engleski

Organic facies and palynofacies of Late Jurassic "Lemeš facies" in Croatia

The Lemeš facies ("Die Lemeš-schichten", Furlani, 1910) was deposited during the Kimmeridgian and Tithonian in the central part of the Adriatic platform where the tectonic movements with an approximate SW-NE orientation formed a relatively shallow intraplatform trough (Velić et al., 2002). The palaeogeographic pattern during that time span is characterised by different facies developments as a consequence of gradual progradation of reefal and perireefal facies followed by oolitic facies resulting in the final infilling of the intraplatform trough and re-establishment of peritidal facies (Velić et al., 1994. Due to this facies development, the Lemeš facies is a diachronic facies ranging through the Kimmeridgian and Early Tithonian. The part of the trough in area of Lika and northern Dalmatia was connected with the open Tethyan realm, and therefore comprises deposits with more pelagic influences. These deposits consist of bedded cherty limestones with radiolarians and ammonites intercalated with bentonite layers and organic-rich shales and marls. For the purposes of the organic facies and palynofacies study, samples were collected in Lika and northern Dalmatia from following localities mapped as Lemeš facies: Poštak Mountain (Dimići and Mijina Glava), Koreničko Vrelo, Korenica-Milatović Draga, Srb-Brotnja, Knin-Rastićevo, Kijevo-Kozjak, Vrlika-Lemeš, Vrlika-Maovice, Vrlika-Dabar, and Muć-Bulići. Most of the analysed samples are found to have TOC greater than 2.0 percent and hydrogen indices in the range 538 to 638, i.e. they were found to be good source rocks. Kerogen type was primarily amorphous. Biomarker analyses were performed for the bitumenes extracted from selected outcrop samples. Steranes (mass fragmentograms m/z 217 and 218), monoaromatic steroids (mass fragmentogram m/z 253), triaromatic steroids (mass fragmentogram m/z 231) and hopanes (mass fragmentogram m/z 191)were studied. According to the biomarker parametres, the studied organic matter was derived mostly from the algal biomass enriched with bacterial biomass. This is consistent with the palynofacies features and the composition of the organic matter under consideration. These high quality petroleum source rocks were deposited in a marine environment at low redox potential, characterised as anoxic to suboxic one which is in accordance with the high portion of lipid-rich amorphous organic matter. Biomarker maturity parametres as well as the fluorescence features of the isolated kerogen showed early to mid mature stage of the related organic matter. It indicates that the related strata were buried deeply during a long geological past at rather elevated temperature resulting in thermal stress recognised in a decent maturity level.

organic facies; palynofacies; kerogen type

nije evidentirano