PROSPECTS FOR EXPLORATION OF HYDROGEN FIELDS IN RIFTOGENE STRUCTURES ..

V. Shestopalov 1,2, O. Lukin2, V. Starostenko3, O. Ponomarenko4, T. Tsvetkova3, I. Koliabina2, O. Makarenko1, O. Usenko3, O. Rud 5, A. Onoprienko6, V. Saprykin1, R. Vardapelian7, 2021 1Radioenvironmental Centre of the National Academy оf Sciences of Ukraine, Kiev, Ukraine 2 Institute of Geological Sciences of the National Academy of Sciences of Ukraine, Kiev, Ukraine 3 S. I. Subbotin Institute of Geophysics of the National Academy of Sciences of Ukraine, Kiev, Ukraine 4 M. P. Semenenko Institute of Geochemistry, Mineralogy and Ore Formation of the National Academy of Sciences of Ukraine, Kyiv, Ukraine 5G. V. Kurdyumov Institute of Metal Physics of the National Academy of Sciences of Ukraine, Kiev, Ukraine 6NDI Foundation, Kiev, Ukraine 7European Centre for Knowledge and Technology Transfer, Brussels, Belgium Received 25 August 2021

Introduction. In this paper, using the example of the Dnieper-Donets Depression, (DDD), located in the southern part of the East European Platform and originated and developed in the Proterozoic and early Paleozoic as a typical rift, we would like to show the promise of such structures to discover elevated concentrations of native hydrogen (Fig. 1).
Although in the former USSR, geological hydrogen has been studied in various structures, at various depths, and of various genesis for a long time, and was first summarized in the English-language publication [Zgonnik, 2020], the most thorough studies of hydrogen in the Earth interior of individual regions and specific genetic types have been done in recent decades [Shestopalov, 2020].
The assumed occurrence of primary hydrogen was substantiated in a number of theoretical studies [Kronig et al.,1946;Stevenson, 1977;Larin, 1993Larin, , 2005Semenenko, 1990;Ma rakushev, 1999;Rumyantsev, 2016 etc.], and was supported by specific studies of manifestations of degassing and migrati on of primary hydrogen [Letnikov, 2001[Letnikov, , 2015Syvorotkin, 2002;Gilat, Vol, 2005, 2012Walshe, 2006;Gufeld, 2012Gufeld, , 2013Murphy, 2016;Ikuta et al., 2019 etc.]. In particular, the re sults obtained by our research group [Shes topa lov, Koliabina, 2019;Shestopalov et al., 2021], indicate the theoretical possibility of hydrogen penetration into olivine-bearing rocks from beneath (coming from the mantle), followed by formation of water and the initiation of serpentinization with additional hydrogen release. Based on the results obtained, serpentinization of olivine with hydrogen release, determined previously in many studies, can be considered the second phase of the whole process. The first phase consists of deep hydrogen penetration into olivine and water formation.
In support of this version of hydrogen primary generation, we provide the results of the application of seismic tomography to study processes in the mantle. According to the developed method for solving the inverse multidimensional problem of seismic kinematics, a three-dimensional velocity model of the mantle was obtained (Fig. 2). The re-vealed location of zones of superdeep fluid activation in the mantle indicates their relationship with the existence of oil-and-gas and probably hydrogen accumulations in the Dnieper-Donets Depression. Dr. Tatiana Tsvetkova and her colleagues [Tsvetkova et al., 2017[Tsvetkova et al., , 2020 have shown that not only plumes but also individual upward flows of deep fluid could be distinguished in the mantle beneath the platform. In particular, the Volyn-Orsha plume and four fluid flows (f2, f3, f4, f12) were distinguished in the vicinity of the Dnieper-Donets Rift (Fig. 3).   Along with the diffusive spreading of hydrogen in the rock mass, a relatively anomalous, often streamline migration of hydrogen in fault zones and fractures is detected. This fault-directed pathway of anomalous hydrogen migration, regardless of its origin, is the migration path of maximum concentration and density of the H 2 fluid.
Results and discussion. Within the Dnieper-Donets Depression, the geologicalgeophysical methods distinguish a significant number of deep faults and ring structures of volcanogenic and explosive origin, which are of interest to discover the increased hydrogen accumulations. Possible locations of the most promising zones of hydrogen concentration in continental platform conditions are confined to faults in rift systems and their nearest fringe, as well as to explosion and volcanogenic ring structures with signs of modern activation (Fig. 4). The prospectivity of such area is determined by research and analysis of a set of geophysical characteristics (thermal, seismic, gravity, electrical conductivity, magnetic) and the corresponding geological and hydrogeological parameters.
Among them, we singled out the most studied Sribne ring structure . A large number of hydrocar-bon fields have been explored and exploited within it, and we have outlined areas that are promising for the study of hydrogen (Fig. 5).
Many hydrocarbon fields have been explored near and within the Sribne ring structure, which is an explosive ring structure of ancient origin that is periodically activated even in the modern period.
The partially detected and supposed pulsatile nature of hydrogen degassing causes intermediate temporary accumulation of hydrogen in formations covered with low-permeable cap rocks, which impede hydrogen degassing, though cannot fully prevent it. Apart from dense igneous rocks (like dolerites in Mali), such low-permeable cap rocks may be formed by thick beds of rock salt at depths of 1-2 km, hydrophilic clay, and intrusive rocks not subjected to metamorphism, etc.
Understanding of the hydrocarbon fields formation and established modern recharging of many of them suggest that their occurrence and maintenance of the certain reserve level is associated with modern activation processes, characterised by intensive migration of hydrogen, which determines the hydrogenation of carbonaceous components. Therefore, oil and gas wells contain the residual hydrogen unconsumed in hydrocarbon formation. However, besides zones of hydrocarbon accumulation and hydrogen consumption for their formation, there may exist zones with conditions of minimising the carbon concentration in geological structure and, consequently, increasing the concentration of uninvolved hydrogen. To select areas located between or beyond hydrocarbon fields (both well-known, as well as not yet discovered), the analysis of local depressions distribution should be used, in which anomalies of hydrogen concentrations are identified by subsoil measurements (see Zgonnik et al., 2015, etc.]).
In the Dnieper-Donets Depression, a variety of local depressions have been decoded from satellite images. The depressions that form well-defined linear chains and ring structures indicating different stages of degassing are well distinguished among them (Fig. 6).
As an example of field work to assess H 2 degassing in a local depression, we present the results obtained 30 km east of Kyiv. The graph clearly shows that outside the local depression, hydrogen concentrations at depths of 0.45 to 1.5 m are close to zero. The maximum values of the H 2 concentration are typical for point 5 near the edge of the depression and they reach 3300 ppm at the depth of 1.5 m (Fig. 7).
The space images were used to identify and quantify the local depressions of the Dnieper-Donets Aulacogene (Fig. 8). About 400,000 local depressions were identified, clustering into linearly elongated lines and spots (shown in white in the Fig. 8). The thickening maxima of these depressions form linearly elongated lineaments (shown in red in the Fig. 8), which coincide well with modern geodynamic areas and some faults identified by other methods. These and other results indicate a clear linkage of local depressions distribution maxima to the fault zones of recent activation, to the areas of highly fractured hard rocks, and to zones of loose rocks decompression.
The clusters of local depressions are mostly located outside the contours of hydrocarbon fields. Most of local depressions are located in the zone of the Southern Near-Edge Fault and the southern slope of the basement. No hydrocarbon fields are discovered in this area. Local depressions are associated with main geodynamic zones and, occasionally, with local fracturing of sedimentary rocks in these zones. Individual measurements of subsoil hydrogen in these depressions show values of up to 11800 ppm. Presumably, the conditions within the area of Southern Near-Edge Fault and the southern slope of the basement are quite similar in setting to those in Mali. In Mali the hydrogen field is located in the near-slope part of the basement, while in DDD, many local depressions resulted from degassing are concentrated in the southern near-slope part of the basement. The main oil-and-gas bearing region is located to the north of Mali, in the deeper part of the trough, while in DDD it is located north of the abovementioned local depressions, in the central part of the graben.
Noteworthy results were obtained in the area of the Northern Near-edge Fault of the mantle origin within the Dnieper-Donets Graben (Fig. 9). A large number of hydrocarbon fields are concentrated near this fault (marked in yellow on the Fig. 9). Relatively maximum  Геофизический журнал № 5, Т. 43, 2021 9 concentrations of local depressions (marked in red on the Fig. 9) allow us to assume the predominant concentration of molecular H 2 clusters outside the hydrocarbon clusters, which are the product of the hydrogen influencing the carbonaceous components of the geological environment. There are no hydrocarbons in the zone of the Southern Near-Edge Fault (see Fig. 6), but this area, including the southern slope of the Dnieper-Donets Depression, contains    Based on the results of the study of geological and geophysical materials, space images and field work, the territories for further more detailed work were allocated within (Fig. 10):

PROSPECTS FOR EXPLORATION OF HYDROGEN FIELDS IN RIFTOGENE STRUCTURES ...
-Sribne and other ring structures, -Southern Near-Edge Fault, -Northern Near-Edge Fault. In this case, the areas for the first-priority study (highlighted in red) and for secondpriority study were identified. Within these areas sites were indicated (red dots) for initial investigation using satellite images, gas sampling (hydrogen, helium, methane, etc.), primary geophysical surveys, with evaluation of intermediate reservoirs and cap rocks. And their results can be used to carry out shallow test drilling and sampling. According to the results obtained and data comparison, the sites for priority drilling and sampling to a greater depth will be selected.
Conclusions. Selection of preferred areas for detailed investigation should be based on a comprehensive analysis of available geophy sical data, study of the geological section with the identification of potential reservoirs and possible low-permeable cap rocks, performing certain types of geophysical studies to specify geophysical characteristics, detailed interpretation of space images to study the features of local depressions and their clusters as well as gas geochemical studies. Based on results of all investigations the most promising areas are selected, as well as a scope of detailed geological-geophysical and gas-geochemical investigations determined. Upon analysis of all materials obtained, sites for exploratory drilling are selected and technical parameters specified.
The following factors provide confidence in the success of the exploration of the hydro gen reservoirs in the Dnieper-Donets Depression: 1. Rift systems in the platforms are characterized by deep activation of the geological environment within the Earth crust and upper mantle with active spreading of volcanogenic explosive ring structures originated in the Earth's deep interior under the action of fluids.
2. Rift systems are characterized by the development of faults originated in the mantle, often under tension stress, which have been active for a long geological time.
3. The high concentration of hydrocarbon fields in the Dnieper-Donets Rift (over 200 proven fields and significant prospects for their increase) indicates a considerable opening of the Earth's deep interior, which facilitates the upward migration of fluids.
4. Hydrocarbon fields within the rift show elevated helium concentrations (total and He 3 /He 4 ratio), which also indicate the active connection of sedimentary cover with the Earth's deep interior (lower crust and upper mantle).
5. Geophysical surveys allow identify ing areas of high electrical conductivity, thermal field anomalies (when measuring the convective component in fault zones), relative mi ni ma of the gravity field, etc., indicating the potential for significant hydrogen accumulations. 6. The investigations of the lower mantle also indicate the manifestation therein of deep upward migration of fluids in the zone of probable influence on the upper mantle be neath the Dnieper-Donets Aulacogene.
7. Detection from satellite images of local depressions concentration in the modern relief and finding their connection with faults indicate the active modern degassing of hydrogen.
8. Some wells drilled for hydrocarbon production have shown elevated hydrogen concentrations (up to 25-40 %).
9. The geological section contains rocks that can store and transmit hydrogen (reservoir rocks) as well as cap rocks that form a barrier above and partially prevent the free upward migration of hydrogen, thus contributing to its temporary accumulation in the reservoir rocks.
All this combined presents an optimistic outlook for the discovery of hydrogen fields in the Dnieper-Donets Aulacogene.
The arguments for the selection of priority areas are: 1. The Sribne depression was selected because it represents deep volcanogenic-ex plosive ring structure that has manifested activation at all major stages of its development, including the modern one.
2. The geology of this area is the most extensively studied. It is characterized by the occurrence of a large number of proven hydrocarbon fields confined to the Upper Carboniferous and Permian deposits and found at depths ranging from 1.2 to 6.5 kilometers.
3. Locally, the elevated helium concentrations (up to 1.78 wt%) were detected in the boreholes.
4. Hydrogen degassing is confirmed by a large number of local depressions related to disjunctive dislocations.
5. Geological section in this area is characterized by occurrence of reservoir rocks and good cap rocks -relatively impermeable rocks involving salt formations with low permeability.
6. The area including the Southern Near-Edge Fault and the southern slope of the Dnieper-Donets Depression was selected because it combines a well-developed network of deep faults and their intersections with the highest density of local depressions, their lin-ear and nodal concentrations, indicating the anomalous degassing in this area.