The characterization of clay minerals in the Kashafrud Formation in a gas field in northeast Iran.

This study focuses on the identification of type, abundance, and distribution patterns of the clay minerals within the Kashafrud Formation, a Middle Jurassic sedimentary rock sequence in the Khangiran gas field, Kopet-Dagh Basin, northeast Iran. This investigation shed light on the identification of type, abundance, and distribution patterns of the clay minerals obtained from 10 representative core samples by employing laboratory techniques, such as powder X-ray diffraction, polarizing light microscopy, and scanning electron microscopy. Results from the XRD analysis revealed quartz as the dominant mineral (49-65%), accompanied by the clay minerals (illite 8-18%, kaolinite 2-6%, chlorite 3-11%), alkali feldspar (7-16%), and plagioclase (3-10%), with occasional presence of ankerite (2-7%) or pyrite (1-3%). Polarizing light microscopy indicated a composition mainly of quartz and feldspar grains, categorized as arkose. Furthermore, the SEM studies highlighted that the pore filling pattern as the prevailing distribution pattern of clay minerals with occasional instances of pore coating pattern. The clay portion of each sample was separated and analyzed by an electron microscope device that identified silicon, oxygen, and aluminum as the primary elements. Furthermore, the findings of this study showed the average weight percentages of clay minerals, including kaolinite, chlorite, and illite in the mentioned formation were 4.3%, 7.4%, and 11.1%, respectively, contributing to an overall clay mineral content of 23.7%. The presence of illite, kaolinite, and chlorite could lead to a significant formation damage. Eventually, the observed distribution pattern and clay mineral content suggest the Kashafrud Formation leans towards a predominantly shaly-sand composition.