Thermal History and Hydrocarbon-Potential of the Alps and the Dinarides in Slovenia.

Slovenia participates in the Eastern Alps, the Southern Alps, the Dinarides and the Pannonian Basin. Maturation history and hydrocarbon generative potential of sediments in the Tertiary Pannonian Basin is well known from previous investigations. In the framework of the proposed project it is planned to investigate the different geotectonic units of western Slovenia (Northern Karawanken; Julian-Savinja Alps, Southern Karawanken; Slovenian Trough and Sava Folds; Adriatic-Dinaric carbonate platform) under a similar aspect. Main goals are: Maturation: Evaluation of the diagenetic to low-grade metamorphic patterns of Permo-Mesozoic sediments of all main tectono-facial units of Slovenia. On this basis paleo-temperatures will be roughly estimated and the relation of time between heating events and tectonic movements (thrusting, strike-slip faulting) should be enlightened. Hydrocarbon potential: Quantity, type and maturity of organic matter in sediments and generation time of hydrocarbons will be determined. Basin modelling: Structural and thermal models of basin evolution will be established, considering changing basin geometries, eroded thicknesses, heat flow variations in time and space and time of hydrocarbon generation. The following methods will be applied: Vitrinite reflectance measurements, biomarker ratios, Rock-Eval pyrolysis, fission-track dating, XRD analysis, fluid inclusion investigation, Conodont Colour Alteration Index, K/Ar dating. This study (together with the result of a former FWF project) will provide the foundation of a better understanding of the thermal evolution of the crust during an orogenic cycle (rifting - collisional thickening - tectonic unroofing - lateral extrusion) the geodynamic evolution of Slovenia, which is a key area within the Pannonian-Carpathian-Dinaric (PANCARDI) region the hydrocarbon potential of Slovenia.

Slovenia participates in the Eastern Alps, the Southern Alps, the Dinarides and the Pannonian Basin. The thermal evolution and the hydrocarbon generative potential of the Tertiary Pannonian Basin are well known from previous investigations. Main goals of the present project were: (1) evaluation of maturation patterns in Carboniferous to Eocene rocks, (2) characterization of the source potential of organic-rich sediments, (3) modelling of the thermal history. More than 1000 samples with different ages and tectonic positions were investigated within the frame of the project using vitrinite reflectance, RockEval pyrolysis, organic geochemistry and clay mineral diagenesis. The maturation of the rocks ranges from diagenesis to low-grade metamorphism. Maturation patterns reflect the structure of the Mesozoic shelf of the Tethyan-ocean, which was characterized by shallow platform areas bordering the deep Slovenian Basin. The topographic relief was a result of Triassic extension and was preserved till the basin was filled with flysch sediments during Cretaceous-early Tertiary times. Therefore, coeval rocks in the Slovenian Basin are characterized by significantly higher maturation than rocks on the adjacent paleo-platforms. The Slovenian Basin subsided along Mesozoic normal faults, which are reflected in maturation maps as areas with high lateral maturation gradients. Some of the Mesozoic faults were reactivated as Neogene strike-slip faults (e.g. Idrija Fault), which displace iso-reflectivity lines. Within each tectonic unit, maturation increases with the age of the rocks. Maximum heating occurred during the early Tertiary. Tertiary nappe piling played only a minor local role for the thermal evolution. Late Cretaceous to early Tertiary heat flow was generally low (e.g. ~50 mW/m in the Southern Karavanke Mts.). This agrees with the geodynamic position of the study area (transition between a passive and an active continental margin). In the area of the Slovenian Basin, where upper Cretaceous rocks were heated to more than 250 C, heat flow was probably slightly higher. As a result of a change in the geodynamic situation (subduction-related magmatism, extension in the Pannonian realm), extremely high heat flows prevailed during Oligocene and Miocene times. Maturation patterns also allow inferences on the thickness of eroded rocks. Maturation gradients in the Slovenian Basin indicate erosion of (lower Tertiary) rocks about 5 km thick. Erosion was significantly less effective within the Julian Alps. Potential (immature to marginal mature) source rocks with algal-rich type II kerogene are restricted to Carnian (upper Triassic) and Ladinian (middle Triassic) levels of the Eastern and Southern Alps. However, most organic- rich rocks are overmature (e.g. Ladinian in the Slovenian Basin). Hydrocarbon generation in these rocks occurred during early Tertiary times. Frequently observed bitumen stainings are a result of the early generated hydrocarbons. Coal seams within the Carnian, Jurassic (anthracite), upper Cretaceous (high vol. bituminous coal) and the lower Tertiary (sulfur-rich sub-bitumnious coal) of the Adriatic-Dinaric carbonate platform are thin and probably do not have any significance as a source for gas.