Paragenesis of Cretaceous to Eocene carbonate reservoirs in the Ionian fold and thrust belt (Albania): relation between tectonism and fluid flow

This paper examines the diagenetic history of dual (i.e. matrix and fracture) porosity reservoir lithologies in Cretaceous to Eocene carbonate turbidites of the Ionian fold and thrust belt, close to the oil-producing centre of Fier-Ballsh (central Albania). The first major diagenetic event controlling reservoir quality was early cementation by isopachous and syntaxial low-Mg calcite. These cements formed primarily around crinoid and rudist fragments, which acted as nucleation sites. In sediments in which these bioclasts are the major rock constituent, this cement can make up 30% of the rock volume, resulting in low effective porosity. In strata in which these bioclasts are mixed with reworkedmicrite, isopachous/syntaxial cements stabilized the framework, and matrixporosity is around 15%. The volumetric importance of these cements, their optical and luminescence character (distribution and dull orange luminescence) and stable isotopic signal (delta(18)O and delta(13)C averaging respectively; -0.5parts per thousand VPDB and +2parts per thousand VPDB) all support a marine phreatic origin. Within these turbidites and debris flows, several generations of fractures alternated with episodes of cementation. A detailed reconstruction of this history was based on cross-cutting relationships of fractures and compactional and layer-parallel shortening (LPS) stylolites. The prefolding calcite veins possess orange cathodoluminescence similar to that of the host rock. Their stable isotope signatures (delta(18)O of -3.86 to -0.85parts per thousand VPDB and delta(13)C of - 0.14 to + 2.98parts per thousand VPDB) support a closed diagenetic rock-buffered system. A similar closed system accounts for the selectively reopened and subsequently calcite-cemented LPS stylolites (delta(18)O of -1.81 to -1.14parts per thousand VPDB and delta(13)C of +1.52 to +2.56parts per thousand VPDB). Within the prefolding veins, brecciated host rock fragments and complex textures such as crack and seal features resulted from hydraulic fracturing. They reflect expulsion of overpressured fluids within the footwall of the frontal thrusts. After folding and thrust sheet emplacement, some calcite veins are still rock buffered (delta(18)O of -0.96 to +0.2parts per thousand VPDB and delta(13)C of +0.79 to +1.37parts per thousand VPDB), whereas others reflect external (i.e. extraformational) and thus large-scale fluid fluxes. Some of these veins are linked to basement-derived fluid circulation or originated from fluid flow along evaporitic decollement horizons (delta(18)O around +3.0parts per thousand VPDB and delta(13)C around +1.5parts per thousand VPDB). Others are related to the maturation of hydrocarbons in the system (delta(18)O around -7.1parts per thousand VPDB and delta(13)C around +9.3parts per thousand VPDB). An open joint system reflecting an extensional stress regime developed during or after the final folding stage. This joint system enhanced vertical connectivity. This open joint network can be explained by the high palaeotopographical position and the folding of the reservoir analogue within the deformational front. The joint system is pre-Burdigalian in age based upon a dated karstified discordance contact. Sediment-filled karst cavity development is linked to meteoric water infiltration during emergence of some of the structures. Despite its sediment fill, the karst network is locally an important contributor to reservoir matrix porosity in otherwise tight lithologies. Development of secondary porosity along bed-parallel and bed-perpendicular (i.e. layer-parallel shortening) stylolites is interpreted as a late-stage diagenetic event associated with migration of acidic fluids during hydrocarbon maturation. Development of porosity along the LPS system enhanced the vertical reservoir connectivity.