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Fractured Inter-bedded Dolomites and Limestones in a Reservoir Analogue: Integrating Carbonate Sedimentology and Structural data from thin Section to Reservoir Scale

Francesco Dati, Monica Girundo, Vincenzo Guerriero, Alessandro Iannace, Stefano Mazzoli, Mariano Parente, Stefano Tavani, Stefano Vitale, Loic Bazalgette, and Christoph Strauss

In low porosity carbonate reservoirs, fracture systems are the main factor controlling the flow of hydrocarbons. Good examples are the Apulian fractured carbonate fields of Basilicata area (southern Italy), the major on-shore carbonate reservoirs in Western Europe. There, hydrocarbons are hosted in interstratified limestones and dolomites of upper Lower to Upper Cretaceous age. The occurrence of fractures at various scales in these fields is well documented by FMI logs. However a profound geological model to explain the distribution of fracture sets and their relative impact on fluid flow in the reservoir is not evidenced, and any study conducted directly on drilling cores presents significant limitations related to observable fracture scales. To overcome such limitations, in the last years a growing interest for geometrical reservoir analogues accrued. The aim of this study is to clarify the role of different geological parameters such as lithology, crystal size and bed thickness, which contribute to the development of stratabound and non-stratabound fracture networks in a carbonate succession that can be considered a good reservoir analogue for the Basilicata oilfields. The study area – Monte Chianello – is located in southern Apennines. The outcropping succession comprises a 1200 m thick sequence of Cretaceous shallow water carbonates characterized by an alternation of dolomitic and calcareous beds, with variable textures and crystal sizes. Comparing non-stratabound fracture density carried out from thin sections and acetate peels, when intragranular microcracks, that are common in dolomite crystals, are not taken into account, higher values result for limestones with respect to dolomites; on the contrary when all fractures (microcracks included) are considered for the estimation, both lithologies show same fracture density values. Dolomites exhibit an inverse relationship between crystal size and fracture density (microcracks excluded), well described by a power law. However, numerous fracture counts on single-bed scan lines provide clear evidence that, generally, bed thickness is the dominating parameter controlling stratabound fracture distribution within the studied outcrops, regardless of lithology and crystals size.

AAPG Search and Discovery Article #90161©2013 AAPG European Regional Conference, Barcelona, Spain, 8-10 April 2013