PARATETHYS · Paleogeography of the Paratethys: Relation to hydrocarbon source rock quality

Paleogeography of the Paratethys: Relation to hydrocarbon source rock quality

The project aims to reconstruct the Oligocene to Early Miocene palaeogeographical and environmental evolution for the Central and Eastern Paratethys and their relations to hydrocarbon source rock quality. To accomplish this goal, the marine and estuarine successions of the Maikop series of the greater caucasus, the Schoeneck formation of the Alpine foreland basin, the Tard Clay of the Pannonian Basin, and of the Bulgarian Ruslar Formation have been studied using a multi-disciplinary research approach. Similarities and differences in organic geochemical composition between the Oligocene Ruslar Formation (Bulgaria) and the existing data of the Schoeneck Formation (Austria) and overlying strata (Dynow marlstone, Eggerding Formation) were outlined. Dysoxic to anoxic conditions and mesohaline to euhaline salinities prevailed during deposition of the Ruslar Formation. Relatively high oxygen contents occurred during early Solenovian times, when brackish surface water favoured nannoplankton blooms and the deposition of bright marls ('Solenovian event' considered as equivalent of the Dynow marlstone). Anoxic conditions with photic zone anoxia were established during late Oligocene times and, probably, reflect a basin-wide anoxic event in the Eastern Paratethys during Kalmykian times. Aquatic and land plant-derived biomass contributed to the organic matter. Relatively high amounts of aquatic organic matter occur in the lower part of the Ruslar Formation and in its upper part, where diatoms are especially abundant. In the Austrian part of the Alpine foreland basin, oil and minor thermal gas are thought to be predominantly sourced from Lower Oligocene horizons (Schoeneck and Eggerding Formations). The source rocks are immature where the oil fields are located and enter the oil window at ca. four km depth beneath the Alpine nappes. Oil-oil and oil-source rock correlations within the Alpine foreland basin of Austria highlighted compositional trends in W-E direction, which reflect differences in source, depositional environment, and maturity of potential source rocks. The enhanced contents of sulfur-aromatic compounds in oils from the western part of the basin would be consistent with a higher contribution of marly shales of the Schoeneck Formation to hydrocarbon expulsion in this area. The observed trend towards lighter carbon isotope composition of hydrocarbon fractions from oil fields in a W-E direction is consistent with lower 13C/12C ratios of organic matter in the black shales of the Schoeneck Formation. The results argue for hydrocarbon migration through highly permeable carrier beds or open faults. The lateral distance of oil fields to the position of mature source rocks beneath the Alpine nappes in the south suggests minimum migration distances between less than 20 km and maximum migration distances more than 50 km. Differences in water column stratification and salinity were identified to govern the deposition and preservation of potential hydrocarbon source rocks. Post-depositional erosion and re-deposition of sedimentary units during basin evolution and subsidence modified the overall hydrocarbon potential in the Alpine realm. Organic geochemical parameters and stable isotope data of organic matter and carbonate within the Tard Clay Formation of the Hungarian Paleogene basin revealed the presence of anoxic cylces leading to the deposition of potential hydrocarbon source rocks. A major contribution of aquatic organisms (green algae, dinoflagellates, Chrysophyte algae) and minor inputs from land plants to organic matter accumulation is indicated. Microbial communities included heterotrophic bacteria, cyanobacteria, chemoautotrophic bacteria, as well as green sulfur bacteria (indicating photic zone anoxia) and methanotrophic bacteria. Higher inputs of terrestrial organic matter occurred during deposition of the lower and uppermost units of the Tard Clay Formation. Environmental conditions changed from marine to brackish, accompanied by oxygen-depletion in the lower parts of the water column. Organic carbon accumulation during this period was a consequence of stagnant bottom water conditions in the Hungarian Paleogene Basin due to salinity stratification. Up to three anoxic cycles are recognised in the drill core sections. The results outlined that geochemical data can be used for intra-formational correlation of the Tard Clay. Organic geochemical and carbon isotope data obtained from a sediment profile of the Maikop Formation in Azerbaijan indicate similarities with the results from the Oligocene Ruslar Formation of Bulgaria. Strictly anoxic conditions due to stratification of the water column and the presence of free H2S in the bottom water are reconstructed during the deposition of organic matter-rich sediments of the Maikop Formation (probably the equivalent of the respective units of the Ruslar Formation deposited during Kalmykian times). The position of the oxic/anoxic boundary (chemocline) within the photic zone of the water column is indicated by the presence of biomarkers from green sulfur bacteria. A position of the chemocline in deeper parts of the water column and enhanced input of land plant-derived organic matter during the deposition of the Upper Maikop sediments are evidenced from our results. Diatoms are major organisms contributing organic matter to the sediments. The results outlined periods and locations within the PARATETHYS during Oligocene times that enabled the deposition and preservation of potential hydrocarbon source rocks. Differences in water column stratification and salinity governed source rock quality. Reconstruction of the thermal evolution of hydrocarbon source rocks and migration pathways enhanced our understanding on the pre-conditions and generation of petroleum deposits within the prospective areas. The results contributed to the understanding of the petroleum systems and, therefore, will increase the successful exploration and sustainable exploitation of hydrocarbons. The understanding of the generation of gas within the Austrian Molasse basin has been changed in the light of recent results supporting the importance of secondary microbial methane formation.

Data: CORDIS, © European Union