Temperature-depth control of petroleum occurrence in the sedimentary section of the South Caspian basin

ABSTRACT This study analyses the temperature and depth control of well productivity (oil and gas), density of hydrocarbons (HC) and gas/liquid HC ratio (GLR) of petroleum fields in the South Caspian Basin (SCB). The database includes more than 700 field data from 54 onshore and offshore fields, which were processed to provide graphical and statistical analysis using standard computer programs. It was established that the peak of initial production and proven oil reserves falls in the temperature range 50-70 oC. The oils of this interval are characterized by medium (about 890 kg/m3) and light (about 806 kg/m3) density. The maximum condensate and gas production are in the temperature range 100-120 oС, associated with the discovery and operation of large gas condensate fields in the deepwater part of the SCB. Liquid HC are represented in this temperature interval by “volatile” oils and condensates with an average density of 777-802 kg/m3. Low values of GLR are typical for temperatures below 70 °C, associated both with poor gas conservation conditions and relatively high volumes of oil production. The carbon isotope composition of methane indicates an exclusively catagenetic (thermal) origin at temperatures >50 °C and both catagenetic and mixed (microbiological-catagenetic) at temperatures