Petrography, diagenesis and geochemistry of the Cambro-Ordovician Khabour sandstones, western Iraq: Implications for reservoir quality and the impact of the Hirnantian glaciation

Deeply buried (2.3–3.5 km) sandstones of the Cambro-Ordovician Khabour Formation are important hydrocarbon reservoirs in western Iraq. An integrated approach incorporating petrography, scanning electron microscope (SEM) and X-ray diffraction (XRD) was applied to investigate diagenesis and diagenetic minerals and their impacts on reservoir quality. Major and trace element distribution using X-Ray Fluorescence (XRF) analysis also was carried out to identify sandstones deposition during or after the Hirnantian glaciation. The sandstones of the Khabour Formation are dominated by sublitharenites and subarkoses. Several diagenetic processes have affected the sandstones of the Khabour Formation. These include early diagenetic porosity occluding events, such as common formation of illite and sericite coatings, along with rare quartz overgrowths, and calcite cementation. This was followed by secondary porosity forming events which include calcite dissolution, leaching of feldspar, alteration of micas and fracturing. The pore systems are dominated by secondary pores, vuggy and fractured porosity, intergranular and intragranular pores and poorly-connected macropores. A decrease in heavy minerals in sandstones from the upper part of the Khabour Formation and an associated decrease in both Zr and Hf concentration, which are typically associated with zircon, supported by a decrease in mica content and low API gamma ray unit values are considered to be a signature of sandstones affected by the Hirnantian glaciation.