DEPOSITIONAL FACIES, PORE  TYPES AND ELASTIC PROPERTIES OF DEEP-WATER GRAVITY FLOW CARBONATES

A. Hairabian ¹*, F. Fournier¹, J. Borgomano ^1,2 and S. Nardon³

¹ Aix-Marseille University, CEREGE, case 67, 3, Place Victor Hugo, 13003 Marseille, France.

² TOTAL S.A., Exploration-Production, CSTJF, Avenue Larribau, 64000 Pau, France.

³ ENI S.p.A. Exploration & Production Division, Sedimentology, Petrography and Stratigraphy Dept., San Donato Milanese 20097, Italy.

* author for correspondence, email: hairabian@cerege.fr

Quantitative petrographic analyses of deep-water resedimented carbonates from the Gargano Peninsula (SE Italy) were integrated with petrophysical laboratory measurements (porosity, P- and S-wave velocities) to assess the impact of sedimentary fabrics and pore space architecture on velocity-porosity transforms. Samples of Upper Cretaceous carbonate came from the Monte Sant’Angelo, Nevarra and Caramanica Formations and can be classified into four depositional facies associations: F1, lithoclastic breccias; F2, bioclastic packtones to grainstones; F3, interbedded grainstones-packstones and wackestones; and F4, (hemi-) pelagic mudstones. Five pore type classes were distinguished: I and II, dominant intercrystalline microporosity; IIIa, dominant intergranular macroporosity; IIIb, dominant mouldic macroporosity; and IIIc, mixed intergranular and mouldic macroporosity. Pore type was found to strongly control velocity-porosity transforms, unlike depositional facies associations.

The equivalent pore aspect ratio (EPAR), derived from differential effective medium models, is proposed to identify pore types from elastic properties. The EPAR originates from the bulk modulus or shear modulus of the samples (K- and m-EPAR, respectively). Regardless of porosity values and depositional facies, microporous samples (type I) and samples with dominant intergranular porosity (type IIIa) are characterized by low values of K- and m -EPAR (<0.22) and by K-EPAR > m-EPAR. By contrast, samples with dominant mouldic porosity (type IIIb) display high values of K- and m-EPAR (>0.25 and 0.4 respectively) and K-EPAR < m -EPAR.

High permeability limestones with dominant intergranular porosity cannot be discriminated from low permeability microporous carbonates. The petrophysical classes derived from elastic properties are shown to be distinct from reservoir property-driven rock types. In the present case, a seismic-based poro-elastic model does not match the reservoir property model. Hence, a sedimentary facies model for the studied carbonates cannot accurately represent the petrophysical properties, which are determined by pores types and pore network architecture.

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