Regional and stratigraphic delineation of deep groundwater systems in Upper Austria using geochemical and thermal data

Sedimentary basins are finite spatial resources, which face increasing competition between different areas of utilization, including groundwater, mineral water, geothermal energy, oil and gas, as well as underground storage. Hence, the knowledge of the deep subsurface is an important aspect for the security of supply, economic development and sustainable management of the underground. Within this context, it is crucial to recognize, define and delimit hydrostratigraphic units, which may occur in one or more lithostratigraphic units.

The Alpine Foreland Basin (AFB) forms the link between the Alpine and Danube macro-regions. It is home to a large population and an important industry location. In the Upper Austrian part, the subsurface of the AFB is intensively used for hydrocarbon and geothermal energy production, for gas storage and for balneological purposes, often from the same geological units. In order to avoid conflicts between different users (national and transnational), a detailed knowledge of hydrostatigraphic units is crucial. Traditionally two hydrostratigraphic units are distinguished in Upper Austria:

- The Malmian aquifer extending from Bavaria to Austria is characterized by complex flow patterns, often low salinity and discharges into shallow marine sandstones of the Oligocene Linz-Melk-Fm. Both aquifers are used for geothermal and balneological purposes.

- Upper Oligocene/Lower Miocene aquifers provide iodide-rich, high salinity waters, which are used for balneological applications.

Considering the great economic importance of the subsurface of the AFB, hydrogeological models have been established. These models have obvious weak points (e.g. poorly defined boundaries), which are caused by low data density, inhomogenous data sets and poor integration of data from water supply, geothermal and petroleum wells.

 

 

Thickness map of the Malm horizon. Simplified thermal water system, regional water flow (modified from Bayrisches Landesamt für Wasserwirtschaft, 1999) and location of oil fields are indicated. Inset presents location of map. From Pytlak et al. 2017 (in press)

 

Therefore, the general aim of the 3-year project is to create a 3D hydrostratigraphy model of the AFB in Upper Austria. To reach the aim, we will create a database with all relevant and available data from petroleum and geothermal industries and water suppliers. Based on these data, an target-oriented sampling strategy will be developed. It is planned to take about 60 samples from different stratigraphic horizons in petroleum, geothermal and water supply wells. The water samples will be investigated regarding hydrochemical composition and stable isotopes. Main targets of the analyses are to define water types, to recognize commingled waters and to apply chemical geothermometers. The molecular and isotopic composition of dissolved gases will be determined to gain information on their origin, migration and alteration. Temperature data from formation tests supplemented by new measurements will be used to improve existing temperature models. The lateral and stratigraphic distribution of water types, possible anomalies and deviations of calculated temperatures from chemical geothermometers from regional trends will be used together with information on dissolved gases to reconstruct upward and downward movement of fluids.

The applicants have a profound understanding of petroleum systems in the AFB. Based on this knowledge, the interaction between petroleum and groundwater systems will be discussed. This approach is especially promising, because on one hand oil data prove a significant influence of hydrodynamic activity on hydrocarbon composition, and on the other hand hydrocarbon gases and oil components are present in balneological waters. In addition, preliminary data indicate identical migration pathways for deep groundwaters and hydrocarbons.

Outcomes of the project will include maps and profiles showing the lateral and vertical distribution of hydrostratigraphic units as well as their delineation and – if applicable – their interaction. The hydrostratigraphic units will be characterized by the distribution of hydrochemical and isotopic parameters and the chemistry of dissolved gases.

The project is relevant for a number of stakeholders (e.g. public authorities, petroleum industry, geothermal energy, balneology). Apart from scientific publications, it is planned to discuss the project outcomes with the stakeholders within the frame of 1-day workshop.