Sedimentary and diagenetic effects on reservoir properties of Upper Cretaceous Ionian Basin and Kruja platform carbonates, Albania

Abstract

This paper aims to study the sedimentary and diagenetic factors controlling the heterogeneity of carbonate reservoir systems and their petrophysical properties in the context of platform to platform-basin transitions. For that, sedimentary series from the Upper Cretaceous carbonate platform of Kruja and its adjacent Ionian Basin (Kremenara anticline structure and Muzina area) in central and southern Albania were chosen as they represent potential analogues for hydrocarbon reservoirs especially since the carbonate gravity deposits of the slope and basin of this system form one of the most important oil reservoirs exploited in Albania, Greece and Italy.In the Upper Cretaceous carbonate platform of Kruja, the main diagenetic processes having an impact on the heterogeneity of reservoir rocks are: (1) micritization and marine cementation; (2) early and late dolomitization and dolomite neomorphism; (3) dissolution and/or neomorphism of aragonite; (4) calcite cementation; (5) mechanical and chemical compaction; and (6) minor fracturing. The petrographic and petrophysical analysis (RMN, Hg-porosimetry, permeametry) of 18 core samples and 71 thin sections of the platform facies (Krujë-Dajt section) revealed that the porosity ranges from 2.1% to 20.3% and the permeability ranges from 2 to 40 mD. The large value of porosity is mainly driven by diagenetic modifications such as dolomitization. The replacement of the mineralogy of bioclasts by dolomitic phases considerably increased the secondary porosity in some grainstone/packstone. While the low permeability values of the same facies have been interpreted to reflect networks of unconnected pores.In the Upper Cretaceous carbonate of the Ionian Basin, 112 samples (Kremenara area: 71 samples; Muzina area: 41 samples) were analyzed for their petrophysical properties and microscopical characteristics. In the Kremenara outcrop, porosity ranges from 1 to 13.9% while the permeability is very low <1 mD. The porosity values of the packstone to grainstone from the interval Tc of the turbidites succession show the highest values and ranges between 4.2 and 13.9%. In the Muzina outcrop the highest porosity values were found in Td facies with a mean value of 10%. Hg-porosimetry reveals a monomodal pore structure, with one pore-type around 0.1 μm. The actual porosity distributions are mainly controlled by the initial sedimentological characteristics and micrite matrix characteristics. The impact of diagenesis on the development of porous networks is very limited. Its role is represented by the effect of neomorphism causing the recrystallization of the micritic matrix into microsparite, allowing the development of significant microporosity in facies with a muddy fraction in their texture.