ORGANIC GEOCHEMISTRY OF CENOZOIC ROCKS IN THE USHUMUNSKOE BROWN COAL FIELD OF THE MIDDLE AMUR SEDIMENTARY BASIN
The relevance of the research is conditioned by the necessity to expand the resource base of the Russian Far East. The study of sedimentary basins and their hydrocarbon potential, the development of new forecast technologies for traditional and unconventional hydrocarbon fields is a priority for the Russian Academy of Sciences for many years. The evaluation of source rocks based on levels of their enrichment with the organic matter and its generation potential will provide valuable data for basin modeling. Besides, given that the Ushumunskoe source rocks have not been studied by organic geochemical methods, the present study sets out to clarify the conditions of the organic matter accumulation, and to reconstruct landscapes and biota in the Cenozoic Era. The aim of the research is to characterize the biological source of the organic matter, conditions of its accumulation, diagenetic settings and the level of the Ushumunskoe source rocks maturity. Objects of the research are the Ushumunskoe coals, clays and siltstones from the Middle Amur sedimentary basin. Methods: comparison of the results of pyrolysis and isotope analysis of the organic matter and characteristics of biomarker hydrocarbons obtained using chromatography-mass spectrometry in saturated and aromatic fractions of bitumen extracts, evaluation of sources, as well as diagenetic and catagenetic conditions of organic matter. Results. The Paleogene-Neogene source rocks of the Ushumunskoe brown coal field in the Middle Amur sedimentary basin were studied by the methods of organic geochemistry for the first time. The organic matter of the studied samples has signs of both terrestrial and sapropelic types. The organic carbon isotopic composition, n-alkanes distribution, the presence of kaurane, beyerane, phyllocladanes (biomarkers of higher plants Podocarpaceae and Araucareaceae) in the saturated fraction of extractable bitumens, retene (biomarker of conifers) in the aromatic fraction indicate the terrestrial (humic) nature of organic matter. At the same time, in most samples the aromatic fraction characteristics correspond to the sapropelic organic matter, which is evidenced by low concentrations of phenanthrenes, high concentrations of mono- and triaromatic steroids, distributions of methyldibenzothiophenes, and probable presence of long-chain alkylnaphthalenes (biomarkers of lake algae Botryococcus braunii). Phytane concentrations are significantly higher, than pristane, and half of the samples according to the hydrogen index (HI) values belong to type II. Due to the absence of marine deposits in the study area, type II is probably a mixture of type I (lacustrine) and III (continental). Moreover, judging by the HI variations throughout the section, depositional environments changed repeatedly from lacustrine to peatbog and vice versa. In some samples, the maxima in the acyclic isoprenoids fall on i-C<sub>21</sub> and i-C<sub>22</sub>, possibly due to the significant contribution of archea to the original living substance. According to the high concentrations of dibenzothiophenes, the organic matter underwent diagenesis with high content of hydrogen sulfide in sediment. Probably, the duality of signatures (terrestrial and sapropelic) is associated with the specificity of the initial biota in this basin and is characteristic of its age and/or geographical localization. The vitrinite reflectance data, pyrolytic characteristics, high carbon preference index in n-alkanes, the presence of biohopanes and hopenes, almost complete absence of typical steranes and terpanes, and distribution of triaromatic steroids are interpreted as indications of immature organic matter, with the latter potentially enhancing such a duality of characteristics.