Πετρογραφικά και γεωχημικά χαρακτηριστικά των μεσοζωικών βιτουμενιούχων σχηματισμών της ζώνης Παρνασσού-Γκιώνας
The scientific interest regarding the hydrocarbon generation potential in Greece is generally focused on the formations of the Ionian Unit in the western areas of the country, where oil-seepages are well known. In contrast, Parnassos-Ghiona geotectonic Unit (PGU), Central Mainland Greece, has mainly...
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| Pubblicato in: | Νημερτής |
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| Autore principale: | |
| Natura: | Articolo |
| Lingua: | inglese |
| Pubblicazione: |
2023
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| Accesso online: | https://hdl.handle.net/10889/26445 |
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| Riassunto: | The scientific interest regarding the hydrocarbon generation potential in Greece is generally focused on the formations of the Ionian Unit in the western areas of the country, where oil-seepages are well known. In contrast, Parnassos-Ghiona geotectonic Unit (PGU), Central Mainland Greece, has mainly been studied for the presence of bauxite ores. However, so-called bituminous limestone strata overlying the three bauxite strata are described in the up-to-date literature, as well as other Upper Cretaceous organic-rich strata including local coal lenses and thin shale layers, occur on top of the b3 bauxite horizon across PGU. In the present study, a series of outcrop and underground sites, as well as lab-core sections, representing all three bauxite strata hosted within PGU and their underlying/overlying formations, have been studied by means of organic petrology and geochemistry, including both Rock-Eval and GC-MS analysis, mineralogy and inorganic geochemistry, as well as microfacies analysis for the case of the carbonate strata. The aim of this study is to interpret the palaeoenvironmental setting during the deposition of the studied formations; the aspects of the palaeoenvironmental evaluation include the determination of the conditions favouring the organic matter accumulation subsequently to the b2 and b3 bauxite formations, as well as the determination of the palaeoenvironmental trend of PGU since the deposition of the Upper Cretaceous b3 strata towards the occurrence of the Paleogene dubmarine fan deposits. In addition, part of the main scope of the present study has been the assessment of the hydrocarbon generation potential of the studied PGU strata. Beginning from the older b1 ore body, the study of the outcrop sections of interest revealed that the bauxite-mud deposition during Middle-Upper Jurassic (Callovian-Oxfordian) was promptly followed by flooding of the karstic cavities at the studied sites, evolving into a shallow-marine depositional setting, corresponding to the D1 and D2 platform stages (see Carras, 1995), depending on the site, of the Distomon limestone strata. The restricted platform interior later evolved into a deeper part of the rimmed platform exterior model, which was established during the Upper Jurassic (Upper Kimmeridgian - Upper Tithonian), as suggested by the pelagic wackstones/ packstone collected at the AG site. The study of the aforementioned “Distomon” limestone strata following the b1 horizon formation do not support the accumulation and preservation of OM during these times. The study of the Upper Jurassic (Kimmeridgian-Tithonian) b2 bauxite strata, at one outcrop section at the western part of the PGU extent (DES6), revealed that the main bauxite paragenesis consists of boehmite and goethite and less of haematite, whereas the transitional bauxite-shale stage is dominated by the presence of chamosite. A thin organic-rich bauxite-shale layer on the top of the b2 horizon is mainly characterized by type IV bituminite and solid-bitumen content and displays a T max value falling within the oil-generation window. Hence, the HC generation from strata deposited right after the b2 ore body formation is not excluded. The sections of interest overlying the Upper Cretaceous (Cenomanian-Turonian) b3 horizon revealed the establishment of different depositional settings, depending on the studied site, including prompt deepening towards a shallow-marine platform interior setting (FZ7-9), during which adequate accumulation and/or preservation of organic matter was in cases favored right after the bauxite-mud deposition, shallow-marine basins with limited water circulation, as well as proximal to the sea paralic mires controlled by high-energy/tidal and oxidizing conditions. The study of the coal lenses revealed the establishment of two different coal-forming modes of organic matter depositional settings, including paralic mires on the top of the lateritic mud deposition, and paralic mires developed upon the Lower Cretaceous limestone bedrock. The low/very low grade humic coal lenses at the Vagoneto and Gouves sites formed by accumulation of mostly herbaceous plants, being characterized by elevated amounts of detrovitrinite and inertinite, and variable amounts of liptinites usually in the form of sporinite and alginite. Accordingly, the Rock-Eval data indicate the occurrence of all kerogen types, with the Types III and IV generally prevailing. The overall coal-petrographic data indicate intense oxidation in the palaeo-mires, possibly related to the dry warm Turonian climate and/or the influence of brackish waters. Τhe thermal maturity of the coal lenses , based on both vitrinite reflectance and Rock-Eval data, indicates Medium Rank Bituminous coals or, in terms of hydrocarbon generation, the early oil window stage. The uppermost Upper Cretaceous (Senonian) limestone strata was deposited under the conditions of shallow-marine platform interior setting (FZ7-9), during which adequate accumulation and/or preservation of organic matter was not favored. During the Upper Cretaceous, the palaeoenvironmental evolution of the PGU reached the FZ6 of the platform margin within the stratigraphic extent of the studied strata, followed by two alterations of carbonate platform settings and flysch-forming environments (reporting the onset of the orogenetic status within PGU during the transition of Maastrichtian and Paleogene), as recorded on the alteration of submarine fan deposits and pelagic limestones, before the final occurrence of the Paleogene sub-marine fan deposits. The Paleogene submarine fan deposits, represented by one site comprising a few-meter-thick sequence of alternating sandstone and marl strata, displayed a small amount of both indigenous and migrated solid hydrocarbons (corresponding to a maximum of 0.5 wt.-% TOC), suggesting a possible migration of hydrocarbons from the Cretaceous organic-rich formations. However, the reservoir features along with the theory of in-situ hydrocarbon generation in this sedimentary sequence belonging to the PGU Paleogene submarine fan deposits needs to be further investigated. |
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