Unconventional hydrocarbon resource assessment for the silurian shales in Diyarbakır Basin, Southeastern Türkiye

Abstract

This thesis investigates the unconventional hydrocarbon generation potential of the Silurian-aged Dadaş shales in the Diyarbakır Basin, located in the Southeastern Anatolia Region of Turkiye. The Silurian-aged Dadaş Formation, like other Silurian shales in North Africa and the Middle East, presents significant potential for both conventional and unconventional hydrocarbon exploration. The Diyarbakır Basin remains relatively underexplored and offers untapped potential that requires further research. In the Paleozoic petroleum system, the Dadaş Formation has been identified as the source rock, while the Hazro and Bedinan sandstone formations have been identified as the reservoir rocks. This study focuses on the thermal maturity and hydrocarbon generation histories of the Silurian Dadaş shales, utilizing a basin modeling approach. In the Diyarbakır Basin, the Dadaş Formation reaches up to 400 meters in thickness, with an average thickness of 70 meters. The Dadaş Formation is divided into three members. The most organic-rich source rock is the lower member, Dadaş-I Formation. This formation covers an area of 12,000 km². The Dadaş-I Formation ranges in thickness from 200 meters and has an average thickness of 30-40 meters. According to geochemical analysis results from previous studies, the total organic carbon (TOC) content of the Dadaş shales is on average 5.9% by weight, though in some regions this value reaches up to 16.68% by weight, thus categorizing it as "hot shale." The kerogen type in the Dadaş hot shale is classified as Type-II, which is highly productive in terms of organic matter, indicating a high potential for oil generation. Additionally, the Hydrogen Index (HI) value ranges from 36 to 510 mg HC/g TOC with maturation. The pyrolysis Tmax values greater than 435°C in the Dadaş shales correspond to vitrinite reflectance values within the oil window. Given the global energy demand and the need for new resources, this research is crucial as it combines geological, geochemical, and basin modeling data to assess unconventional hydrocarbon potential. The basin model was calibrated using data from previous studies and created using PetroMod software (Schlumberger). The burial, temperature, thermal maturation, and hydrocarbon generation processes of the Dadaş hot shale were examined using 2D basin modeling techniques. The model was calibrated with data from four exploration wells in the region. The calibrated model results indicate that the Dadaş hot shales are within the oil generation window and that the main hydrocarbon generation occurred before the Alpine orogeny (uplift and erosion) phase. The modeling results provide insight into the burial history, maturity history, hydrocarbon generation history, and hydrocarbon content of the Silurian-aged Dadaş shales in the region. Calculations were made based on sample results from two wells over an 80 km-long geological section of the Silurian-aged Dadaş hot shale. The Dadaş hot shale has an average TOC content of 6% by weight and is characterized by significant amounts of oil, with minor amounts of gas. The pyrolysis Tmax values range from 434°C to 462°C, which correspond to vitrinite reflectance (%Ro) values ranging from 0.84 %Ro to 1.18 %Ro. These maturation levels play a significant role in the potential hydrocarbon generation of the shales. The surface temperature has been determined to be 20°C, with a geothermal gradient of 25°C per kilometer. According to the geological cross-section, the Dadaş hot shale has been buried deeper from south to north, leading to an increase in formation temperature ranging from 75°C to 120°C in the same direction. According to the modeling results, hydrocarbon generation is lower in the southern part of the Diyarbakır Basin and higher in the northern part. This is due to the deeper burial of rocks in the north and higher levels of maturity. Therefore, it has been determined that oil generation increases and the onset of oil formation occurs earlier from south to north. The modeling results indicate that the oil generation of the Dadaş hot shale started approximately 6 million years ago and reached its peak around 1 million years ago. According to the modeling, hydrocarbons generated in the Dadaş hot shales have migrated to reservoir rocks such as the Hazro and Bedinan sandstones. Maturity and oil-in-place (OIP) calculations were made based on the analysis of Dadaş hot shale samples from previous studies. The oil volume in the Dadaş hot shale is estimated to range between 11.6 and 84 billion barrels based on minimum and maximum maturity values. Considering that 2-10% of this oil can be produced using unconventional methods, the technically recoverable oil is estimated to range between 0.23 and 8.4 billion barrels. Thus, it has been concluded that the Dadaş hot shales contain a significant amount of economically recoverable oil. In unconventional methods, horizontal drilling and hydraulic fracturing are performed for hydrocarbon extraction from the source rock. Analyses of the Dadaş hot shales have shown that they are rich in clay. A potential disadvantage of this is that the clay-rich nature of the Dadaş shales may lead to technological challenges in hydraulic fracturing required for the extraction of this oil. To better understand the hydrocarbon potential of the region and evaluate the effectiveness of production methods, further field studies, drilling, and geochemical analyses are recommended.