Naslov
Organska petrologija Vanjskih Dinarida, Hrvatska
(Organic petrology of the External Dinarides, Croatia)

Autori
Troskot-Čorbić, Tamara ; Španić Darko

Vrsta, podvrsta i kategorija rada
Sažeci sa skupova, prošireni sažetak, znanstveni

Izvornik
4. Hrvatski geološki kongres / Knjiga sažetaka / Abstracts Book / Horvat, Marija - Zagreb : Hrvatski geološki institut, 2010, 300-301

ISBN
978-953-6907-23-6

Skup
4. Hrvatski geološki kongres

Mjesto i datum
Šibenik, Hrvatska, 14.10.2010. - 15.10.2010

Vrsta sudjelovanja
Poster

Vrsta recenzije
Međunarodna recenzija

Ključne riječi
Organska petrologija, Dinaridi, organsi facijesi, mati~ne stijene, bitumen, zrelost
(Organic petrology, Dinarides, organic facies, source rock, bitumen, maturity)

Sažetak
The External (Karst or Outer) Dinarides are a complex folded, faulted and imbricate belt which developed along the northeastern margin of the Adriatic Sea. They are characterized by very thick carbonate succession that ranges in age from the Middle Permian (or even Late Carboniferous) to the Eocene. The entire carbonate succession was deposited within carbonate platform environments (VELIĆ et al., 2002, TARI, 2002, VLAHOVIĆ et al., 2002, 2005). Platform carbonate deposits are characterized by frequent lateral and vertical alternations of different facies mostly associated with diverse shallow marine environments, but there are also carbonate slope deposits and those representing temporarily drowned platform facies and intraplatform troughs (TIŠLJAR et al., 2002). All these carbonate deposits are the long-term objective of detailed organic geochemical characterization (BARIĆ et al., 2003). The aim of this paper is to represent organic petrology as useful tool for defining organic matter. Outcrop samples of either laminated and fine-grained limestones and shales that contain autochthonous organic matter (kerogen and associated bitumen) or dolomites and limestones with pores, fissures and cavities filled with migrated bitumen were collected from localities of Dinarides (Gorski Kotar, Lika, Dalmatia). Total organic carbon of all samples was analyzed because TOC is the basic parameter which categorizes whether rocks will be additionally analyzed. Based on detailed microscopic description, appearance, maceral composition, type and origin of organic matter organic facies are determined. Maturity stage of organic matter is defined by vitrinite and bitumen reflectance measurements, color of the fluorescence and thermal alteration indices (STACH et al., 1982, JONES, 1987, TYSON, 1995, TAYLOR et al., 1998). Organic rich sediments are classified as source rock, oil shale and coal (HUTTON, 1987, COOK & SHERWOOD, 1991, HUNT, 1995). The results of the analyses show that all organic rich rocks of the Permian and Triassic age are in higher catagenetic to metagenetic or even metamorphous stage of thermal transformation (VR 1.7-2.70%, vitrinite anisotropy). Organic matter in clastic sedimentary rocks is of terrestrial origin mostly represented by macerals of vitrinite group. In siliciclastic carbonates vitrinite particles are incorporated in amorphous organic matter while in carbonates amorphous organic matter is dominated. Micrinite structure of amorphous organic matter could be connected with bituminite and its algal-bacterial origin. These rocks are overmature source rock. Regular appearances of pyrobitumen (impsonite) in samples confirm hydrocarbon generation in the past. Organic rich sediments of the Jurassic megafacies of mudstones and wackestones deposited in lagoon and subtidal and/or isolated and restricted lagoon/deeper subtidal areas of the inner platform as well as megafacies of “limestones with cherts”, deposited within intraplatform troughs with temporary or continuous connection to the open sea (TIŠLJAR et al., 2002) are the main objective of study. Optical examination of the Jurassic organic-rich laminated limestones reveals absolute predomination of amorphous organic matter. Lower Jurassic laminated carbonates contain thermally altered amorphous organic matter. Organic matter is in higher catagenetic to metagenetic stage. Micrinite texture of amorphous organic matter could be associated with bituminite origin. In maceral composition of Upper Jurassic laminated carbonates and calcitic shales (“Lemeš facies”) dominate amorphous unstructured organic matter sporadically with traces of algal liptinite (mainly dinoflagellata cysts) and rarely fusinite. Organic matter in these sediments is typical oil prone. According to microsolubility and strong yellow to yellow-orange fluorescence effect, organic matter is a mixture of algal, bacterially degraded kerogen and migrated bitumen. Bitumen reflectance is from 0.15 to 0.30 % Ro (asphaltite group). Examinations of whole rock samples reveal dominantly presence of bituminite and in smaller portion of lamalginite. Generally, organic matter is in the diagenetic stage of thermal transformation or have just reached onset of oil generation (catagenesis). High biomass productions as well as high percentages of organic-bonded sulfur are evidence that organic matter formation took place in marine, carbonate environment (intraplatform trough) with contemporaneously sulfur incorporation in a kerogen macromolecular structure. Bitumen is a product of early generation from sulfur rich kerogen. During the Cretaceous, restricted lagoons and shallow, intraplatform lakes were favorable for formation of the fine-grained, laminated, organic rich carbonates (limestones and dolomitised limestones). This type of facies is most abundant during the Albian-Cenomanian, Late Santonian-Early Campanian and Maastrichtian. Source material for organic matter is of microbial and algal origin, which forms lamalginite and bituminite. In the deeper parts of intraplatform depressions and in slope deposits (especially during the Late Santonian and Early Campanian transgression) some strong fluorescing alginite (mostly cysts of dinoflagellates) is incorporated in bituminite (or “organic-mineral groundmass”). Organic facies is typical oil prone and also enriched in sulfur incorporated in kerogen structure which additionally points out carbonate-evaporite environments and anoxic conditions during deposition. This kind of organic facies enabled hydrocarbon generation at lower degree of thermal transformation. So, this is the reason why microscopic examinations of whole rock reveal increased solid bitumen content in all samples. Bitumen color of the fluorescence is orange to brown and reflectance below 0.2 %Ro. According to microscopic characteristics, migrabitumen in pores, fissures and cavities of the Jurassic and Cretaceous rocks in most cases belong to asphaltite group. Asphaltites are derived from heavy, asphaltenes rich oils (JACOB, 1989). However, it can be presumed that they are product of early generation from sulfur rich kerogens. After short migration viscous liquid filled the cavities in the rocks and transform into solid bitumen by devolatilization and polymerization. These bitumens have characteristics of determined oil-prone Jurassic and Cretaceous source rocks. In the Paleogene sediments, especially in the subtidal and lagoon environments, continued occurrence of immature marine amorphous organic matter of algal and bacterial origin is determined. On the other hand, development of terrestrial and fresh water to brackish facies is evidenced by increased expansion of coal facies. Coals are dominantly of terrestrial origin in the peat to sub-bituminous stage. Sapropelic coal is sporadically also present.

Izvorni jezik
Engleski

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