V.A. Kashirtsev

Biomarker hydrocarbons in the organic matter of Paleogene sediments in southern West Siberia

Results of geochemical studies of chloroform extracts from the organic matter (OM) of argillaceous-aleuritic Paleogene rocks drilled by wells in southern West Siberia are reported. Judging from the prevalence of odd over even n-alkanes, as well as the simultaneous presence of terpenes, biohopanes, and regular petroleum hopanes (in 17a(H),21β(H) configuration), the OM of the sediments passed the step of the diagenetic transformation but did not exit from the protocatagenesis zone, i.e., it did not reach the oil window. Taraxerene and olean-12-ene were detected at depths of 90–120 m. At depths greater than 400 m, oleanane and a number of quite rare triterpenoids, such as 28,30-bisnorhopane-17(21)-ene and its reduced analog 28,30-bisnorhopane, appear in trace amounts. Most of the array of triterpenoids are characteristic of terrigenous OM, whereas the pattern of the distribution of tricyclic cheilanthanes in some samples is typical of marine sediments. In character, it is close to the set of the biomarker molecules identified in extracts of Holocene and Miocene deep-sea sediments from Baffin Bay.

Biomarkers in the pyrolysis products of asphaltenes from ancient crude oils of East Siberia as indicators of source rock depositional environment

The hydrocarbon composition of the pyrolysis products of asphaltenes isolated from ancient crude oils of East Siberia was determined by the gas chromatography-mass spectrometry technique and was compared with that for Oman oils. The study resulted in the conclusion that the source rocks of both East Siberia and Oman had been deposited under identical facial conditions with similar compositions of source biota.

New and less common sterane and terpane hydrocarbons in oils from the Nepa-Botuoba anteclise (Eastern Siberia)

A number of previously unknown or less common sterane and terpane hydrocarbons in crude oils of the Nepa-Botuoba anteclise (NBA) have been found by gas chromatography-mass spectrometry. The former include pregnane structures demethylated at C10 (principal fragment ion at m/z 203) and C26-C29 tricyclic secosteranes (m/z 219). By the m/z 123 mass chromatogram, a number of bicyclic sesquiterpanes with the drimane or tricyclic norisopimarane structure have been identified for the first time in NBA oils. The origin of these terpenoid hydrocarbons is usually associated with the involvement of higher plant debris in sedimentary source rocks. Their presence in Neoproterozoic oils suggests other precursors of these hydrocarbons. Like 10-desmethylpregnanes, 12- and 13-monomethylalkanes, which are unique biomarkers for Neoproterozoic oils of the Siberian platform, have been found in Vendian and Lower Cambrian oils of the Arabian platform and provide evidence for a close similarity between biotas that existed in these ancient basins.

Terpanes from oil shows of Lake Baikal

The composition and distribution of terpanes in oil shows that emerge from the bottom of Lake Baikal were studied. The oil samples from three sites of Baikal showed different extents of their bacterial degradation, from practically unaltered to heavily degraded oils. In the unaltered samples of oils and moderately degraded differences, a unique variety of biomarkers (sesquiterpanes, secohopanes, new regular hopanes with still unknown structure, oleonane, β-carotane, etc.) was discovered. It was shown that the concentration of triterpanes and β-carotane increases with a growth in the degree of bacterial degradation and the amount of alkanes and sesquiterpanes is simultaneously reduced. The array of the obtained data suggests that the source of Baikal oils is the organic matter buried in freshwater reservoirs. In this organic material, together with the remnants of the living matter of lacustrine organisms, a significant role was played by the remnants of higher plants brought from the land. The age of source rock cannot be older than the Cretaceous, i.e., the time of appearance of angiospermous plants on the land (presence of oleanane).

Steranes in neoproterozoic oils from the Nepa-Botuoba anteclise in the Siberian platform and the South Oman salt basin in the Arabian platform

A number of rare sterane hydrocarbons untypical of younger Phanerozoic petroleum systems have been found by gas chromatography-mass spectrometry in crude oils from the Nepa-Botuoba anteclise of the Siberian platform and the South Oman salt basin of the Arabian platform (Huqf family oils). Among regular steranes, a continuous series of C21 to C30 hydrocarbons including the previously unknown C23-C25 isomers have been identified. It has been noted that short-chain C21 and C22 steranes are represented by the most thermally stable pregnane species diginane and methyldiginane. Another series of steroids is represented by A-norsteranes, the structures demethylated at C10 (main fragment ion in the mass spectrum at m/z 203). Tricyclic hydrocarbons C26-C29 identified by m/z 219 have been attributed to the homologous series of 8,14-secosteranes. These chemofossils have a wide stratigraphic range of occurrence and, like modern minor secosterynes, can be associated with lipid membranes of sponge and their bacterial synbiotics.

Heteroaromatic compounds of bitumenoids from the Cambrian oil shale formation

The molecular composition of NSO heterocyclic aromatic compounds in the lube stock fractions of chloroform-extractable bitumenoids isolated from Cambrian shale formation rocks of Kuonam oil shale, which differ in their organic carbon content, was studied. Of the sulfur compounds, C3–C4 alkylbenzothiophenes; dibenzothiophene, its C1–C5 alkyl and naphtheno derivatives; naphthobenzothiophene; and methylnaphthobenzothiophenes were identified. Among the oxygen compounds, unsubstituted dibenzofuran, unsubstituted naphthobenzofuran, and their C1–C4 and C1–C2 homologues, respectively, were detected. The nitrogen compounds present in the test fractions were supposed to be 7H-benzo[c]- and 7H-benzo[b]acridin-12-ones and methylbenzoacridin-12-ones.

Isolation of adamantane hydrocarbons from Cenomanian oil of the Russkoe oilfield

Adamantane hydrocarbons have been isolated from Cenomanian heavy naphthenic oil of the Russkoe field using the thiocarbamide adduction method. Steam distillation of the oil has given a fraction (boiling range 105–150°C) containing 0.36 wt % adamantane, from which a concentrate containing 18.2 wt % C10–C14 adamantane derivatives has been obtained. Adamantane and its derivatives in the crude oil, oil fractions, and concentrate have been identified, and adamantane has been quantified using the gas chromatography—mass spectrometry technique.