Clay mineralogy and geochemistry of three offshore wells in the southwestern Black Sea, northern Turkey: the effect of burial diagenesis on the conversion of smectite to illite

The conversion of smectite to illite has long been studied by numerous researchers because of its importance as a diagenetic metric. Interpreting the pressure, temperature, and age of the sequences in which this conversion occurs provides the possibility to identify the historical maturation parameters of hydrocarbon sources. The Black Sea Basin is known to be an area that can provide source rocks for oil and gas production. The purpose of this study was to determine the clay minerals and their abundances, to establish a stratigraphic correlation among three wells, which is useful to select specific stratigraphic horizons for hydrocarbon exploration, and to predict paleotemperature ranges in the wells by using the conversion of clay minerals. The determination of the clay mineralogy and chemical composition of the three wells in the Black Sea Basin was done by several methods of analysis. These methods include powder X-ray diffraction (XRD), X-ray fluorescence spectroscopy (XRF), and environmental scanning electron microscopy (ESEM). All 54 samples were processed by XRD and XRF and 6 representative samples were selected for ESEM analysis. Based on the XRD results, the clay minerals determined in the samples are illite, smectite, and mixed-layer illite/smectite (I/S), which are the most abundant minerals calculated by the method described in Underwood and Pickering, plus kaolinite and chlorite. The chemical results of major oxides acquired from XRF analyses show that the changes in Na2O and K2O, which are the main actors in the conversion of smectite to illite, do not gradually increase or decrease. Since the Black Sea Basin is considered a rift basin, the maximum temperature ranges of the conversion were calculated by considering the maximum and minimum depths of the samples. These temperature ranges are 111-154 °C, 147-208 °C, and 48-59 °C for Well-1, Well-2, and Well-3, respectively.