Studies of biomarkers in the organic matter of the seafloor sediments in the northern and middle Caspian Sea

Abstract

The present day approach to the investigation and typification of organic matter (OM) in rocks and fossil fuel deposits is based largely on the determination of individual compounds. This approach is the result of significant progress in the studies of the structure and stereochemistry of individual compounds of oil, in which more than 1000 various hydrocarbons (HC) are currently known. It is important to note that this new level of studies was reached through chromatomassspectrometry, which allowed for the opportunity for much finer analysis with a high precision of separation of individual hydrocarbons. Many of these compounds may be considered as chemical fossils that preserved obvious structural relations with initial bioorganic molecules. During diagenesis, initial organic matter is transformed into oil HC, the evidence of which is left in the general composition of the HC systems in the relative concentrations of certain compounds, as well as in their structures. Our studies were focused on seafloor sediment (SFS) sampled from a water area of the northern Caspian Sea, as well as from northern and western parts of the Middle Caspian. We were looking for possible hydrocarbon exhalations of deposits in subsurface conditions relying on the behavior of molecule biomarkers in the northern Caspian sediments. The background distribution of HC and OM in recent sediments was compared with anomalous distributions of HS and OM that had possibly migrated. The geochemical, genetic, and catagenetic features of recent sediments were studied to identify possible sources of migrated organic matter (supposed oil pools). Our interpretation is based on distinguishing between syngenetic OM (immature OM of recent sediments) and migrated OM. Firstly, investigated OM of SFS should manifest features of recent sediments. One of such features, found in the distribution of individual HC, may be the sharp domination of odd alkanes C 25, 27, 29, 31 [1]. Another feature is the domination of biological markers, that is, OM with traces of an insignificant restructuring of biogenic structures that occurred at the beginning of diagenesis. Migrated or redeposited OM may be recognized by high parameters of maturity in HC distribution not particular to recent sediments and by the presence of compounds developed during the catalytic thermal maturation of organic matter that occurred during fossilization when sediment was being transformed into rock. We investigated 40 seafloor sediment samples gathered by the Caspian Fisheries Research Institute (KaspNIRKh) from the water areas of the northern and middle Caspian Sea (its northern and western framing). The samples were analyzed by the method of chromatography using a magnetic chromatomass-spectrometer of high resolution, Thermo Finnigan MAT 900 XP. Chromatomass-spectrometry data is interpreted based on the identification of individual organic compounds.