Diagenetic impact on porosity evaluation of middle-late Eocene carbonate aquifer (Pila Spi Formation): Insights from the Bjeel area,Iraqi Kurdistan Region

Abstract

The porosity evolution and diagenetic controls of the middle-late Eocene carbonate rocks of the Pila Spi Formation in the Khanaga section, Bjeel area, were investigated to understand their impact on aquifer potential and quality, which is an important issue for water resource management in carbonate aquifers. The formation, with an overall thickness of 37.5 m, is composed of fractured limestone, marly limestone, and dolomitic limestone, with thin beds of marl and mudstone characterised by karstic fissures and pores. The study is based on petrographic analysis of 25 thin sections, supported by quantitative porosity assessment and diagenetic interpretation. A petrographic analysis of thin sections from the Pila Spi Formation revealed that the matrix is predominantly micrite with minor microspar, containing skeletal grains indicative of a shallow marine environment. While non-skeletal grains are poloids, intraclasts, and monocrystalline quartz. Numerous diagenetic processes, such as micritisation, dolomitisation, physical compaction, solution, cementation, neomorphism, silicification, pyritisation, iron oxidation, and fracturing, influenced the formation. Five porosity types were identified in the Pila Spi carbonate, most of which are of secondary origin. They are fenestral and intraparticle porosities (primary) and moldic, vuggy, and fracture porosities (secondary). Quantitatively, secondary porosity constitutes particularly vuggy and moldic porosities that represent the most common types, with average values of 2.25% and 0.99%. Solution and fracturing processes enhanced porosity, particularly secondary types, in the Pila Spi Formation carbonates. Early dolomitisation also contributed to the development of fenestral porosity. In contrast, cementation, compaction, neomorphism, silicification, and pyritisation reduced porosity. Karstic fissures and pores are the main factors enhancing the aquifer potential of the formation, whereas iron oxidation negatively affects the groundwater quality. This study provides a comprehensive and quantitative framework linking diagenetic processes to porosity evolution, significantly improving the understanding of aquifer behaviour and groundwater quality in the Pila Spi Formation.