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Abstract: Offshore Exploration of Mesozoic and Paleogene Deposits on the "Adriatic Carbonate Platform" (Adriatic Sea, Croatia)

Lucic, Dubravko - INA-Naftaplin

The exploration area, in terms of the regional plate tectonic, belongs to the Adriatic (Adria, Apulia) plate region. The entire plate is encompassed with seismically active borders; Dinarides extend along NW-SE border, at the most SE (towards Albania) is active Budva-Cukali zone, Alps are on the NW and Apennines at the W. In this paper schematics are presented only for stratigraphical relations in the southern part of the Adriatic platform "South Adriatic Block"(Fig.1).

The "embryo" of the Adriatic plate originated during the Pangea breakdown, while the plate was separated from other plates of the West Tethys realm. Post-Hercynian structures during Triassic were covered with epicontinental clastic-evaporites and thin Anisian carbonate platform which overlay the mayor part of the area. At the end of that period during the Triassic phase of rift-extension, the Anisian platform was destroyed and numerous rift extension grabens were filled with Ladinian-Carnian deposits. From Norian to the Middle Eocene, the carbonate shelf was stabilized. According to intensive subsidence very thick portions of carbonate rocks (3500-4000m or more) were deposited. Deposits consist of limestone and dolomite with sporadic evaporitic and shale intercalation deposited mainly in the shallow shelf environments (RSP, OSP, Tidal flat, Reef etc.). According to structural features, in places such as on slopes and in the intarplatform basins, carbonate turbidites and pelagic limestone were deposited (Maja-1 Fig.1). During the period of platform structuration, several episodes are significantly marked out. Upper Lias: Ionian and Scaglia-Biancone Basin were generated. Upper Jurassic: according to regional faulting, deep Lemes Trough was formed. Albian-Lower Cenomanian: global drop of sea level. Upper Cretaceous: initial destruction and folding of the platform, period of long carstification and bauxite deposition. Lower Eocene: transgression. Final termination and sedimentary break of carbonate deposition in the scale of the regional stable shelf was marked by Upper Eocene synorogene flysch deposition during the Tertiary folding phase. After folding, during post-orogene transpression, the area was affected by normal faulting and wrench type tectonic. Thick accumulation of fine Neogene Molassa type clastics covered the area.

During 40 years of exploration approxmiately 60 wildcat wells were drilled through Mesozoic and Paleogene deposits with maximum depth of 7000 m. Forty shallow gas wells (1800-2200 m) were drilled through Tertiary clastics. Until today, there was no commercial oil discovery. However, oil occurrences in deep wells were identified at the base of Eocene clastics, in the unconformable contact between Upper Cretaceous rudist limestone and Lower Eocene nummulitic limestone, within the dolomitic-anhydritic sequence of Albian-Cenomanian age. Potentional source rocks of the entire platform are Upper Jurassic; Lemes beds-organic reach carbonate shales (TOC>7%); Upper Cretaceous shales (TOC>15%), Albian bituminous stromatolitic dolomite and bituminous ostracodal MDST and WKST (TOC-1.75%), Upper Cretaceous oligosteginidic limestone (TOC-5.92%), Eocene chalky limestone (1.08% TOC) etc. Kerogene is mainly type II. These source rocks unfortunately have only local distribution and small volume. In the "South Adriatic Block", source rocks are Eocene and Creatceous in age (TOC=3.78, 6.18, 6.02 and 1.39). Kerogene type II. Porosity in Cretaceous limestone is (22-24%) and in Eocene (19%). In the regional scale caprocks are thick Neogene clastic deposits.

AAPG Search and Discovery Article #90933©1998 ABGP/AAPG International Conference and Exhibition, Rio de Janeiro, Brazil