Natural fracture, cleat, and strong adsorption impact on low oil and condensate retention in the Carboniferous shales and coals of the western Black Sea Basin of Turkey

Abstract

Total organic carbon (TOC) values of the Carboniferous shales and coals of the western Black Sea Basin of Turkey range from 0.72 to 71.52 wt. %, with an average of 32.3 wt. %, and generation potential chance from 0.33 to 184.64, with an average of 82.47 mg HC/g rock. Hydrogen index (HI) values of the shales and coals range from 24 to 384 mg HC/g TOC. The HI versus pyrolysis temperature at the maximum rate of kerogen conversion plot indicates that the organic matter is type II–III. The calculated vitrinite reflectance equivalent values of the shales and coals range from 0.63% to 1.32%. Thus, the shales and coals of the Carboniferous formations are represented by immature, effective oil and condensate generation windows. The shales and coals of the Alacaagzi Formation contain high quartz and lower clay. Oil saturation index (OSI) values of the shales and coals have been calculated as being 0.7 and 12 mg HC/g TOC, indicating possible lack of producible oil and condensate. The OSI values are lower than those of other coal-bearing and marine shales of the world. First, strong adsorption capacity may be a cause of the lower oil and condensate retention. Second, faults, natural fractures, and cleats related fluid flows may be important factors in these lower OSI ratios. The extensional, collisional, and inverse tectonic stages and diagenetic processes of coalification formed multiple interconnected faults, natural opening-mode fractures, and cleats in the shales and coals. Thus, 15 coalbed methane exploration wells failed to produce any economical coalbed methane.