Privalov V. Tectonothermal evolution of the Donets Basin

The thesis is devoted to solving problem of the Donbas tectonothermal evolution. The new geodynamic model of the Basin is being proposed. The Donets basin is the most anomalous segment of the Pripyat-Dnieper-Donets rift and located as a deformed prismatoidal block on the intersection of NNW striking Early Proterozoic weak lineament belt within the Sarmatian Shield. Being essentially a "solitary" block within continuous rift system, the Donbas responded much more sensitively to plate motion stresses accommodating them in shuttle anti-clockwise and clockwise rotations of the Donbas megablock. A number of tectonic phases affected the Donbas and associated with sign-variable rotational regimes are being distinguished. The significant tectonic element of intrabasional architecture is the WNW-ESE trending principal displacement zone (PDZ) consisting of a set of dextrally en echelon arranged deep basement faults with oversteps superimposed on depressions (pull-aparts) in the basement. In contrast with adjacent rift sectors in the Donbas during the Carboniferous, extensive coal-forming environments have been developed periodically being triggered by interplay of PDZ strike-slip motions and shuttle sign-variable rotations of the Donbas megablock. This caused recurrence from swampy coastal-marine plains with huge peatbogs to shallow sea environments and resulted in accumulation of thick paralic coal-bearing Carboniferous formation containing more than 300 coal seams and layers. The hydrogen index of Donbas coal samples ranges from 229 to 306 mgHC/gTOC. This suggests that some seams can be considered gas- and gas-oil-prone. Thermal maturation of concentrated in coal beds and dispersed in the Carboniferous host rocks organic matter has led to formation of an enormous methane resource. Most of methane in coal beds is sorbed onto microporous surfaces of coal beds and dispersed coaly matter in rock massive. The significant methane resource is expected as result of conventional trapping related with gas migrationand permeability contrasts of tectonic and lithological origin. Variations of methane content in coal beds and country rock massive in the Donbas reflect a consequence of events from kerogen breakdown to gas generation, migration and formation of reservoirs and local entrapments. Hydrocarbon and, more specifically, methane generation is a function of kerogen composition, burial and thermal history. However, the final balance of retention of gas is critically dependant on timing of hydrocarbon generation pulses and their relationships with inversion episodes, and following temporal and spatial distribution of tectonic stress regimes. Maximum burial occurred during early Permian time and was followed by a major exhumation phase. First and principal pulse of gas generation in the Donbas occurred mainly during the Carboniferous and Early Permian deep burial on the background of heat flow in the range of 40-75 mW/m2. A major part of the Donbas has been uplifted dramatically and deformed with prominent NNE-SSW shortening grain during the latest phases of Hercynian orogeny. This was in response to stresses generated by convergence of plates in the Ural orogenic belt, transmitted to the Donbas and re-actively amplified here because of sinistral shearing and following anti-clockwise rotation of the Donbas megablock. The basin uplift and compressive deformations caused intensive gas migration and even partial loss. However, the significant volumes of gas were retained in dilatational domains and bends. Modelling of the fission track data, combined with modelling of vitrinite reflectance data, reveals that large parts of the basin were affected by a Permo-Triassic (~250 Ma) heat flow event, which was related to Permo-Triassic andesitic magmatism. Our results suggest that coal bed methane was generated not only during maximum burial, but also afterwards. Second gas generation phase postdates maximal burial time and Permian inversion and can be considered as an inevitable result of Permian-Triassic thermal event, which epicentres at level of 125-200 mW/m2 appear to be associated with releasing sectors along the PDZ and rotational jogs on the boundaries of transversal deep faults. Obviously, the late generated gas has a higher preservation potential. Therefore, the gas potential of the Donets Basin should be re-evaluated in the light of the new findings.The secondary Early Cretaceous uplift episode affected mainly the SE margin of the basin. During this event the full de-methanisation of anthracites adjoining Rostov block (Russia) took place, and sorption niche in this region was re-occupied by carbon dioxide. Whereas rocks west of the city of Donetsk (Krasnoarmeisk Monocline) experienced Permo-Triassic temperatures in the range of 90 to 105 °C, rocks northeast of Donetsk (Tchystyakovo-Snezhnyansky region) were heated to up to more than 240 °C. Jurassic temperatures northeast of Donetsk were in the order of 90 to 100 °C. Cimmerian and Alpine tectonic events resulted in sectorial syn- or transpressional reactivations of reversed, normal and lateral slip movements on fault planes, deplanation of strata, formation of linear and subconcentric shallow dextral shear belts superimposing onto principal Hercynian structural trends in the sedimentary cover. The results indicate that increased levels of elastic energy accumulated in rock massive due to recent tectonic stress are likely to contribute into localizing areas of gas-and-coal outbursts activity. Constraints resulted from Rock-Eval data suggest the possible ways to predict potential outbursts hazards.