On the Decomposition of Water in Sedimentary Rocks of the Earth’s Crust

S. L. Shvartsev

(Presented by the scientific seminar of the Department of Hydrogeology and the Problem Geological Laboratory)

The constant of decomposition of water with the release of free hydrogen and oxygen at room temperature is characterized by an insignificantly small value, equal to \(10^{-83.1}\) (Garrels, Christ, 1968), which makes water one of the most stable compounds in nature, practically incapable of spontaneous decomposition under the conditions of the earth’s crust. Therefore, the decomposition of water in the sedimentary cover is considered possible only as a result of such processes as photolysis, photosynthesis, oxidation of elements of variable valence (mainly Fe and S), and hydrolysis (Sinitsyn, 1970). In all other cases, when studying the character of various geochemical processes and phenomena occurring in the earth’s crust, water is regarded as thermodynamically stable.

However, everything stated above is valid only for free (gravitational) waters and becomes inaccurate when we are dealing with bound waters, which have thermodynamic parameters different from those of free waters. Most geochemical processes in the earth’s crust occur with the participation not of free but of bound waters, which are practically ubiquitous. The content of bound waters is especially high in clays and clayey rocks, which distinguishes these formations among all other rocks of the earth’s crust.

Up to the present time, bound waters remain poorly studied from the thermodynamic point of view. However, the data already available are sufficient to raise the question that bound waters, in comparison with free waters, are less stable thermodynamically and therefore, under certain geological conditions, are capable of decomposing with the release of free hydrogen. The solution of this problem has important geological consequences, which until now have not been taken into account by science. Therefore we shall dwell on this question in more detail and, first of all, try to determine in which direction the point of absolute neutrality of water shifts when it passes from the free state to the bound state.

Absolute Neutrality of Bound Waters

As M. J. Pourbaix (Pourbaix, 1963) showed, the absolute neutrality of free water at room temperature is characterized by the following indicators: \(\mathrm{pH}=7.0\), \(\mathrm{Eh}=+0.4\ \mathrm{V}\), \(P_{\mathrm{O}_2}=10^{-27.86}\ \mathrm{ata}\), \(P_{\mathrm{H}_2}=10^{-27.56}\ \mathrm{ata}\). All the values given for the parameters considered are mutually related, and a change in one of them leads to a change in the others.

In order to understand in what direction the absolute neutrality of bound water will change, let us recall that, in natural mineral systems, bound water is affected by surface forces, which change its physical properties in the same direction as does an increase in temperature. Therefore water in thin films (pore water, film water, etc.) differs from free (gravitational) water in density, viscosity, surface tension, vapor elasticity, thermal conductivity, electrical conductivity, freezing temperature, degree of molecular dissociation, dielectric constant, etc. (Blokh, 1970). The difference between the properties of bound and free water at one and the same temperature is the greater, the more: a) dispersed the mineral system; b) the lower its moisture content; c) the higher the hydrophilicity of the minerals composing it. It is important to emphasize here that “even at very high moisture contents of sediments, the pore solution contained in them has an elevated surface temperature and, correspondingly, physical properties different from those of a solution of the same composition but located outside the sphere of influence of surface forces” (Blokh, 1970), i.e., waters with altered physical properties are widely distributed in the Earth’s crust, beginning with the pore solutions of oceanic and marine muds and ending with film or capillary waters (in the terminology of V. I. Vernadsky) of metamorphic and magmatic rocks.

Thus, the transition of free waters into the bound state is accompanied by a change in their properties in the same direction as upon heating. Heating of water, however, leads to a sharp increase in its decomposition and dissociation constants (Table 1). Consequently, bound water must also be characterized by elevated values of these constants. This latter

Table 1

Change in the dissociation and decomposition constants of water as a function of temperature

proposition is confirmed by the results of experimental studies. Thus, J. J. Fripiat and co-workers (Fripiat et al., 1965) showed that the dissociation constant of bound water in clays increases by \(10^6\). Later, these same researchers (Tuillaux et al., 1968), using methods of magnetic resonance, conductometry, and dielectrometry, established more precisely that water adsorbed by Na—Ca—montmorillonite has a dissociation constant equal to \(10^{-7,15}\), i.e., it exceeds the dissociation constant of ordinary water by almost seven mathematical orders of magnitude.

The author is not aware of experimental data on the determination of the decomposition constant of bound water into hydrogen and oxygen. However, using the well-known van ’t Hoff equation, which establishes the dependence between the equilibrium constant of a reaction and absolute temperature (Garrels, Christ, 1968), it can be shown that, in the case where the dissociation constant (\(K_g\)) of water is equal to \(10^{-7,15}\), its decomposition constant (\(K_p\)) must be equal to \(10^{-17,0}\). In turn, knowing these two quantities, we can calculate the free energy of formation of bound water and, consequently, its absolute neutrality (Table 2).

Without guaranteeing the absolute accuracy of the figures given for bound water, since their value depends on the degree of bonding of water with the particleך

Table 2

Comparison of certain thermodynamic constants of absolutely neutral bound and free water at room temperature

of the rock, its moisture content, mineralogical composition, etc.; it is important to emphasize the direction of the change in the physical and thermodynamic properties of water as it passes from the free state into the bound state. As can be seen from the data in Table 2, such a transition is accompanied by a sharp increase in the concentration of \(\mathrm{H}^{+}\) and \(\mathrm{OH}^{-}\) ions, an increase in the partial pressures of oxygen and hydrogen, a decrease in the value of the oxidation–reduction potential and, what is very important, a sharp decrease in the thermodynamic stability of water. In the bound state, water, in terms of its stability, already at ordinary temperatures approaches many other natural compounds, such as carbonic, phosphoric, and silicic acids, hydrogen sulfide, and other gases. And this, as we shall see below, has important geological significance.

Thus, the transition of water from the free state into the bound state is accompanied by a sharp increase in the partial pressure of both hydrogen and oxygen, and also by an increase in the solution in the concentration of \(\mathrm{H}^{+}\) and \(\mathrm{OH}^{-}\) ions. Both are possible only with the spontaneous decomposition of water. Consequently, the very binding of water is itself accompanied by its partial decomposition and transition into a less stable state. This, in turn, should facilitate the participation of water in various geochemical processes. Let us consider, in particular, how water behaves when it is buried in clayey sediments on the floors of marine basins.

Behavior of water in buried marine sediments

During the formation of clayey sediments in marine basins, a considerable amount of free water is bound. It is sufficient to note that the moisture content of silty sediments forming on the sea floor, as a rule, is 60–80%, and in individual cases reaches 100 and even 140% (Tageeva, Tikhomirova, 1962; Shishkina, 1971). Part of this silty water is free (gravitational), but the greater part is in a bound state. The degree of binding of the different layers of water with the silt is not the same, and it is greatest for the water located in the interpacket space of clay minerals. A considerable part of the bound water enters the lattice of these minerals in the form of hydroxyl ions, i.e., is in a decomposed state.

Most clay minerals, as is known, are represented by hydromica, montmorillonite, kaolinite, and, to a lesser extent, chlorite. These minerals contain 10–20% water in the form of hydroxyl ions, which amounts to approximately one-fifth of the water of silty sediments. All this indicates

The fact that the scale of water decomposition at the stage of formation of clayey sediments is sufficiently large must be taken into account in various geochemical constructions.

As the sediment-accumulation zone subsides and the weight of the overlying rock mass increases, free water is squeezed out into lateral or overlying rocks. As a result, the proportion of bound waters in the sediment increases and the proportion of free waters decreases. This, in turn, should lead to an even greater shift of the point of absolute neutrality of water into the region of higher partial pressures of hydrogen and oxygen. Consequently, in the forming clayey sediments a geochemical environment will be created that is fundamentally different from that observed in free waters. The principal difference of this environment consists in the fact that here water is in a considerably less stable state than gravitational water. It is important to emphasize once again that this new environment is most clearly expressed in clays, and not in sandstones, since in the latter unbound, free water predominates.

In the process of diagenesis and catagenesis of clayey sediments, a new group of clay minerals—authigenic minerals—is formed (Strakhov, 1962; Sarkisyan, Kotelnikov, 1971), the formation of which, among other things, requires the participation of hydrogen ions, that is, the decomposition of water. Diagenesis and catagenesis of previously formed clays also, in a number of cases, proceed in a direction that requires the participation of water molecules. Thus, in the detailed work of G. Müller (1971) it is indicated that at the stage of catagenesis of clayey sediments montmorillonite gradually disappears, while the chlorite content increases. The reaction equation for the formation of chlorite with the addition of the necessary quantities of MgO and H₂O has the following form:

$$ 1.7\mathrm{Al}_2\mathrm{O}_3 \cdot 0.9\mathrm{MgO} \cdot 8\mathrm{SiO}_2 \cdot 2\mathrm{H}_2\mathrm{O} + 9.2\mathrm{MgO} + 6\mathrm{H}_2\mathrm{O} = $$

$$ = 10.1\mathrm{MgO} \cdot 1.7\mathrm{Al}_2\mathrm{O}_3 \cdot 6.4\mathrm{SiO}_2 \cdot 8\mathrm{H}_2\mathrm{O} + 1.6\mathrm{SiO}_2 . $$

Such a transformation of montmorillonite into chlorite, as is evident from the equation given, is accompanied by the binding and decomposition of a considerable quantity of water.

Consequently, sedimentation, diagenesis, and catagenesis of clayey rocks lead to the binding and decomposition of significant volumes of buried waters. If one takes into account that among sedimentary rocks clays predominate, their mass being estimated approximately at \(1.0 \cdot 10^{18}\) tons, and if one assumes that on average 10% of the water in clays is in an ionized state, then the mass of decomposed water will be \(1.0 \cdot 10^{17}\) tons, which is an impressive figure. In fact, this figure is larger, since clays contain more than 10% ionized water and, in addition, water is decomposed in the course of other oxidation–reduction reactions. Thus, according to V. I. Molchanov (1968), oxidation of ferrous iron to ferric iron alone should have led to the decomposition of \(3.08 \cdot 10^{15}\) tons of water, which is approximately 0.002 of the present mass of the hydrosphere.

Mechanism of Water Decomposition

Thus, a considerable quantity of water decomposes in clays. But by what pathway does this decomposition occur? What is its mechanism?

It would be most logical to imagine the decomposition as the simple dissociation of water into \( \mathrm{H}^{+} \) and \( \mathrm{OH}^{-} \) ions. However, taking into account the circumstance that for strongly bound water the dissociation constant increases by only seven mathematical orders of magnitude, whereas the decomposition constant by 64 (Table 2), one must assume that water decomposition may occur by the liberation of free hydrogen, and not its proton. About

This is also evidenced by the data of hydrogeological observations. Indeed, if the formation of the \(H^+\) ion were taking place, then silt waters, as well as pore waters of clayey sediments, would have to have an acidic reaction, which in fact is not observed (Kryukov, 1971; Shishkina, 1972). Consequently, the decomposition of water does not consist in its simple dissociation, but has a more complex character.

Taking into account what has been said, as well as the fact that clayey rocks, as a rule, are characterized by reducing geochemical conditions, we suppose that the principal mechanism of water decomposition in clays is the following reaction:

\[ 2H_2O + 2e = H_2 + 2OH^-. \]

The free hydrogen formed in this way is spent partly on the reduction of the sulfate ion of groundwaters to hydrogen sulfide, partly accumulates in the gas phase of groundwaters (which is established by the corresponding analyses), but the main source of its absorption is the organic matter of sedimentary rocks. Here we come to a very important point, namely the possibility of hydrogenation of the organic matter of clays at the expense of hydrogen released during the decomposition of water.

Indeed, the above reaction of water decomposition is reducing in its nature, since it requires an influx of electrons. The source of the latter in clays cannot be any of the chemical elements having variable valence (sulfur, iron, etc.), since at the moment of burial of the sediment these elements are in an oxidized state and, for their own reduction, also require an influx of free electrons. The source of the latter can only be organic matter, which is always present in clays of marine origin. In this case an oxidation–reduction system must arise in the clays, in which the organic matter, being oxidized, acts as the reducing agent, while water, being reduced, acts as the oxidizing agent. It seems to us that precisely the emergence of this system is the cause of the transformation of the organic matter of sedimentary rocks into hydrocarbons. The source of the hydrogen necessary for this is water, as many investigators have previously indicated. However, we consider the mechanism of hydrogen formation from water to be different from what was assumed earlier (Sokolov, 1972).

It is important to draw attention here also to the circumstance that the most favorable conditions for water decomposition, and consequently for the generation of hydrocarbons according to our scheme, are created in clays capable of swelling, since interpacket water is characterized by the least stability. The connection of oil formation precisely with this type of clay has repeatedly been emphasized by many petroleum geologists (Vassoevich, 1972).

Thus, water in clays, under the appropriate conditions, the details of which are at present unclear and require appropriate investigations, can decompose, most probably by releasing free hydrogen. A number of the following geological facts speaks in favor of such a mechanism.

The first thing to which attention should be paid is the wide distribution in sedimentary rocks of hydrogen sulfide, the genesis of which is usually explained by the processes of reduction of sulfate ions of groundwaters by sulfate-reducing bacteria. However, by the present time data have accumulated which cannot be explained from the standpoint of only the biogenic genesis of hydrogen sulfide. Thus, hydrogen sulfide is widely distributed in brines with a total mineralization of more than 300 g/l, where sulfate-reducing bacteria have not been found. There is observed

the discrepancy between the distribution of hydrogen sulfide of modern genesis and the distribution of sulfate-reducing bacteria in the silty waters of modern marine basins (Shishkina, 1971). Hydrogen sulfide is also formed in deeper horizons of sedimentary rocks, where bacterial activity is impossible. All these contradictions can be resolved if it is recognized that in clays the reduction of sulfates occurs at the expense of free hydrogen formed during the decomposition of water, while sulfate-reducing bacteria, under certain conditions, use the energy of this process for their vital activity. It is also possible that both pathways of hydrogen sulfide formation exist in nature.

From the standpoint of water decomposition, the problem of the formation of a reducing geochemical environment in clays and other poorly permeable rocks is resolved in a new way. The presence under these conditions of a gley or hydrogen-sulfide environment is usually explained by hindered water exchange and the action of microorganisms (Perelman, 1968). However, hindered water exchange and the consumption of oxygen in oxidative reactions by themselves cannot lead to a sharp decrease in the value of Eh, since the absolute electroneutrality of free water is established at Eh equal to \(+0.4\) (Table 2). Sulfate-reducing bacteria, which can also decrease Eh, are far from ubiquitous in their distribution.

In our opinion, there is a more universal mechanism for lowering Eh in poorly permeable rocks, which is caused by the transition of water into a new state characterized by elevated partial pressures of hydrogen, which partly extends also to gravitational waters occurring in poorly permeable rocks.

The best confirmation of the existence of precisely such a mechanism is the fact of the wide distribution of free hydrogen in clays and clayey rocks. Thus, according to P. A. Levshunov (1972), a close relationship is observed between the distribution of hydrogen in clays and the amount of organic matter: clays containing elevated amounts of organic matter also contain elevated amounts of hydrogen. It is interesting that the hydrogen content decreases in those clays where hydrocarbons are present. These data are sufficiently convincing evidence for the reality of the existence of a mechanism of water decomposition with the release of hydrogen and its subsequent participation in the formation of hydrocarbons.

The decomposition of water in particular cases is apparently so significant that it leads to an increase in the mineralization of free waters, which should be manifested most fully in areas of oil fields. And indeed such a fact has long been known, and it has still not received a sufficient explanation in any of the existing hypotheses for the formation of the chemical composition of groundwaters. The list of such problems, the solution of which is more convincing from the standpoint of recognizing the fact of water decomposition according to the scheme proposed by us, could be continued; however, this lies beyond the scope of the present work.

Conclusion

The materials presented in this work convince us that, within the sedimentary shell, and above all in clays, the decomposition of significant volumes of water takes place. The best evidence of this is provided by the composition of the clay minerals that form. Such decomposition is possible because the bound water in clays is characterized by sharply elevated values of the decomposition and dissociation constants. One of the most important factors stimulating this decomposition is the presence in clays of organic matter, with respect to which

therefore water may act as a strong oxidizing agent. The decomposition of water creates in clays a distinctive geochemical environment that promotes the formation of hydrocarbons, hydrogen sulfide, and other hydrogen compounds.

The scale and exact mechanism of water decomposition are at present unknown and require further study. However, even now it may be assumed that the decomposition of water is accompanied by the release of free hydrogen. This circumstance, in our opinion, has important geochemical significance, and research in this direction will help direct scientific thought along a new path, which ultimately will aid in solving many controversial problems of the genesis of oil, geochemistry, lithology, hydrogeology, etc.

REFERENCES

1. A. M. Blokh. On the properties of bound water at elevated temperatures. Lithology and Mineral Resources, No. 5, 1970.

2. N. B. Vassoevich. Initial substance for oil and gas. In: Origin of Oil and Gas and the Formation of Their Deposits. “Nedra,” 1972.

3. R. M. Garrels, Ch. L. Christ. Solutions, Minerals, Equilibria. “Mir,” 1968.

4. P. A. Kryukov. Mountain, Soil, and Silt Solutions. “Nauka,” 1971.

5. P. A. Levshunov. Hydrogen and its significance in the processes of transformation of hydrocarbons. In: Origin of Oil and Gas and the Formation of Their Deposits. “Nauka,” 1972.

6. V. I. Molchanov. Sedimentation and free hydrogen. Doklady of the Academy of Sciences of the USSR, vol. 182, No. 2, 1968.

7. G. Müller. Diagenesis and catagenesis of clayey sediments. In: Diagenesis and Catagenesis of Sedimentary Formations. “Mir,” 1971.

8. G. B. Naumov, B. N. Ryzhenko, I. L. Khodakovsky. Handbook of Thermodynamic Quantities. Atomizdat, 1971.

9. A. I. Perelman. Geochemistry of Epigenetic Processes. “Nedra,” 1968.

10. S. G. Sarkisyan, D. D. Kotelnikov. Clay Minerals and Problems of Petroleum Geology. “Nedra,” 1971.

11. V. M. Sinitsyn. Fossilization of gases in the sedimentary shell and its significance for the evolution of the Earth’s atmosphere. Bulletin of Leningrad University, issue 1, No. 6, 1970.

12. V. A. Sokolov. Processes of formation of oil and gas. In: Origin of Oil and Gas and the Formation of Their Deposits. “Nedra,” 1972.

13. N. M. Strakhov. Foundations of the Theory of Lithogenesis. Publishing House of the Academy of Sciences of the USSR, 1962.

14. N. V. Tageeva, M. M. Tikhomirova. Geochemistry of Pore Waters during the Diagenesis of Marine Sediments. Publishing House of the Academy of Sciences of the USSR, 1962.

15. O. V. Shishkina. Geochemistry of Marine and Oceanic Silt Waters. “Nauka,” 1972.

16. J. Fripiat, A. Jelli, G. Poncelet, I. Andre. Thermodynamic properties of adsorbed water molecules and electrical conduction in montmorillonites and silicas. J. Phys. Chem., v. 69, No. 7, 1965.

17. M. I. Pourbaix. Atlas d’équilibres électrochimiques a 25°C. Paris, 1963.

18. R. Touillaux, P. Salvador, C. Vandermeersche, I. I. Fripiat. Study of water layers adsorbed on Na—and Ca—montmorillonite by the pulsed nuclear magnetic resonance technique. Israel J. Chem., 6, No. 3, 1968.