Introduction

The U.S. Geological Survey (USGS) assessed the potential for undiscovered, technically recoverable conventional oil and gas resources within the greater Carpathian area (fig. 1). The greater Carpathian area for this assessment consists of assessment units (AUs) within the northern Carpathian Basin, southern Carpathian Basin, Moesian Platform, Varna Trough, Transylvanian Basin, Great Hungarian Plain of the Pannonian Basin, Danube Basin, and Vienna Basin (fig. 1).

The greater Carpathian area has a long record of oil and gas exploration dating back more than 100 years (Kotarba and others, 2019). Many of the producing areas are at a mature level of exploration for conventional oil and gas resources.

The greater Carpathian area is a complex mosaic of tectonic elements that record the tectonic evolution of one of the most complex regions in the world. Decades of focused research have shed considerable light on the evolution of the greater Carpathian area, which records multiple phases of deformation in the Tethyan realm between the eastern European continent and the northern margin of Gondwana since the Jurassic. The following is a brief summary from the literature of the key points in the tectonic evolution of the greater Carpathian area (Morley, 1996; Corver and others, 2009; Krézsek and others, 2010; Matenco and Radivojević, 2012; Horvàth and others, 2015; Krézsek and others, 2023; Schleder and others, 2023; Tari and others, 2023; Zielińska and others, 2023). From the Jurassic through the Paleogene, several terranes rifted from the Gondwanan and European margins and translated to the north and east, resulting in subduction that closed the paleo-Tethys Ocean. The diachronous movement of these various terranes opened several strands of the neo-Tethys Ocean. The terranes were made up of various continental and marine assemblages (Csontos and Vörös, 2004). The Alcapa and Tisza-Dacia terranes rifted from the southern margin of the European craton, moved northeast, and eventually entered the Carpathian embayment, where they began to collide with the eastern European margin. The Adria terrane collided with the southern margin of these terranes, leading to differential movement of the Alcapa and Tisza-Dacia terranes and the diachronous west-to-east formation of the Alpine and Carpathian fold and thrust belts. In the late Miocene, rollback of the south-facing subducting slab led to backarc extension of the terranes, forming the Miocene Pannonian Basin, an area largely coincident with the Greater Hungarian Plain Reservoirs AU (fig. 1). Thermal subsidence of the Pannonian Basin resulted in the deposition of several kilometers of sediment from the Pliocene to Holocene. In the late Pliocene, northeast movement of the Adria terrane caused contraction and inversion of many Miocene extensional structures (Morley, 1996).

Total Petroleum System and Assessment Units

The USGS defined the Mesozoic–Cenozoic Composite Total Petroleum System (TPS) to encompass oil and gas potentially sourced from several organic-rich marine shales and marls in the greater Carpathian assessment area. Nine AUs were defined within the Mesozoic–Cenozoic Composite TPS: (1) North Carpathian Basin Reservoirs AU, (2) South Carpathian Basin Reservoirs AU, (3) Moesian Platform Reservoirs AU, (4) Varna Trough Reservoirs AU, (5) Transylvanian Basin Postsalt Reservoirs AU, (6) Transylvanian Basin Presalt Reservoirs AU, (7) Greater Hungarian Plain Reservoirs AU, (8) Danube Basin Reservoirs AU, and (9) Vienna Basin Reservoirs AU.

The major source rocks in the North Carpathian Basin Reservoirs AU, South Carpathian Basin Reservoirs AU, Moesian Platform Reservoirs AU, and Varna Trough Reservoirs AU are Oligocene to Miocene organic-rich dysodile shales and the Oligocene to Miocene Menilite Shale (Kotarba and others, 2019; Körmös and others, 2021; Wójcik-Tabol and others, 2022; Krézsek and others, 2023; Rybár and Kotulová, 2023). Other potential source rocks are the shales of the Berriasian to Albian Spas Formation and Upper Jurassic marine marls. Source rocks in the Greater Hungarian Plain Reservoirs AU and Danube Basin Reservoirs AU are the Oligocene Tard Clay Formation (Badics and Vető, 2012; Körmös and others, 2021) and Miocene shales equivalent to the Oligocene to Miocene Dysodile Shale and Menilite Shale. The Triassic Kössen Formation is a viable source rock but may be limited in extent within the greater Carpathian area. The source rocks for biogenic gas in the Transylvanian Basin Postsalt Reservoirs AU and Transylvanian Basin Presalt Reservoirs AU are Miocene to Pliocene shales (Krézsek and others, 2010; Baciu and others, 2018). The major source rock in the Vienna Basin is the Jurassic Mikulov Formation (Rupprecht and others, 2017). The assessment input data for the nine AUs are summarized in table 1 and Schenk (2026).

Undiscovered Resources Summary

The USGS quantitatively assessed undiscovered conventional oil and gas resources in nine AUs in the greater Carpathian area (table 2). The estimated mean resources are 208 million barrels of oil (MMBO), or 0.2 billion barrels of oil, with an F95 to F5 range from 90 to 412 MMBO; 4,123 billion cubic feet of gas (BCFG), or 4.1 trillion cubic feet of gas, with an F95 to F5 range from 1,846 to 7,766 BCFG; and 89 million barrels of natural gas liquids (MMBNGL), with an F95 to F5 range from 41 to 162 MMBNGL. The potential volumes of undiscovered conventional oil and gas reflect the mature level of exploration in the greater Carpathian area.

For More Information

Assessment results are also available at the USGS Energy Resources Program website, https://www.usgs.gov/programs/energy-resources-program.

Greater Carpathian Area Assessment Team

Christopher J. Schenk, Tracey J. Mercier, Phuong A. Le, Andrea D. Cicero, Sarah E. Gelman, Jane S. Hearon, Benjamin G. Johnson, Jenny H. Lagesse, and Heidi M. Leathers-Miller