Biomarkers in organic matter of Jurassic deposits in the western part of the Yenisei-Khatanga regional trough, Russia

Rodchenko A.

Novosibirsk State University, Novosibirsk, Russia

[e-mail]

In connection with the discovery of the Vankor, Suzun, Lodochnoye and Tagulskoye gas-oil fields with large oil pools, it became necessary to refine the prediction for oil and gas content. For this purpose, modern geochemical methods have been used to study core samples from wells drilled in the last decades.

In this paper, geochemical studies of organic matter were carried out in 50 samples of core material from wells Nanadyanskaya-310, Payutskaya-1, Pelyatkinskaya-15 and Ushakovskaya-1, located in the western part of the Yenisei-Khatanga regional trough. In the stratigraphic range, Middle Jurassic (Vym, Leontievskoye and Malyshevka Formations) and Middle-Lower Cretaceous (Golchikha Formation) sediments were studied. The core material was selected by the workers of the A.A Trofimuk Institute of Petroleum Geology and Geophysics, SB RAS. The content of extractable bitumen in the rocks was determined by cold extraction using centrifugation. The saturated hydrocarbon fraction of bitumen extracts, isolated by liquid-adsorption chromatography, was studied by the methods of gas-liquid chromatography and gas chromatography-mass spectrometry. The study of normal and isoprenoid alkanes was carried out using chromatograph Hewlett Packard 5890 with a flame ionization detector. Analysis of cycloaliphatic biomarker hydrocarbons was carried out on chromatograph Hewlett Packard 5890 - mass spectrometer 5972A MSD with the computer system (ChemStation) of recording and processing of information HP GI034.

Below, the distribution of biomarkers in each of the homologous series of hydrocarbons is considered.

Normal and isoprenoid alkanes. Bitumen extracts from wells Pelyatkinskaya-15 and Ushakovskaya-1 in the Golchikha Formation are sharply dominated by C₁₇ n -alkanes (the average value of the ratio n -C₂₇/n-C₁₇ < 0.60). This indicates the plankton-algal source of initial organic matter. These bitumen extracts are also characterized by high concentrations of acyclic isoprenoids: pristane and C₁₇ normal alkane are present in approximately equal amounts (Pr/n-C₁₇ = 0.84-0.98). As a rule, the organic matter of terragene type is dominated by n -C₂₁- n -C₂₃ n -alkanes. Similar distribution is characteristic of only one sample from well Nanadyanskaya-310 in the Malyshevka Formation (3423 m). The other bitumens are characterized by the predominance of C₁₇-C₁₈ n -alkanes or a wide-peak maximum in C₁₇-C₂₅ n-alkanes. This distribution can be explained by the mixed nature of source organic matter or catagenesis of source terragene organic matter [2].

The predominance of pristane over phytane (average value of Pr/Ph ratio is equal to 2.79), revealed in all the bitumen extracts, shows the burial of organic matter under suboxidizing conditions.

To determine the degree of organic matter catagenesis, the CPI for C₂₄-C₃₄ normal alkanes was calculated. The values of this parameter are close to unity that indicates the maturity of organic matter.

Steranes. By the distribution of steranes, bitumen extracts are divided into two groups. The first group, comprising nine samples of the Golchikha Formation from wells Pelyatkinskaya-15 and Ushakovskaya-1, are characterized by approximately equal concentrations of C₂₇, C₂₈ and C₂₉ steranes (an average ratio of C₂₉/C₂₇ = 1.11). This is indicative of the relation of bitumen extracts with the lipids of aquagene organic matter. The larger second group consists of 41 samples. In this group, C₂₇-C₃₀ steranes are sharply dominated by ethylcholestane concentrations (C₂₉/C₂₇ ratio varies from 2.29 to 6.08), which is typical of the bitumen extracts of terragene organic matter.

Hopanes. Among terpane hydrocarbons, C₂₇-C₃₅ hopanes are present in the bitumen extracts in the greatest concentration. Hopane C₃₀ is found to be in the maximum concentration. There have also been identified rare occurring hopanes: C₂₉ diahopane (17α(H), 21β(H) 30-nordiahopane) [3], hZ hopane (C (14α)-homo-26-nor-17α-hopane) [4], and hY hopane of unknown structure [6]. All the bitumen extracts have high concentrations of C₃₀ diahopane. It is believed that the diahopane source is produced by bacteria in the clay sediments, which accumulated under oxidizing conditions [5].

To assess the degree of catagenesis, trisnorhopane (Ts) and trisnorneohopane (Tm) ratio was used. This biomarker parameter reflects catagenesis of organic matter better than the rest parameters of the sampling. The value of this parameter increases with depth (Ts/Tm ratio varies from 0.34 to 15.65) and reaches 6.00 at the beginning of the main zone of gas generation.

In the studied bitumen extracts, the concentration of C₃₄ homohopane is higher than that of C₃₅ homohopane (C₃₅/C₃₄ ratio < 1), indicating the burial of organic matter in suboxidizing conditions.

Tricyclanes. The bitumen extracts of the Golchikha Formation from wells Pelyatkinskaya-15 and Ushakovskaya-1, show a marked predominance of C₂₁, C₂₃ cheilanthanes. The values of tricyclane index I_тс = 2(C₁₉+C₂₀)/(C₂₃+C₂₄+C₂₅+C₂₆), proposed by A.E. Kontorovich [1], are lower than or close to unity (0.66-1.13), indicating the aquagene nature of the source organic matter. Among cheilanthanes of terragene bitumen extracts, the maximum concentrations of C₁₉-C₂₁ hydrocarbons are present. The values of tricyclane index are higher in unity (I_тс = 3.93-15.30).

Moretanes. The S to R ratio of C₃₂ moretanes was used to assess the degree of organic matter catagenesis. Samples from well Payutskaya-1in the Leontieskoye Formation and part of samples from well Ushakovskaya-1 in the Vym Formation are characterized by higher degree of catagenesis (S/R C₃₂ > 1). In other samples, the S/R ratio of C₃₂ moretanes varies from 0.10 to 0.91.

Our studies suggest that organic matter from wells Nanadyanskaya-310, Payutskaya-1, Pelyatkinskaya-15 and Ushakovskaya-1 in the Middle Jurassic Formations is assigned to terragene type. Organic matter from wells Pelyatkinskaya-15 and Ushakovskaya-1 in the Golchikha Formation contains a significant proportion of aquagene component. The formation of organic matter occurred under suboxidizing conditions.

On the basis of data on geological zonations of wells from well logging data and performed geochemical studies, it can be concluded that the Golchikha Formation is of polyfacies nature and formed in the coastal marine environments. The lower part of the formation (samples from wells Nanadyanskaya-310 and Payutskaya-1), containing organic matter of terragene type, formed under the conditions of input of lipids of higher land plants into sediments. The upper part of the formation (samples from the wells Pelyatkinskaya-15 and Ushakovskaya-1) with the organic matter of aquagene type seemed to form at greater distances from land and the sediments were filled mostly by the remains of plankton-derived and bacterial nature.

The study was supported by grant of the President of Russian Federation for the young scientists (МК-4893.2012.5).

References:

1. Kontorovich A.E., Bakhturov S.F., Basharin A.K., et al. Heterochronous centers of naphthid formation and sedimentation in the North Asian craton // Russian Geology and Geophysics. – Novosibirsk, 1999. V. 40, no. 11, p. 1676-1693.

2. Petrov Al. A. Hydrocarbons of oil. – Moscow: Nauka, 1984. – 263 p.

3. Moldowan J.M., Fago F.J., Carlson R.M. et al. Rearranged hopanes in sediments and petroleum // Geochim. et. Cosmochim. Acta, 1991, V. 55, p. 3333-3353.

4. Nytoft H.P., Bojesen-Koefoed J.A. 17 α, 21 β(H)-hopanes: natural and synthetic // Organic Geochemistry, 2001, V. 32, P. 841-856.

5. Peters K.E., Walters С.C., Moldowan J.M. The biomarker guide. 2nd ed. New York, Cambridge University Press, 2005, V. 2, 1155p.

6. Zhu Y., Hao F., Zou H. et al. Jurassic oils in the central Sichuan basin, southwest China: Unusual biomarker distribution and possible origin // Organic Geochemistry, 2007, V. 38, P. 1884-1896.

Gas Geochemical Evaluation of the Petroleum Potential of the Birofeld Graben of the Middle Amur Sedimentary Basin (Russian Far East)

Yatsuk A.V. and Gresov A.I.

V.I. Il’ichev Pacific Oceanological Institute FEB RAN, Vladivostok, Russia

[e-mail]

The gas geochemical investigations in the northeastern part of the Birofeld graben revealed that the soil layer contains anomalous fields and zones of hydrocarbon gases confined largely to the southeastern fault and its splays. It is established that the molecular mass and weight reflecting the composition of the C₁–C₄ hydrocarbon fractions represent reliable characteristic parameters of the genetically different hydrocarbon gases in the Birofeld graben. The use of the gas geochemical coefficients (quantitative parameters) allows for the discrimination of the main genetic groups of natural gases.

No surface natural oil and gas showings are registered in the Birofeld graben, which indicates the good high properties of the cap rocks and represents a positive factor. This inference is substantiated by the results of the snow cover and soil gas surveys: most of the defined hydrocarbon anomalies are usually below fivefold values of the background parameters (0.0010%), while amounting occasionally to 0.002–0.004% (20–40 ppm).

Proceeding from the data on the reserves of coal seams [1], the minimal predicted resources of hydrocarbon gases contained in the coal seams of the northeastern and central areas of the Birofeld graben and in their gas-saturated host rocks should be as high as 5.4 × 10⁹ and 1.1 × 10⁹ m³, respectively (or 6.5 × 10⁹ m³ in total). According to the geological- technological criteria and the economic significance, this corresponds to a small gas field (1–10 × 10⁹ m³) with prospects for its development for local needs.

Fig. Schematic map of the gas geochemical zoning of the genetically different natural gases in the northeastern Birofeld graben.

It is established that the hydrocarbon gases from the igneous rocks, condensate gas, gas condensate, and oil gas accumulations migrate through fault zones from the Cretaceous sequences, which serve as both oil–gas source and oil–gas generating formations that stimulate the accumulation of hydrocarbons and the formation of their pools in the Cenozoic sequences developed in tectonically closed fault line structures along the southeastern wall of the graben and on dome shaped uplifts. The discovery of large economic gas reservoirs in the Cenozoic sedimentary complex in the graben areas under consideration seems problematic, although the defined gas geochemical properties of the primary organic matter and natural gases in the Cenozoic and Cretaceous gas source rocks imply the development of small and, presumably, medium gas, condensate–gas, gas–condensate, and oil–gas accumulations.

The anomalous zones of genetically and compositionally different hydrocarbon gas accumulations defined by the preliminary gas geochemical investigations are promising for the planning detailed oil exploration work in the studied area of the Birofeld graben (water intake well) and in the southern part of the Dimitrovo uplift (Fig.). In addition, it should be taken into consideration that the oil and gas fields in neighboring China are localized in the zone of the main graben forming fault bordering the inner uplift.

Judging from the obtained geochemical gas data, the area under consideration is highly promising for the discovery of gas and, probably, gas–condensate accumulations. The defined gas geochemical parameters, combined with the seismic exploration materials, form a basis for revealing and outlining promising graben areas appropriate for oil and gas drilling exploration works.

References:

1. Middle Amur Sedimentary Basin: Geological Structure, Geodynamics, and Fuel-Energy Resources, Ed. by Kirillova G. L. and Khanchuk A.I. (DVO RAN, Vladivostok, 2009), 424 pp.

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