocks, and they integrated into two facies associations relating to their environmental interpretation: back reef/lagoon and patch reef. A new eight pore types were distinguished in the carbonate rocks of Khurmala Formation: interparticle, intraparticle, growth framework, intercrystalline, moldic, vuggy, fracture, and stylolite porosities. Most of the primary porosity was reduced by compaction and cementation due to effect of intensive diagenesis. Secondary porosity is predominant in both studied sections, the most common being the moldic porosity associated with different microfacies fabrics. CL investigations revealed that dolomitized lime wackestone microfacies has two types of different cementations in the late stages of blocky calcite cement which are discontinuous light to dull orange luminescence without zoning, and pure calcite cement of coarse to very coarse blocky calcite cement with characteristic bright orange luminescence and tight zoning.