GEOCHEMISTRY AND CARBON ISOTOPE CHARACTERISTICS OF ASSOCIATED GASES FROM OILFIELDS IN THE NW GREATER CAUCASUS, RUSSIA

The NW Greater Caucasus and surrounding areas have a long history of petroleum production dating back to the mid-19th century. However the origin of the oil and gas is still a matter of debate. This paper focusses on a study area to the north of the western Greater Caucasus covering the West and East Kuban Basins and neighbouring structural highs where oil and natural gas occur in reservoir units of Neogene, Paleogene, Cretaceous and Jurassic ages. The study is based on the results of organic geochemical and stable carbon isotope analyses of 21 samples of associated gases from 16 oil fields. The gas samples' compositions were investigated together with the delta C-13 values of C-1 to C-5 alkanes. Twenty of the gas samples were of thermogenic origin and one was mixed (thermogenic and secondary microbial). delta C-13 values for CH4 for all the gas samples varied over a wide range from -62.3 to -30.0 parts per thousand, indicating major variations in the thermal maturity of the respective source rocks. In a previous study, three genetic types of oil were identified in the study area and are referred to here as Types 1, 2 and 3. Gaseous hydrocarbons associated with Type 1 oils were generated by source rocks in the Middle Eocene Kuma Formation and the Oligocene (lower) part of the Maikop Group which are inferred to have been deposited in anoxic, deep-marine conditions. C-2-C-5 alkanes in gases associated with Type 1 oils have the lowest (most negative) delta C-13 values of the samples studied, and the lowest iso-butane: n-butane and iso-pentane: n-pentane ratios. These gas samples were recovered from wells in oilfields located in the south and SE of the West Kuban Basin, in the Akhtyrskaya fault zone which extends along the northern flank of the Greater Caucasus foldbelt. Higher delta C-13 values of C-2-C-5 alkanes were found in gas samples associated with Type 2 oils which were recovered from wells in the central West Kuban Basin. The associated source rocks are interpreted to have been deposited in weakly oxidizing subaqueous conditions and to contain mainly marine organic matter but with an input of higher land plant material. The precise age and stratigraphic identity of the source rocks is unresolved but they may comprise the Upper Oligocene - Lower Miocene (middle and upper) part of the Maikop Group. In gases associated with Type 3 oils, delta C-13 values of ethane to pentane are similar to those of gases associated with Type 2 oils. These gases were recovered from wells in oilfields located on the Adygeya Swell which divides the southern East and West Kuban Basins and from the Armavir-Nevinnomyssk Swell to the north. The precise source of these gases and oils is not known but the oils are probably older than Late Cretaceous on account of the absence of oleanane.