The Neogene Pannonian Basin formed part of the Central Paratethys and is underlain by the Eocene and Oligocene sediments of the Hungarian Palaeogene Basin (Tari et al., 1993). In the Pannonian Basin, there are six known hydrocarbon systems and the Palaeogene hydro-carbon system belongs to the Hungarian Palaeogene Basin (Dolton, 2006), which is located in the north-central part of Hungary. The Hungarian Palaeogene Basin is interpreted as a retro-arc flexural foreland basin, where the depositional facies migrated towards the east-northeast according to the present position (Tari et al., 1993; Kováč et al., 2016). The tectonic evolution has been related to normal faulting and strike-slip regimes (Palotai, 2013 and references therein). The term Palaeogene Basin comprises all sedimentary sequences, forming a single cycle from the Eocene to the Early Ottnangian (Sztanó and Tari, 1993).
The Palaeogene sediments are unconformably overly the Mesozoic basement. The generalized lithostratigraphy begins with sediments showing a continuous upward transition from a terrigenous to a lagoonal environment, the Eocene Kosd Formation (Gidai, 1978; Less, 2005). The Kosd Formation is followed by the shelf deposits of the Szépvölgy Limestone Formation (Kázmér, 1985). The continued subsidence and the east-northeast trending of the depositional facies caused the deposition of the Buda Marl Formation under oxygen-depleted conditions (Less, 2005; Nagymarosy and Báldi-Beke, 1988; Ozsvárt et al., 2016). The sediments of the Tard Clay Formation accumulated in a euxinic basin (Báldi, 1984, Bechtel et al., 2012) then the Kiscell Clay Formation settled down in a well-oxygenated depositional environment
(Báldi and Báldi-Beke, 1985). The sedimentary sequence of the Hungarian Palaeogene Basin is covered by thick Neogene sediments.
In the last few decades, several hydrocarbon reservoirs have been discovered in the Hungarian Palaeogene Basin. Hydrocarbons were detected in the fractured and weathered Mesozoic basement rocks in the research area. The basal conglomerate and breccia and the sandstone sequence of the Kosd Formation also form reservoirs and host substantial amounts of hydrocarbons. The karstified Szépvölgy Limestone and turbidite sandstone bodies within the Kis-cell Clay Formation and the clastic sediments of the Miocene formations also serve as targets of exploration activities (Dolton, 2006). The reservoirs were discovered along with various structural, stratigraphic and combination trap types within the study area (Kokai, 1994;
Dolton, 2006).
Fine-grained, organic matter-rich sediments within the Eocene and Oligocene succession were recognized as potential source rocks (e.g., Badics and Vető, 2012; Bechtel et al., 2012; Brukner-Wein et al., 1990; Milota et al., 1995). It is widely accepted that the Lower Oligo-cene Tard Clay Formation provides the most important source rocks (Bechtel et al., 2012; Brukner-Wein et al., 1990; Hertelendi and Vető, 1991; Milota et al., 1995). However, the Late Eocene Buda Marl Formation (Sachsenhofer et al., 2018a, 2018b) and Late Oligocene Kiscell Clay Formation (Milota et al., 1995) were also considered as potential source rocks.
Boreholes, drilled in the early 2000s in the south-central part of the Hungarian Palaeogene Basin, penetrated Eocene mixed siliciclastic-carbonate sequence, forming hydrocarbon reservoirs and coal-bearing sequences of the Kosd Formation beneath the Oligocene source rocks. The Oligocene source rocks have been investigated by several authors (e.g., Badics and Vető, 2012; Bechtel et al., 2012; Milota et al., 1995; Sachsenhofer et al., 2018a, 2018b), but the Eocene Kosd Formation remained largely uninvestigated. Comprehensive organic geochemical studies have only been performed on sediments of the Tard Clay Formation (Bechtel et al., 2012), whereas in-depth investigations are still missing for the Kosd, Buda Marl and Kiscell Clay formations. Furthermore, the detailed analysis of crude oils and thor-ough oil-to-source rock correlation are also absent. Besides the missing organic geochemical analyses of the potential source rocks and oil-to-source rock correlation, the Eocene mixed siliciclastic-carbonate reservoir section also waits for detailed investigation. These reservoirs are characterised by diverse reservoir quality. The initial high production capacity of the reservoirs can only be maintained by multiple workover activities (Radovics et al., 2017). To maintain favourable production capacity, a detailed investigation of diagenetic processes characterising the Eocene mixed siliciclastic-carbonate reservoir rocks and their surroundings is needed as a prerequisite for appropriate reservoir description.
This study focuses on the southern-central part of the Hungarian Palaeogene Basin. The aims are to (i) enhance the understanding of the depositional environment and organic matter sources of the coal-bearing Kosd Formation, (ii) estimate the hydrocarbon potential