Hans-Gert Linzer

Neogene uplift and erosion in the Alpine Foreland Basin (Upper Austria and Salzburg)

Neogene uplift and erosion in the Alpine Foreland Basin (Upper Austria and Salzburg) In the present paper we apply a multi-technique approach (shale compaction data, seismic stratigraphy, isopach maps, moisture content of lignite, fission track data) to assess timing and amount of uplift and erosion of the Alpine Foreland Basin. The combination of the different techniques allows us to discriminate the effects of two different erosion events during the Neogene: (1) Seismic stratigraphy and isopach maps indicate a Karpatian (Early Miocene) regional tilting of the basin to the west (slope of about 0.5 %) and a minor erosion phase. (2) Moisture content of lignite combined with fission track data provides evidence for extensive regional uplift after deposition of Late Miocene fluvial deposits. It is estimated that sediments, 500 to 900 m thick, have been eroded. Shale compaction data derived from sonic logs indicates additional uplift of the eastern part of the basin (near the river Enns). Here, 300 to 1000 m of sediments were additionally eroded (giving a total erosion of about 1000 to 1900 m!), with a general increase of erosion thickness towards the northeast. While the regional uplift is probably related to isostatic rebound of the Alps after termination of thrusting, the local uplift in the east could be affected by Late Neogene E-W compressional events within the Alpine-Pannonian system. Both, tilting and erosion influence the hydrocarbon habitat in the Molasse Basin (tilting of oil-water contacts, PVT conditions, biodegradation).

Gas accumulations in Oligocene–Miocene reservoirs in the Alpine Foreland Basin (Austria): evidence for gas mixing and gas degradation

Two petroleum systems are present in the eastern (Austrian) sector of the Alpine Foreland Basin. Whereas oil and thermogenic gas in Mesozoic and Eocene reservoir rocks have been generated beneath the Alps in Lower Oligocene source rocks, relative dry gas in Oligocene–Miocene clastic rocks deposited in the deep marine basin-axial channel system (Puchkirchen Channel) is interpreted as microbial in origin. Detailed investigations of the molecular and isotope composition of 87 gas samples from 86 wells, representing all producing fields with Oligocene and Miocene reservoir rocks, suggest that the presence of pure microbial gas is rare and limited mainly to the northern basin flank (e.g., KK field). All other fields contain varying amounts of thermogenic gas, which has been generated from a source rock with oil-window maturity. A relation with the underlying thermogenic petroleum system is obvious. Upward migration occurred along discrete fault zones (e.g., H field) or through low-permeability caprocks. Local erosion of Lower Oligocene sediments, the principal seal for the thermogenic petroleum system, as well as a high percentage of permeable rocks within the Puchkirchen Channel favored upward migration and mixing of thermogenic and microbial gas. All gas samples in Oligocene–Miocene reservoirs are biodegraded. Biodegradation and the formation of secondary microbial gas resulted in gas drying. Therefore, the gas samples analyzed in this study are relative dry, despite significant contributions of thermogenic hydrocarbons. Biodegradation probably continues at present time. The degree of biodegradation, however, decreases with depth.

Light Hydrocarbon Geochemistry of Oils in the Alpine Foreland Basin: Impact of Geothermal Fluids on the Petroleum System

Oil is produced in the Austrian sector of the Alpine Foreland Basin from Eocene and Cenomanian reservoirs. Apart from petroleum, the basin hosts a significant geothermal potential, which is based on the regional flow of meteoric water through Malmian (Upper Jurassic) carbonate rocks. Oils are predominantly composed of n-alkanes, while some samples are progressively depleted in light aromatic components. The depletion in aromatic components relative to abundant n-alkanes is an effect of water washing. Waters coproduced with oils that are affected by water washing show a progressive reduction in salinity and depletion in 2H and 18O isotopes, indicating that the degree of water washing is mainly controlled by the inflow of meteoric water from the Malmian aquifer. In some fields with Cenomanian reservoir rocks, a hydraulic connectivity with the Malmian aquifer is evident. However, water washing is also recognized in Eocene reservoirs and in areas where the Malmian aquifer is missing. This shows that existing flow models for the regional Malmian aquifer have to be modified. Therefore, the results emphasize the importance of combining data from the petroleum and geothermal industry, which are often handled separately.

Diagenesis in Cenomanian Clastic Reservoir Rocks of the Alpine Foreland Basin (Austria)

The North Alpine Foreland Basin is a minor oil and moderate gas province. It´s major Reservoir horizons are for oil Cenomanian and Eocene non- and shallow marine sandstones, while mainly Oligo-/Miocene deep water sediments comprise gas. In this contribution, the diagenetic history of Cenomanian reservoir sandstones is presented. Commonly, facies and primary mineralogy controls diagenesis. The lower units of the Cenomanian green-sandstones are poorly cemented and clay minerals (illite, kaolinite) prevail as pore filling. On the contrary, the upper unit is enriched in carbonate cement, identified as pure calcite. Occasionally clay minerals have been replaced by carbonate cement and are therefore interpreted as primary matrix or very early cement phase. Carbonate cements of Cenomanian sandstones exhibit isotope values of about d13C: -13.6 to 3.3‰ and d18O: -23.9 to -4.2‰ [PDB]. The trend of d18O towards lighter isotope ratios relative to sea water is attributed to a meteoric flush. Within one well, biogenic influence is indicated by light d13C ratios of -32.7 to -25.0‰. Reservoir quality is affected by carbonate cementation. High porosity values are caused by poor cementation, whereas permeability is decreased by the presence of clay minerals.

Light Hydrocarbons Geochemistry of Oils Trapped in the Austrian Foreland Basin

The Alpine foreland basin is a minor oil and moderate gas province in central Europe and is considered as mature in terms of hydrocarbon exploration. During this time many studies have been performed to understand the petroleum system in the basin. Within this context, organic geochemical, biomarker and stable isotope data have been used to characterize potential source rocks and their lateral variability in facies and maturity, as well as in oil-source rock correlations. However, till now all studies have focused on the C15 hydrocarbon fraction of oil samples. Hence, the aim of the present study is to explore the information provided by the light hydrocarbon fraction with respect to maturity, kerogen type and hydrocarbons alteration. Results from the Austrian foreland basin reveal that hydrocarbons are derived from kerogen type II, which is in agreement with results from “classical” biomarkers studies. Maturity estimates based on the light hydrocarbon fraction correlate very well with estimates based on aromatic hydrocarbons. Water washing, which is difficult to trace using higher hydrocarbons fraction, can be clearly identified applying light hydrocarbons. Oil-water interaction yields information on hydrodynamic mechanisms in active aquifers, which are important for drilling issues, oil production as well as hydrothermal potential estimations.