Naslov
Organic facies of the Upper Jurassic sediments in the Poštak Mountain, Croatia

Autori
Troskot-Čorbić, Tamara ; Španić, Darko

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

Izvornik
65th Anual Meeting of the ICCP, Sosnowiec, Poland / Ciesielczuk, Justyna ; Fabianska, Monika ; Misz-Kennan, Magdalena ; Szopa, Krzysztof - Sosnowiec : University of Silesia, 2013, 121-122

ISBN
978-83-934-005-2

Skup
65th Anual Meeting of the ICCP

Mjesto i datum
Sosnowiec, Poljska, 25.08.2013. - 31.08.2013

Vrsta sudjelovanja
Poster

Vrsta recenzije
Međunarodna recenzija

Ključne riječi
organic facies, Poštak Mt. , Jurassic

Sažetak
Carbonate deposits of the Karst (External) Dinarides ranging from the Lower Jurassic to Cretaceous were formed on the Adriatic Carbonate Platform (AdCP). The Late Jurassic megafacies of “limestones with cherts” was deposited within tectonically formed intraplatform troughs which had temporary or continuous connection to the open sea. During recent investigations the most complete succession of Kimmeridgian deeper marine deposits has been documented at the slopes of Poštak Mt. New informal stratigraphic unit Poštak is proposed composed of three members: Rastičevo, Dimići and Lemeš. The objective of this study is detailed organic geochemical characterization of the Dimići member in Poštak Mt. Standard organic geochemistry analyses were performed on more than 100 outcrops. An integral interpretation of this data was made in the sense of comprehensive organic facies study in order to define organic matter types, source rocks and their generative potential as well as genetic correlations. Based on the integration of analytical data organic facies AB/B (after Jones 1987) it is recognized as follows: Organic facies AB/B is characterized with high organic matter content, excellent hydrocarbon potential and high transformation ratio. Type II kerogen dominates in the organic-rich laminated limestones. Occasionally, the type I kerogen is present as well. Organic facies AB/B is of marine, algal-bacterial origin. 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 in the range from 0.15 to 0.30% Ro (asphaltite group). Organic facies AB/B is characterized with high organic sulfur content (from 6 to 10%). Stable carbon isotope values are in range from 13C -27.79 to -24.65‰ in kerogen and from -29.53 to -25.55‰ in bitumen. Correlation is mainly positive. These values are frequent in the Jurassic rocks and they are generally attributed to the specific algal and bacterial lipids, as well as to the developed water anoxia. The Upper Jurassic fine-grained, laminated limestones have an excellent generation potential and represent very good to excellent oil-prone source rocks. Organic matter content is in range from 0.35 and 11.9%, in average 2.97% TOC). Petroleum potential is from good to excellent (between 1.17 and 67.00 mg HC/g rock, in average 18.25 mg HC/g rock). Amorphous organic matter gradually changes from lamalginite to bituminite I and finally to organo-bituminous-mineral groundmass, i.e. to matrix bituminite. Solid bitumen is incorporated in all structural types. Phytoplankton is represented by prasinophycean phycomata and dinoflagellate cysts. Throughout the geological columns quantitative variations in cyanobacterial and liptinite/liptodetrinite content are microscopically observed. Variations in C27 and C29 regular sterane were documented as well. These dark organic-rich laminated limestones (mudstones) with cherts form the older part of Lemeš trough deposits. Organic matter was deposited in deeper trough of the Adriatic Carbonate Platform. Deposition occurred in the low energy environment within intraplatform trough. Water stratification and bottom waters euxinia were occasionally disturbed by inflow of the open sea water masses. The laminated organic matter in a form of lamalginite was deposited in calm, stable environment with developed water mass stratification, under oxygen–depleted conditions. In relatively disturbed unstable conditions organic matter deposited as lamalginite became decomposed and bacterially degraded which led to a gradual transition into bituminite I and finally into organo-bituminous-mineral groundmass, matrix bituminite. According to maturity parameters (Tmax, PI, TAI, fluorescence color, vitrinite reflectance, bitumen reflectance, biomarker maturity parameters M/H, 22S/(22S+22R), Rc, Rm(MDR), 20S/(20S+20R), Ts/Tm), the organic facies AB/B generally reach onset of oil generation. High percentages of organic-bonded sulfur confirm that organic matter formation took place in the marine, carbonate environment with contemporaneous sulfur incorporation into a kerogen macromolecule structure. This kind of organic facies enabled hydrocarbon generation at lower degree of thermal transformation. Consequently, the total bitumen yields are high. The extractable fractions are dominated by resins and asphaltenes which reflect these specific organic facies enriched in sulfur, but also some degradation effects due to the atmospheric influence. Gas chromatograms of alkane fraction of bitumen show predominance of n-alkane in lower molecular range, domination of phytane over pristane and n-alkane over iso-alkane, respectively. These profiles are typical for bitumen of algal-bacterial origin and anoxic (reducing) depositional environment and correspondent to the defined maturity level. Source rock extracts and bitumens or bituminous coatings show high degree of similarity which indicates short migration paths. Migrabitumen, which fill fissures, cavities and pores in the rocks, is classified as the natural asphalt. That bitumen is product of early generation from sulfur and organic rich kerogen. In the absence of the cap rocks, bitumen reached the surface, and was degraded and transformed from primary viscous liquid into a solid bitumen.

Izvorni jezik
Engleski

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