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Organic facies of the Upper Jurassic sediments in the Poštak Mt., Croatia

During the Late Oxfordian and Early Kimmeridgian the intraplatform Lemeš trough was formed in the area of todays Lika, Western Bosnia and Herzegovina and Northern Dalmatia, which had continuous connection with the open sea. Deeper marine 'Lemeš Beds' were described in 1909 by Schubert from the Lemeš pass on the Svilaja Mt. During recent investigations the most complete succession of Kimmeridgian deeper marine deposits has been documented at the slopes of the Poštak Mt., where it is clear that deposits from the Lemeš locality correspond to the upper part of the complete succession. Therefore, a new informal lithostratigraphic unit named Poštak is proposed ; composed of three members: Rastičevo, Dimići and Lemeš. The Rastičevo member is composed of alternation of light-brown thick-bedded limestones with rare bivalve and echinoderm bioclasts and thin intraclastic-peloid-skeletal intercalations. These deposits of probably Lower Kimmeridgian age were deposited in newly formed intraplatform trough connected to the open sea. The Dimići member is characterized by alternation of light grey and greenish-grey limestones with radiolaria and dark grey, thin-bedded to laminated, mostly recrystallized radiolarian limestones enriched in organic contents. These middle Kimmeridgian rocks were deposited in the deepest parts of the intraplatform trough, and therefore are spatially restricted. The Lemeš member is composed of well-bedded light grey to light brown limestones with rare chert nodules. Limestones of Lemeš member were deposited during the gradual infilling of the intraplatform trough, but planktonic fauna, especially relatively frequent layers with ammonites of the Upper Kimmeridgian to Lower Tithonian age indicate still preserved connection with the open sea. The objective of this work is detail organic geochemical characterization of the Dimići member. Based on the integration of analytical data performed on more than hundred samples organic facies AB/B (after Jones, 1987) is recognized. Organic facies is characterized with high organic matter content (from 0.35 to 11.9%, in average 2.97% TOC), excellent hydrocarbon potential (between 1.17 and 67.00 mg HC/g rock, in average 18.25 mg HC/g rock) and high transformation ratio. Type II kerogen dominates in the organic-rich laminated limestones. Occasionally, the type I kerogen is present as well. Organic matter is a mixture of algal, bacterially degraded kerogen and migrated bitumen. 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 sections quantitative variations in cyanobacterial and liptinite/liptodetrinite content are microscopically observed. Variations in C27 and C29 regular sterane were documented as well. Bitumen reflectance is in the range from 0.15 to 0.30% Ro (asphaltite group). Organic facies 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. These values are common in the Jurassic rocks and 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. 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 reached onset of oil generation. Migrabitumen, which fill fissures, cavities and pores in the rocks, is classified as the natural asphalt, being the product of early generation from sulfur and organic rich kerogen. Deposition of these dark organic-rich laminated limestones with cherts occurred in the low energy environments 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, leading to a gradual transition into bituminite I and finally into organo-bituminous-mineral groundmass, matrix bituminite. According to available geological data original dimensions of the Lemeš trough, in which the source rock was deposited, can not be reasonably estimated. Preserved volume of Dimići member is low, discovered on a small number of sites with respect to presumed total area of the trough sediments. Deposits are subsequently significantly tectonically disintegrated and are recently found in several structural units separated by thrust and strike-slip faults with displacements in tens of kilometers. In order to estimate the real perspective of further hydrocarbon exploration in this area and evaluation of petroleum systems, based on proven source rock characteristics and source rock-bitumen correlation, the results should be verified and supplemented by geophysical measurements and potential exploration drillings.

Organic facies, Upper Jurassic, Lemeš, Croatia

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