New evidences for the Burano formation source rock facies in the Croatian sector of Central Adriatic

Abstract

In wider Adriatic area Late Triassic sediments are known as “Burano formation” consisting of Carnian-Norian evaporitic member and Rethian dolomitic member. Burano formation is better described in the Italian sector of the Adriatic based on more evidences available. Despite sparse Triassic section well&outcrop data on the Croatian side, lithostratigraphic division of Late Triassic is generally in accordance with the “Burano formation”. However, after summarizing Croatian data sets it has been observed that Ladinian-Carnian evaporites of “Vlasta-Komiza” facies seems to be specific for the Croatian side of Adriatic.

The objective of this study is detailed facies and source rock characterization of the Triassic sediments of Vlasta-1 and Vlatka-1 wells which were used as starting points for the Burano facies modelling and areal reconstruction.

Early to Middle Carnian deposition in the Vlasta-1 well is dominated by subaquatic halite with thin layers of carbonates, anhydrites and clay. Sedimentation took place in relatively large, shallow, enclosed bays or lagoons or in marine areas which due to tectonics and sea level variability remained relatively isolated from the open sea. Supratidal environment was established in Late Carnian where carbonates and anhydrites were deposited in typical regressive sabkha microcycles composed of three members (organic rich carbonates, then early diagenetic dolomites and anhydrites and finally gypsum and anhydrite).

Up to the Upper Norian, deposition of carbonate-anhydrite rocks in supratidal environment with established sabkha conditions continued. The youngest part of the late Norian deposition took place in intratidal-supratidal environment with no sabkha conditions.

Source rock potential has been recognized in Late Carnian organic rich laminated limestones in Vlasta-1 well within 5400-5600m interval. Analyses showed the autochthonous organic matter with organic matter content from 0.15 to 1.80% (in average 0.83%). Limestones have good to excellent hydrocarbon potential (up to 13 mg HC/g rock). Kerogen is type II to II-S. Organic matter is a mixture of algal, bacterially degraded kerogen and migrated bitumen. Amorphous organic matter changes from lamalginite to dominantly bituminite. Stable carbon isotope values are within range from δ13C -27 to -28‰ in kerogen and from -30 to -31‰ in bitumen. These values are typical for the Triassic rocks and they are generally attributed to the specific algal and bacterial lipids as well as to the anoxic conditions. Biomarker analyses confirmed algal-bacterially origin of organic matter and deposition in the low energy, highly anoxic, lagoonal, carbonate-evaporitic environment. Late Triassic laminated limestones have an excellent generation potential and represent very good to excellent oil-prone source rocks.

According to all maturity parameters those source rocks generally reached onset of oil generation. However, only small oil volume was recovered from the same Late Triassic section. Oil is positively correlated with Late Triassic source rock.

In Vlatka-1 well few Norian intervals have simmilar source rock characteristics.

An attempt has been made to map described source rock horizon in wider Central Adriatic area. Public available Italian well data have been used to complement the Burano formation spatial reconstruction. Despite detailed well&outcrop data analysed, it has to be emphasized that the proposed model is constrained by the variable 2D seismic quality and complex geology with strong Triassic (Late Permian?) halocinetic footprint

AAPG Datapages/Search and Discovery Article #90226 © 2015 European Regional Conference and Exhibition, Lisbon, Portugal, May 18-19, 2015