Conceptions and Indicators of the Abiogenic Oil and Gas Origin and Its Significance
B. M. Valyaev, S. A. Leonov, G. A. Titkov, and M. Yu. Chudetsky
Oil and Gas Problems Institute Russian Academy of Sciences
Isotopic geochemistry methods received wide acceptance in different branches of geology in the second half of the XX century. In oil and gas geology, research on carbon isotopes of organic matter and bitumen not only generally corroborated the traditional views on the organic concept of the genesis of oil and gas, but also provided significant criteria for separating stages of hydrocarbon generation of various composition in agreement with the stages of sedimentary rocks transformation in diagenesis and catagenesis. However, geological data on the relation between hydrocarbon accumulations and deep faults, the wide vertical range of oil and gas accu-mulation in individual deposits, the priority of vertical migration testified to the common and deep character of hydrocarbons genesis in the whole sedimentary section. This disagreement between conclusions derived from isotopic geochemical and geological data could be settled only by a new alternative interpretation of isotopic geochemical data.
First alternative interpretations made by us belonged to two isotopic systems: carbon (δ13C) -hydrogen (δD) in oils, gases and various kinds of carbonaceous formations (Valyaev and Titkov, 1985) and carbon in methane (δ13Cm) - carbon in carbonic acid (δ13Ca) in different natural gases (Valyaev and Grinchenko, 1985). In both cases, a wide range of natural carbonaceous formations was considered which is related not only to near-surface settings of diagenesis and catagenesis but also encompasses deep processes of mantle matter differentiation, magmatism, metamorphism and hydrothermal activity. On double diagram δ13C/δD, order distribution of isotope dotted areas (IDA) was revealed of various morphological groups of natural gases including the so-called biochemical ones in near-surface deposits, and two major branches of natural gases, i.e. hydrocarbon and carbonic acid were set apart. Such order of IDA of natural gases and other carbonaceous compounds may be explained most adequately by the common nature of the deep genesis of all these formations. Features causing this order overweighed the distorting effects of the age and lithological composition, poorer or richer content of one or other type of organic matter in the rocks. On the second double diagram δ13Сm/δCa, with order position of isotope dotted areas stressed by the agreement with equilibrium isotherms, two branches of natural gases are also separated. The latter branch (carbonic acid) is associated with the manifestations of modern or Quaternary magmatism (volcanism), and the former is associated with the products of amagmatic, "cold" degassing (P.N. Kropotkin). As a result of some constructions, the necessary agreement was achieved in the interpretation of geological and isotopic geochemical data from the viewpoint of the deep genesis of hydrocarbons. However in oil and gas geology, previous interpretations related to isotopic carbon - hydrogen system in conformity with traditional view-points on the organic genesis of oil and gas remained unchanged.
Moreover, they were supported with the use of data on isotopic helium (3He/4He) and carbon - helium (C/3He) systems [Jenden, et al., 1993; and others]. For these isotopic systems, alternative interpretations of the results of research may be possible. Above all, first data reports on the isotopic system 3He/4He showed differences and separation of two groups of IDA data with sharply different isotopic relationships for a major part n×10-6 for carbonic acid (high-temperature) gases and n×10-8 for hydrocarbon gases, i.e. in this system two branches of deep de-gassing were separated. In previous data interpretations, as the mantle single standard of parameter C/3He the constant value ~2×109 was used, which is characteristic of basalts of mid-ocean ridges where carbonic acid prevails. As the analysis shows , a more differentiated pattern is observed if for the number of carbon atoms (C) neither value (Ca) is used for carbonic acid nor (Cm) for methane in the relationship numerator, but value (CΣ) for total carbon. In this case, a wide range of values of relationship CΣ/3He is typical of mantle rocks. In mid-oceanic ridges basalts, the values in the range from 1 to 8×109 prevail; in xenolith and island basalts of the ocean the values range from 2×109 to 2×1011, in the xenolith of Baikal rift the values amount up to 2×1012, in the xenolith of Pannon basin they make 1×1013, in alcali basalts enclosing xenolith they range from 1×1012 to 4×1014 [Lokhov and Levsky, 1993; Truel et al., 1993]. Evidently the con-clusion can be made about the heterogeneity of mantle matter (mantle derivatives) by parameter CΣ/3He. The nature of the heterogeneity deserves further investigation.
Summarization of published data and correlation of isotopic helium 3He/4He and carbon - helium C/3He systems showed that correlation trends between these parameters in the plot are sharply different for carbonic acid and hydrocarbon fluids both in inclination angles and in IDA position . At the same time, correlation trends for hydrocarbon gases of different regions show similar features (parallelism). Trend shift may be explained by different tectonic settings and thermal dynamics conditions. The results of the research suggest principally different models to account for the isotopic geochemistry variety of hydrocarbon - helium systems. The drawback is remedied of previously proposed models of shifting (to study the genesis of methane in hydro-carbon accumulations) which did not take into consideration the fluid carrier of mantle helium.
One more argument and a very strong one that has been forwarded until now in favor of the organic genesis of oil is isotopically light (IL) composition of methane carbon in near-surface accumulations of hydrocarbons which is considered to be the indicator of biochemical genesis of such methane. The share of IL methane in global hydrocarbon reserves appeared to be surprisingly large, making more than 20% (Rice, 1993). Methane in gas hydrate accumulations falls in the subgroup of isotopically light. On land and on the floor of the oceans of the world, the geological settings of IL methane accumulations testify to its deep nature and the intrusion of hydro-carbon in near-surface horizons by disjunctive dislocations governing the localized flows of deep hydrocarbon fluids [2 - 4]. The only way to bring to an agreement a significant difference between the interpretations of geological and isotopic geochemical data on the nature of IL methane is searching for an alternative interpretation of isotopic geochemical data. The results of research specially conducted to achieve this goal are presented in a paper recently published by us . For a major part they mean that methane generated biochemically is presented in two separate groups extremely IL either by δ13C or δD of methane which are not related to the formation of commercial oil and gas accumulations with IL methane. The traditional ideas of the IL methane generation at the expense of organic matter with decisive role of either processes of CO2 reduction or fermentation (Whiticar, et al., 1986) have little force now. In the paper quoted above , three likely mechanisms are proposed of IL methane generation. In this case, deep hydrocarbon fluids are considered to be the major supplier of IL methane in the deposit; additional amounts of methane can be formed in the process of these fluids transformation (after their penetration into the sedimentary section) and at the expense of bacterial (biochemical) processes.
Thus traditional interpretations of isotopic geochemical data supporting the generation of oil and gas from organic matter in the process of diagenesis and catagenesis of sedimentary work prove to have an alternative. This alternative is associated with considering oil and gas formation processes and the above-mentioned isotopic geochemical data in the context of a wider spectrum of geological processes including not only the sedimentary section but also the Earth's crust and the mantle of the deep interior of oil- and gas-bearing regions. Moreover, alternative constructions are in a better agreement with geological data on the conditions of occurrence and likely mechanisms of the formation of hydrocarbon accumulations of different types including gas hydrates. Significant modifications should be made in the use of isotope geochemical indicators and criteria in the estimate of reserves and prediction of oil-and-gas presence.
1. Valyaev B. M., Titkov G. A., Isotope composition of the hydrogen of methane in hydro-carbons accumulations. Transactions Russian Academy of Sciences, 1997, vol. 357, no. 6, pp. 808 - 811 (Russian).
2. Valyaev B. M., Leonov S.A. The new isotope – geochemical criteria for hydrocarbon genesis. – New Ideas in Geology and Geochemistry of Oil and Gas. Materials of the sec-ond international conference. M.; MSU, 1998, p. 41-43. (Russian).
3. Dmitrievsky A.N., Valyaev B.M. New data on the gas hydrates in the World Ocean and the prospects of their exploration // Proceedings Indo-Russian Joint workshop on gas hydrates under ILTP. – New Delhi, Department of ocean development Geoverument of India, 2002, p.127-134.
4. Valyaev B.M., Titkov G.A., Chudetsky M.Yu. About genesis of the isotopic light (δ13C, δD) methane of the oil and gas fields. – Degassing of the Earth and genesis of hydrocarbon fluids and deposits. – M. GEOS, 2002, p.108-134 (Russian).
5. Dmitrievsky A.N., Valyaev B.M. Principal results and perspectives of the investigation on the problem “Outgassing of the Earth”. - Outgassing of the Earth: Geodynamics, Deep Flu-ids, Oil and Gas. Proceedings of the International Conference in Commemoration of Academician P.N. Kropotkin, May 20-24, 2002, Moscow. M.: GEOS, 2002, p. 3-6 (Russian).