Geology and Geophysics Institute, Azerbaijan National Academy of Sciences, Javid Ave., 119, Baku, Azerbaijan, AZ1143: email@example.com
Summary. This paper details aspects of isotope-geochemical features for oil and gas on the N-W flank of South Caspian basin (Gobustan, western Absheron regions and adjacent offshore area), where natural oil and gas seepages are associated with tectonic faults and mud volcanoes (MVs). Oils concerned with MVs are biodegraded, having predominantly naphthenic-paraffinic base and less than 0.5% sulfur content. Oil samples and hydrocarbon extracts of source rock were analyzed using gas chromatography, mass spectrometry and their combination (biomarker analysis). Oil–oil and oil–source rock correlations are used. The values of the complex biomarker parameters of selected oil samples correspond to Tertiary source rocks with the similar organic facies accumulated in deep-water conditions with some terrestrial contribution. Oils have low to moderate organic maturities (Ro = 0.75-0.85). Relationship between the isotope composition of carbon (ICC) for aliphatic and aromatic fractions of oils and rock extracts (oil-rock correlation) shows that the source of commercial oil reserves in the Productive series (lower Pliocene) is mainly Miocene deposits (Diatom, Chokrak and Upper Maykop strata). The source of gases is from the sedimentary cover as is evidenced by the 3He/4He ratio values. They also have been derived from mixed marine organic-rich source facies. Such gas is not from the thermal destruction of oils. Calculated depth of oil source corresponds to 6.1-8.6 km (gas source – from 8.1 to 15.8 km). The isotopically heavy ICC of СО2 in gases is most probably the microbial destruction of hydrocarbons at shallow depths (no deeper than 2 km).
Keywords: oil, gas, source rocks, organic matter, geochemistry, correlation, South Caspian basin
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