The Kashafrud Formation, located within the Khangiran gas field of northeast Iran, has drawn the attention of geologists and hydrocarbon specialists alike due to the complex nature of its clay mineral composition. A recent study has shed light on the type, abundance, and distribution of these clay minerals, offering new insights relevant to petroleum exploration.
Significantly, the study reveals quartz as the dominant mineral present, comprising between 49% and 65% of the overall composition. Accompanying this are several clay minerals: illite appears at 8% to 18%, kaolinite makes up 2% to 6%, and chlorite spans 3% to 11%. Other notable minerals include alkali feldspar (7% to 16%) and plagioclase (3% to 10%). The identification was based on rigorous laboratory tests, including powder X-ray diffraction (XRD), scanning electron microscopy (SEM), and polarizing light microscopy, which together paint a clearer picture of this complex reservoir structure.
The research team, led by P.N. Ardebili and colleagues, focused on ten core samples extracted from well #3, which was drilled down to the Kashafrud Formation, reaching a drilled thickness of 433 meters. They found the average weight percentages of clay minerals to be kaolinite at 4.3%, chlorite at 7.4%, and illite at 11.1%. These findings contribute to the formation's overall clay mineral content of 23.7%, raising concerns about the potential for formation damage due to the presence of these minerals.
Of note is the influential role of clay minerals on reservoir qualities, as the study pointed out, emphasizing the need for extensive analysis of clay occurrence in hydrocarbon-rich formations. “Even when present only in small quantities, clay minerals can exert significant impacts on reservoir characteristics, including porosity and permeability,” the authors noted. Their findings reflect not only on the mineralogical composition but also on the geochemical and physical processes affecting hydrocarbon generation.
The geological setup of the Khangiran gas field is equally compelling. Located approximately 180 kilometers northeast of Mashhad and 25 kilometers northwest of Sarakhs, this region displays unique tectonic features due to the collision of the Iran Microplate and the Laurasian plates. The Kashafrud Formation is the latest drilled formation within this gas field, known historically for its hydrocarbon potential. According to Joe Ardebili, “The Kashafrud Formation serves as one of the main sources of gas, with stratigraphic studies confirming its viability as a reservoir.”
The testing procedures employed for this research were methodical. XRD analysis was performed at angles from 4 to 70° 2θ using the advanced Bruker D8 diffractometer, providing comprehensive mineral identification across the selected core samples. Within the survey’s sample analysis, extensive examination through polarizing light microscopy revealed the composition primarily consisted of quartz and feldspar grains, classified as arkose—a type of sandstone containing more than 25% feldspar. This classification highlights the potential complexity of extracting hydrocarbons due to the variable reservoir quality.
Further, the SEM analysis brought to light the microstructural aspects of clay minerals. Observations revealed the presence of kaolinite, illite, and chlorite forming concrete-like structures, predominantly showing pore-filling patterns. “The morphology of these minerals, particularly the fibrous nature of illite, critically impacts fluid permeability,” explained one of the researchers. These findings are pivotal as they address the challenges engineers face when attempting to optimize hydrocarbon extraction under varying geological conditions.
Another finding discussed by the authors is related to the average size of the clay particles. With sizes ranging from 1.4 µm to 13.7 µm, variations exist both within and among samples, drawing attention to the diagenetic processes at play. Understanding the impact of these size differences on fluid migration and retention is indispensable, especially considering the reservoir's complex behavior during extraction.
The study concludes with significant insights about the Kashafrud Formation's reservoir potential. The clay mineralogy not only determines reservoir quality and behavior but also impacts the geochemical pathways of hydrocarbon generation. The comprehensive analysis by Ardebili et al. is expected to serve as a benchmark for future studies on clay mineralogy related to petroleum geology, paving the way for more effective extraction techniques across similar geological formations worldwide.