Abstract
Eocene and Lower Oligocene rocks are potential source rocks for crude oil accumulations in the Hungarian Palaeogene Basin. To enhance the understanding of the hydrocarbon system, this study (i) assesses the petroleum potential of Palaeogene formations, (ii) characterises the source rock facies of the accumulated oils, and (iii) provides an oil-to-source correlation. Rock-Eval data of samples from three boreholes (W–1, W–9 and W–12) show that most Palaeogene formations are mature at depths exceeding 2.1–2.5 km. The coal-bearing Kosd Formation includes good to excellent gas- (and oil-) prone source rocks. The overlying Buda Marl Formation is typically organic matter-lean but contains oil-prone rocks with up to 2.3 wt% TOC and a fair petroleum potential in borehole W–9. The Tard Clay Formation in W–12 reaches up to 1.9 wt% TOC and shows HI values up to 440 mg HC/g TOC, characterising the deposits as good petroleum source rocks. Based on low TOC contents, the Kiscell Clay Formation is not considered a source rock. Molecular parameters of 12 crude oil samples indicate a shaly source rock deposited in a marine/brackish environment. Salinity stratification, causing the development of oxygen-depleted conditions, is likely. The organic matter is dominated by aquatic biomass, including algae, dinoflagellates and chemoautotrophic bacteria. Minor angiosperm-dominated organic matter was transported into the basin from the shoreline. Specific V-shaped compound-specific carbon isotope patterns of n-alkanes observed in crude oils and extracts from the Tard Clay prove the dominant source rock. Minor differences between biomarker ratios are related to vertical and lateral facies variations within the Tard Clay Formation. The accumulated oils are slightly more mature than the Tard Clay in borehole W–12.
Highlights
During the Palaeogene, the closure of the Mesozoic Tethyan basin and subsequent basin isolation caused the birth of the Paratethys Sea (Schulz et al, 2005)
This study focuses on the central-southern part of the Hungarian Palaeogene Basin
The study aims are to advance the understanding of the petroleum system by (i) characterising the maturity and source rock potential of the Upper Eocene (Kosd and Buda Marl formations) and Lower Oligocene (Tard Clay and Kiscell Clay formations) succession, (ii) determining the source rock facies of crude oils produced in several oil fields in the Hungarian Palaeogene Basin, and (iii) correlating these crude oils to a specific source rock formation
Summary
During the Palaeogene, the closure of the Mesozoic Tethyan basin and subsequent basin isolation caused the birth of the Paratethys Sea (Schulz et al, 2005). Whereas it is widely accepted that the Lower Oligocene Tard Clay Formation provides the most important source rocks (Fig. 2; Bechtel et al, 2012; Brukner-Wein et al, 1990; Hertelendi and Veto, 1991; Milota et al, 1995), the Middle Eocene Kosd Formation The study aims are to advance the understanding of the petroleum system by (i) characterising the maturity and source rock potential of the Upper Eocene (Kosd and Buda Marl formations) and Lower Oligocene (Tard Clay and Kiscell Clay formations) succession, (ii) determining the source rock facies of crude oils produced in several oil fields in the Hungarian Palaeogene Basin, and (iii) correlating these crude oils to a specific source rock formation. Oil-to-source rock correlation is based on biomarker and compound-specific isotope data from oil samples, which are compared with new source rock data and source rock data published by Bechtel et al (2012) and Kormos et al (2020), who discussed the origin of the source organic matter, established on biomarker parameters, in the Tard Clay and Kosd formations, respectively
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