Fluid circulation in the Ionian fold and thrust belt (Albania): implications for hydrocarbon prospectivity

Abstract

Within the Ionian zone, which belongs to the external Albanide fold and thrust belt, a surface reservoir analogue was studied with the aim of unravelling the processes which exert an effect on the dual porosity network. Matrix porosity preservation in the Cretaceous to Eocene carbonate turbidite beds relates to early cementation, where the presence of a mixture of micritic and mono-crystalline bioclasts (crinoids and rudists) controls the framework-stabilising development of cements. The presence of these cements explains why these lithologies have not been entirely compacted (as happened for the overlying micrite-dominated lithologies). The underlying bioclast-rich layers also do not possess any porosity since they were entirely cemented.Present-day oil production is mainly fracture-related. Several vein generations have been recognised as well as other tectonic features such as conjugate high-angle or tension joints and stylolites. With respect to the latter, bed parallel (i.e. compactional) and bed perpendicular (i.e. layer parallel shortening or tectonic) stylolites have been differentiated. The spatial relationships between these stylolites and the veins allow differentiation of pre-, syn- and post-kinematic fluid fluxes, which have been characterised based on their petrographical (i.e. cathodoluminescence), geochemical (trace and stable isotope) and fluid inclusion features. This exercise shows that the fluid flow system was rock buffered until the folding and thrusting period. From then onwards the fluid flow system reflects the interaction with fluids which reacted with evaporites which occur as decollement horizons as well as fluid flow systems in relation to the maturation of hydrocarbons. Finally after folding and faulting, the studied anticlinal structure was temporarily emergent during the Miocene. During this period sediment infilled karst cavities formed. Since the latter sediment infill is not compacted but porous, they contribute to the total oil-in place.