THE CARBON AND OXYGEN ISOTOPE CHARACTERISTICS AND PALEOENVIRONMENTAL SIGNIFICANCE OF LOWER-MIDDLE CARBONIFEROUS CARBONATE ROCKS OF THE EASTERN CASPIAN BASIN

Carboniferous carbonate rocks provide important oil and gas reservoirs in the eastern Caspian Basin, Carbon and oxygen isotopes record the paleoenvironment and diagenetic environment of reservoir formation. Carbon and oxygen isotope analyses of 76 lower middle Carboniferous carbonate rocks from the Zanarol area were combined with petrographic analysis to investigate the paleoenvironment during formation of the carbonate rocks. Most of the 613CpDB values are between 2 parts per thousand and 6 parts per thousand, with an average of 0.67 parts per thousand. Most of the delta O-18(PDB) values are between -7 parts per thousand and -4 parts per thousand, with an average of -6.13 parts per thousand. These values indicate that paleoenvironmental information is preserved in spite of some diagenetic changes. The values of Z are 120-134, consistent with formation of the carbonate rocks in normal to saline seawater salinities within a marine environment. The paleotemperature was 20 degrees C-28 degrees C, which indicates a warm subtropical climate at the time of deposition. The carbon and oxygen isotope values indicate that the B layer underwent diagenesis in a near-surface environment, and that the G and D layers underwent diagenesis in medium deep burial environments. The delta 1(3)C(PDB) values of the B layer samples are higher than those of the G and D layers, which is attributed to changes in the sedimentary environment. The main facies represented by the G and D layers is an open platform facies, while the B layer formed on an open continental shelf. An abrupt change in carbon and oxygen isotope ratios at the top of the B layer is attributed to tectonic movement in the eastern part the study area associated with the Ural orogeny during the late Carboniferous. At this time, water in the sediments became and organic activity declined as the area of the open sea shrunk and the water stagnated, with a consequent increase in water salinity and an abrupt increase in the delta C-13(PDB) and delta O-18(PDB) isotopes.