The paper deals with results of the performed isotopic-geochemical and chromato-mass-spectrometric analyses of oils from 14 fields located in shelf and onshore of the south flank (Turkmenian sector) of the South-Caspian basin (SCB). These oil fields are corresponded to the anticlinal structures of three tectonic zones: Pribalkhanian zone of uplifts (Jdanov, LAM banks, Cheleken, West Cheleken, Kotur-Tepe, Barsa-Gelmes, Burun, Komsomolskaya, Kumdag), Kizilkum trough (Erdekli) and Gograndag-Okarem (Korpeje, S. Kamishlija, Ag-Patlaukh) zones of uplifts. The low prystane-phytane (Pr/Ph) ratios (1.17-1.63 in oils and 1.45-2.08 in condensates), predominance of the low molecular alkanes in the general series of alkanes’ distribution (С31:С19=0.08:0.63), the prevalence of the even alkanes over the uneven ones in unweathered oils as well as the clear naphthenic nature of oils (naphthenic index = 1.9-6.7) testify to mainly marine nature of the initial organic matter (OM). The isotopic composition of oil carbon also points to that. The clear tendency of isotopic lightening of oils is observed in the eastern direction, from shelf fields towards the onshore fields. According to the physical-chemical properties, the oils from single fields located in the various structural zones and the production targets within one field differ significantly. Oils and condensates have very low degree of maturity; calculated by biomarker data in vitrinite equivalent (Ro,%) they make 0.55-0.65% and 0.62-0.75% accordingly. The oil generation mainly from diatom organic matter of clayey sediments of marine delta is sub-stantiated. Moreover, the summary contribution of the Middle- and Upper Miocene complex into oil formation is higher than of the Paleogene-Lower Miocene one; the oil and condensate maturity is low. The supply of structures with hydrocarbons in the shelf of the east flank of the SCB took place as from kitchen areas located within the SCB (Kizilkum trough) and from sources in the Middle Caspian (Kelkor trough).

Keywords: oil, organic matter, isotopic-geochemical analysis, biomarkers, maturity, South-Caspian basin, Turkmenian sector

 

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