By Philip S. Prince, Virginia Tech Active Tectonics and Geomorphology Lab

The Vienna Basin is home to an interesting hydrogeothermal energy initiative that has an interestingly close connection to oil and gas exploration. I first heard of these projects while preparing a lecture on alternative energy about a year ago. To me, this is a really interesting story in terms of its geologic setting as well as its context within the search for energy resources in the past, present, and future. I put together a couple of simple models for the lecture to try to illustrate the context of the Vienna Basin within surrounding features and provide a framework for understanding some interesting cross sections of the project area that show up again and again. I will say up front that I am far from an expert in the geology of the region and I don’t know much about the present state of the hydrogeothermal projects, so I try to provide lots of links to more focused discussions of the area. Hopefully these will add to my simplified narration of the basin’s development and its zones of geothermal potential!

Red dot is Vienna; orange dot is Bratislava. The eastern Alps and western Carpathians are essentially a continuous thrust front; it is locally buried by a few kilometers of sediment where it has collapsed into the Vienna Basin. Note that there was “piggyback” sedimentation on the thrust belt prior to the basin’s formation, so the thrust faulting and depositional history is VERY complex. It is highly simplified here.

The Vienna Basin is a rhomb-shaped pull-apart basin that separates the northern Calcareous Alps west of the city from the western Carpathians on the other side of the basin (see the Intro here). The basin appears to “cut” and separate the two mountain belts, which it indeed does.

Image source here. The rhomb shape with the “2” in it is the Vienna Basin, which has disconnected the mountain topography of the eastern Alps from the western Carpathians.

When the normal faults that bound the basin formed, they cut across the pre-existing continuous thrust belt. This caused a portion of the thrust belt to subside and be buried by up to a few kilometers of sediment fill.

The elongated light blue structures are anticlines above thrust faults. These structure formed during compression of the layer pack. Moving the right of the model toward the top and the left of the model toward the bottom produces a pull-apart basin along a fault stepover. The thrust-related structures collapse into the basin, where they are buried by sediment. They are still there; they’re just cut by normal faults and out of sight.

As a result, there are “blocks” of thrust faulted compressional structures underneath the flat basin surface. The video link here tries to provide a highly simplified visualization for the whole process, focusing on the cross section appearance of the buried blocks.

 

It’s reasonable to think of this area as a thrust wedge with a “sag” developed in it due to the pull-apart extension. The cross section below, which is from the eastern end of the basin, paints this picture well and is a reasonable comparison to the model. Yes, all the text on the cross section is backwards; I reflected it to match the model!

This simple comparison illustrates the concept of a thrust belt (wedge) with a local “sag” in it due to extension and normal faulting. Sediment fills the sag, which is the depositional basin. The black lines show the basal fault of the thrust system, which warps downward beneath the basin. Underneath the basin fill, the thrust structures are still present in largely intact blocks. As usual, the model is too short and thick; this is a result of the light gray microbead sliding surfaces not being quite weak enough.

The blocks of sedimentary thrust belt beneath the basin fill have long been of interest to oil and gas exploration. Subsurface imaging, drilling, and comparison to un-buried portions of the thrust belt have provided a good understanding of the rock units and structures that are present. A detailed cross section of a buried thrust block produced by Zimmer and Wessely (1996) (Fig. 9 here) has been sampled by numerous later authors and repeatedly shows up if you look around for Vienna Basin geology information online. Note that most sources provide cross sections looking east across the basin; I made the models from a west-looking perspective, and have reflected some cross sections to match, like the one below.

The top image (here) shows gas in red and liquid hydrocarbon in green… it is based on the Zimmer and Wessely (1996) section. The complex structures hosting the hydrocarbons are Alpine thrust structures beneath younger Vienna Basin sediments. The model image immediately above the caption shows a roughly corresponding zone in the model, where part of the thrust wedge has subsided on normal faults and been buried by sediment.

The top image above shows that drilling for hydrocarbons has been extensive, allowing folks to have a well-informed understanding of what is down there. One aspect of the buried thrust blocks revealed by drilling is the presence of lots of high-temperature water, which is known to be present in some rock layers that did not yield oil or gas (page 3 here). This is where the alternative energy application enters story…

If the water is sufficiently hot and flows out of the rock and into a well at a sufficient rate, it could be brought to the surface where its heat could be extracted through a heat exchanger. The heat energy can be converted to other energy forms through a variety of processes. With sufficient temperature, electricity can be generated, but much of the Vienna Basin project appears directed at home heating, which is a major generator of CO2. The cooled water would then be re-injected into the formation, where it would keep pressures up and can eventually absorb more heat to continue the process.

Image source here. Red wells produce heated waters; the blue well returns water back to the producing formation after heat has been extracted.

This process can’t be applied everywhere on Earth. An appropriate geothermal gradient and potential for water flow during extraction and injection are necessary, and oil and gas exploration has revealed that parts of the Vienna Basin offer both. Initiatives are underway to take advantage of this potential energy resource.

I think the hydrogeothermal plans for the Vienna Basin are particularly interesting because some projects intend to target specific deformed sedimentary layers within the buried thrust structures. Carbonate sedimentary units are of particular interest because they should permit adequately rapid flow of waters into the producing wells and out of the re-injection wells. The targets (green areas in the figure below) are also sufficiently deep to offer elevated temperature (>100 degrees C). The same Zimmer and Wessely (1996) cross section that was originally prepared to show hydrocarbon plays has actually been re-purposed to show hydrogeothermal target layers in a few reports.

The green layers are potential hydrogeothermal targets. As previously stated, this cross section and entire perspective on the area were born out of oil and gas exploration and associated drilling. Image source here. This is an expanded version of the Zimmer and Wessely (1996) section above, showing oil and gas targets. You can see the structures shown a few photos up beneath the greatest density of wells (scroll back up and take a look).

The box shows a roughly corresponding part of the model. Black lines represent wells to extract thermal waters; they would really be well pairs (doublets) to accomplish extraction and re-injection. There likely isn’t enough heat here to generate steam energy, but it would be very effective in offsetting home heating needs.

I have looked through several documents regarding the intended development of these potential hydrogeothermal resources, but I can’t really get a feel for whether or not the project continues to move ahead. A 2019 paper suggests interest and work continues, but one quickly gets a sense of the challenges facing the project. Many of the thermal water targets are deep–up to 3.5 km (11,500 ft) or so. This makes simply drilling the production and injection wells a costly gamble should the project not work out. Additionally, the project would have to avoid interfering with oil and gas production, which is still active in the area. Formation waters that do not originate from surface recharge have high mineral content, which may present a challenge during cooling in the heat exchangers. Production would also have to avoid interfering with surface discharge of thermal water, which is extensively used for bathing in the region (or “balneological purpose,” a new word for me!).

Balneological pursuits in Vienna. Book your trip! (Not really)

Despite all of these challenges, I really liked the idea of assessing information gathered through oil and gas exploration and re-applying it to develop alternative energy schemes. I think it represents taking full advantage of available information and developing creative ways to use it. In effect, the hydrogeothermal project is doing just what oil and gas exploration does–looking for and extracting desirable fluids in the subsurface. In this case, the desirable fluid is hot and readily flowing water, but the visualizations and ultimate extraction methods are remarkably similar to, and informed by, past oil and gas work. This approach, combined with the complex tectonic history of the area and its hydrogeothermal targets, makes for a really good example of the sophistication and practicality of geoscience applications in the 21st century.

This post was originally published on The Geo Models blog.