Integrated Approach for Efficient Reservoir Characterization and Completion of the Silurian Unconventional Formation in Southeastern Turkey
Authors: Özkul Canalp , Yilmaz Ismail Omer , Bensenouci Fethi , Ezhov Kirill , Yazaroglu Mehmed Ekrem , Mengen Ahmet Ergun 
Published: February 2024

The Silurian age Dadaş Shale in the Diyarbakır basin, SE Anatolian Basin are the most prospective unconventional reservoir in Turkey. However, the potential of this unconventional formation still remained ambiguous. Thus, sedimentological analysis on cores and cuttings, petrographic analysis on thin sections and scanning electron microscope (SEM) images, well log data interpretation, natural fracture characterization using Borehole Images (BHI) and cores are conducted to unlock the potential of Dadaş unconventional reservoir. The results revealed that the Dadaş-I member of the Dadaş formation has potential of source rock for the Paleozoic oil system and has considerable unconventional reservoir potential.

Dadaş-I member is characterized and subdivided into four lithofacies L1, L2, L3, and L4 using the well logs information, supported by core data such as porosity, permeability, saturation, pyrolysis, SEM and X-Ray diffraction analysis (XRD) interpretation results. Petrophysical properties of the highlighted lithofacies reveal that L1 (High TOC & Quartz rich zone) and L3 (TOC rich zone) have the highest hydrocarbon potential whereas L4 (Carbonate rich zone) shows reasonable properties. However, hydrocarbon potential of the L2 (Lean Shale zone) is poor and not considered to be a good candidate for unconventional reservoir.

Considered as the main permeability driver in unconventional reservoir, natural fractures characterization from logs and cores is performed in the four lithofacies. Borehole images results show that fracture orientations are unique in facies L4, striking parallel to an E-W major faults group whereas fractures in other facies are striking NNE-SSW, parallel to the second group of faults in the area. As the E-W faults are known to be formed first, L4 facies seems to be naturally fractured before the rest of facies, due to its higher elastic properties and brittleness. The two major fault groups are clearly defining the natural fractures orientations in the study area. However, the second faulting episode seems to have more impact on fracture intensity due to higher stress conditions. Many fractures interpreted as open fractures on image logs are in fact fully or partially infilled with calcite in cores. This inconsistency is explained by the different stress conditions near wellbore, where natural fractures can reopen locally under Hoop stresses, and far field conditions captured better in this case by coring. The fracture aperture from borehole images have been corrected after comparison with direct measurements on cores and based on high definition core photos. Overall, the petrophysical and geomechanical properties as well as fracture analysis, show that the four facies behave as distinct mechanical stratigraphy units. The integration of natural fractures and in-situ stresses shows that fractures oriented NNE-SSW with lower dips angles are more critically stressed than the rest. The post-mortem analysis of offset wells has proven that only combination of several factors, like high values of Gross Thickness, Pore Pressure, Dynamic Porosity, Total Organic Carbon (TOC), Brittleness as well as the presence of critically stressed fractures can increase the chance of success by defining the best interval to be targeted by stimulation operation, and enhance production from Dadaş shale reservoirs.