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Hydrocarbon Source Rock of the Dinarides: an Overview and Future Challenges

The Dinarides are a complex folded, faulted and imbricate belt along the NE margin of the Adriatic Sea. This mountain chain consists of a thick rock succession ranging from Carboniferous to Quaternary. The coastal zone and islands are mainly composed of carbonates that belong to the several palaeogeographic entities (VLAHOVIĆ et al., 2005). The widespread organic matter occurrences throughout area has attracted attention and encouraged petroleum exploration since 1970s. During years, a comprehensive geochemical evaluation is performed on more than 5000 outcrops and boreholes samples. Based on this huge data set, potential source rocks are verified in several stratigraphic levels. More importantly, source rock-hydrocarbons genetic correlations and petroleum system evaluations are established, as well. In the Northern Adriatic offshore biogenic gas accumulations of Plio-Pleistocene system are under production. The aim of this overview is to represent source rock potential of the rest of the area. Organic-rich siliciclastics and carbonates of the Carboniferous and Permian age are source rock with no potential due to metagenetic or metamorphous stage of thermal transformation. Organic-rich sediments of Triasssic and Early Jurassic age generally have poor source rock characteristics. They are in the higher catagenesis with transition to metagenesis. Regular appearances of pyrobitumen in these rocks indicate hydrocarbon generation in the past. On contrary, Upper Triassic organic-rich laminated limestones of Vlasta-Komiza facies are very good to excellent oil-prone source rocks that have reached onset of oil generation (COTA & BARIĆ, 1998). Organic matter is a mixture of algal, bacterially degraded kerogen and migrated bitumen. Kerogen is type II to II-S. Stable carbon isotopes and biomarkers confirmed origin and deposition in the low energy, highly anoxic, lagoonal, carbonate-evaporitic environment. However, only small oil volume was recovered. Oil is positively correlated with the Triassic source rock. An attempt has been made to map this source rock horizon in relation to Burano formation (Italy) throughout the wider Adriatic area. Determined differences of Triassic successions have highlighted the necessity of detail sedimentary basin evolution of the extensional/rift basins, facies distribution and understanding of the pre-platform rift systems and their thermal subsidence history as a prerequisite to evaluate the survivability of syn- to post-rift petroleum systems. The lack of high-quality genetic model hampered the otherwise valiant attempts to chase the Mesozoic carbonate oil play in the Dinarides. The best source rock of the Dinarides are Upper Jurassic dark organic-rich laminated limestones with cherts deposited in the older part of Lemeš trough (TROSKOT-ČORBIĆ, 2011). Organic facies is characterized with high organic matter content, excellent hydrocarbon potential and high transformation ratio. These very good to excellent oil-prone source rocks contain kerogen type II of marine, algal-bacterial origin. Amorphous organic matter gradually changes from lamalginite to bituminite. Solid bitumen is incorporated in all structural types. The organic facies generally has reached onset of oil generation. High percentages of organic-bonded sulphur confirm that organic matter formation took place in the marine, carbonate environment with contemporaneous sulphur 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. Bituminous coatings in nearby area show high degree of similarity indicating short migration pathways. Cretaceous laminated organic rich carbonates, deposited in the restricted lagoons and intraplatform troughs, are also excellent source rock. Organic matter is of microbial and algal origin. Kerogen is type II to I. Organic matter is immature. Organic facies is typical oil prone and enriched in sulphur incorporated in kerogen structure enabling hydrocarbon generation at lower degree of thermal transformation. These Cretaceous oil prone source rocks are also distributed over the central Adriatic offshore area (COTA & BARIĆ, 1998). At depths of 4, 500 to 5000 m sediments have reached oil window. According to stable carbon isotopes and biomarkers these low maturity Cretaceous source rocks are sourcing heavy hydrocarbons that are registered in non-commercial quantities in few Adriatic offshore wells or as oil shows (migrabitumens) that filled fissures, cavities and pores in widespread area. The wider Mediterranean area hosts a significant number of petroleum provinces (ZAPPATERRA, 1994). Active source rocks, genetically related oil and gas and all the geologic elements and processes that are essential, if an oil and gas accumulation is to exist, are evidenced in the Dinarides, as well. Understanding the thermal history and the structural-tectonic relationships that influenced it are fundamental in future research.

source rock, generative potential, Dinarides

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