engleski

Petroleum Potential of the Adriatic Offshore, Croatia

The studied area extends from the Istrian Peninsula to the north-west, Dubrovnik offshore area to the south-east, and from the Croatian coast-line to the offshore boundary with Italy. Exploration activities in the area began in 1968 and with minor interruptions continued for 20 years. They resulted in the several gas field discoveries in the northern Adriatic while in the central and southern part, oil, gas and bitumen shows were recognised. The whole region is characterised by geological, hydrodynamic and thermal complexity. The Adriatic offshore belongs to relatively "cool" basins with present day geothermal gradients varying from 1.0 to 2.8 ^0C/100m. Positive thermal anomalies were established locally, toward the offshore boundary with Italy in the farthest north, central and southern part of the area (Britvi};, et al., 1989). Geochemical analyses have been performed on core and cutting samples from more than 40 wells in order to assess the source rock presence, organic facies type and maturity. Detailed geochemical studies were used for hydrocarbon determination. Geochemical prospecting results have been presented within the context of the stratigraphic framework. The analytic included deposits in the range from the Lower Triassic to Pliocene-Quaternary age. The oldest sediments analysed in the area are Lower Triassic clastics deposited in the post-Hercynian phase. No source rock potential has been determined in this unit so far. Clastic rocks and carbonates deposited as a result of transgression overly this unit. Transgression reaches its maximum in Anisian with the limestone deposition. The remarkable rifting event accompanied by volcanism took place in Ladinian. Upper Triassic post-rifting phase is characterised by dominantly restricted shelf deposition (sabkha facies) of the Ladinian - Carnian age. Certain source rock potential has been recognised within this unit. Low organic matter content and unfavourable maceral composition is in accordance with frequent changes in depositional environment. Oil shows and the increased quantities of bitumen were encountered within the same unit in one deep exploration well from 5500 m to 5550 m. Numerous analyses have shown unusual characteristics of hydrocarbons, possibly as a result of the source rock facies diversity. From Norian-Rhaetian to Middle Eocene carbonate platform regime was established in the area, with an average of 5000 m sediments being deposited. Upper Jurassic and Lower Cretaceous carbonate-evaporitic sediments are generally low in indigenous organic matter content although a considerable number of migrated hydrocarbon shows was observed in them. Particularly interesting are the significant hydrogen sulphide rich gas manifestations registered in several deep wells. Sedimentary sequences of Lower and Upper Cretaceous (especially within the Dugi Otok Depression) have been recognised as source rocks with good potential for petroleum. Organic matter is type II kerogen with the amorphous algal remains as main compounds, implying marine origin (Bari};, et al., 1987). All maturity parameters indicate immature to marginally mature stage of thermal alteration. Numerous hydrocarbon shows were encountered within this section and from one well in the "Dugi Otok Basin" area, even a low rate flow was obtained. Oils from the Lower to Upper Cretaceous sequence are generally low API gravity, high in sulphur (10, 2 - 15, 6%) and low in saturated hydrocarbons. Carbon isotope ratio of d13C -22 ^0/_00 and biological markers indicated algal-bacterial origin of precursor deposited in highly reducing environment. Interesting oil and gas manifestations have been observed within the carbonate platform sediments of the Upper Cretaceous - Eocene age (Mid Adriatic area). Migrated hydrocarbons fill the limestone and dolomite fractures. Extract analyses indicate algal sapropelic organic matter in the mature, katagenic stage of alteration, confirmed by the wet gas composition. In the Southern Adriatic region, oil shows related to vugular and fracture porosity have also been detected in the same stratigraphic unit. The oil has migrated from the mature, presently unknown source. Subsidence of the Adriatic basin took place in Neogene and Quaternary with the predominant siliciclastic deposition. Great economic significance can be assigned to gas accumulations from Pliocene-Quaternary sands in the Northern Adriatic. The main gas constituent is "pure" methane (C_2+< 0.2%) with a high negative carbon isotope ratio (d13 -74 ^0/_00 to -76 ^0/_00) indicating bacterial - diagenetic origin. Heterogeneity of organic facies and different type of hydrocarbons within the Adriatic offshore can be attributed to complex geological characteristics. However the obtained data support the possibility of finding new hydrocarbon accumulations in the area. REFERENCES Bari}; G., Mari~i}; M. and Radi}; J.(1987) Geochemical characterization of organic facies in the Dugi Otok Basin, the Adriatic Sea. In Advances in Organic Geochemistry, 1987 (Edited by Mattavelli L. and Novelli L.), Org. Geochem. Vol.13, NOS 1-3, pp. 343-349 Britvi}; V., Cota L., Radi}; J.(1991)Thermal history, maturity and generating potential of source rocks in the Dinarides and the Adriatic Sea area, The second international symposium on the Adricatic carbonate platform, Zadar, Croatia, 1991, Abstracts Book, pp. 94.

source rock; kerogen type; organic facies; maturity parameters; oil; bitumen; biological markers

nije evidentirano