Organic geochemistry and element distribution in coals formed in Eocene lagoon facies from the Eastern Black Sea Region, NE Turkey

Abstract

The amount and type, the inorganic element content, and the maturity of organic materials of Eocene coals, shaly coals, and coaly shales exposed in the Gümüşhane and Bayburt districts of the Eastern Black Sea Region of Turkey were investigated. The depositional environments and hydrocarbon potentials were also interpreted. The total organic carbon concentrations in the studied samples ranged from 0.50% to 63.08%. The samples from Özyurt, Kayadibi, and Tarhanas contained types II and III kerogen, and those from Sökmen and Manas contained type III kerogen. The samples contained Co, Cs, Ga, Hf, Th, U, Y, Mo, Be, Cd, Sb, and La, with average values similar to those of standard brown coals. The samples showed average contents of Co, Ga, Nb, Rb, V, Y, Cu, Pb, Zn, As, Be, and Se, similar to those of other Turkish coals. The sediment source of Eocene samples in the five areas was characterized by rocks with intermediate or mafic geochemical characteristics. The terrigenous/aquatic ratio of coal and shaly coal samples of the areas in question is >1. The sterane distribution was C29 > C28 > C27 and C29 > C27 > C28 for the Özyurt and Tarhanas areas, respectively. The average Tmax values for samples are between 424 °C and 460 °C. For samples Oz-1 and Ta-2, 22S/(22S + 22R) homohopane (C32) ratios are 0.48 and 0.61, respectively; 20S/(20S + 20R) sterane (C29) ratios are 0.18 and 0.53, respectively; and Ts/(Ts + Tm) ratios are 0.015 and 0.64, respectively. The Pr/Ph ratios of the samples are >3. The studied samples have low sterane/hopane and high (C19+C20)/C23) ratios without anoxic biomarkers (17α(H)-28,30-bisnorhopane). Based on these data, the coals, shaly coals, and coaly shales were probably deposited under an oxic-suboxic mixture of marine and terrestrial environment conditions; these materials contain terrestrial organic matter and cannot generate hydrocarbon.