Petroleum potential of the Adriatic offshore, Croatia

The Adriatic offshore of Croatia extends from the Istrian peninsula in the northwest to the Dubrovnik offshore area to the southeast and from the Croatian coast-line to the offshore boundary with Italy. Geochemical prospecting comprised analyses of Permian through Pliocene-Quaternary sediments. Integrated results of geochemical analyses indicate the presence of source rocks in Triassic (Ladinian-Carnian) and Lower to Upper Cretaceous units. Triassic source rock sequences within the thin limestone streaks from the lithologically mixed sabkha facies are evidenced in the southeastern portion of the Adriatic carbonate platform complex. Algal-bacterial organic matter of the predominantly marine origin is in the early stage of thermal maturation. Properties of analysed bitumen and oil samples imply a direct relationship with the associated source rocks. Cretaceous source rock sequences with good generating potential were determined in the central part of the Adratic offshore. Layers of micritic limestone and bituminous laminated limestone contain mixtures of oil prone, types I and II, high sulfur kerogen. All the maturity parameters indicate immature to marginally mature stage of thermal transformation, resulting from extremely low geothermal gradients in the area coupled with the observed burial history. In addition to oil, biogenic gases with a predominance of inorganic compounds (CO2 and H2S) were recovered from the Cretaceous carbonates. The results of oil and gas analyses define the depositional environment during source rock accumulation and the subsequent thermal conditions. It can be concluded that the biogenic gas accumulations in the Pliocene-Quaternary sandstones in the northern Adriatic area are the most prospective accumulations in the area.