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Reservoir Development of Deep Water Gravitary Carbonates: Examples from the Cretaceous Apulian Platform Margin (SE Italy)

Borgomano, Jean *1; Hairabian, Alex 1; Nardon, Sergio 2
(1) Geology of Carbonate Systems and Reservoirs, University of Aix-Marseille, Marseille, France.
(2) Eni S.p.A. - Exploration & Production Division, Milano, Italy.

Although deep water gravitary carbonates are hydrocarbon producing reservoirs in a limited number of petroleum provinces, they have the potential to become important exploration plays, especially in the deep water offshore domains. As typical platform or reefal carbonates, their reservoir development is characterized by great complexity and heterogeneity that could represent significant exploration challenges: within a basin, some resedimented carbonate units present great porosity, others are completely tight and non porous. Geological characteristics and controlling factors of such carbonate reservoirs are presented in the light of well studied outcrops on the Apulian Platform Margin, which are analogues to the Aquila Field (Italy) and the Pozza Rica Fields (Mexico). The Cretaceous base-of-slope interval (1500 m thick) in the Gargano area is characterized by bioclastic and conglomeratic aprons interbedded with pelagic mudstones. The outer shelf domain, which was the prolific source of rudistid bioclastic aprons, is not preserved. The platform is mainly built of mud-dominated transgressive systems tracts, whereas highstand systems tracts are developed as bioclastic base-of-slope aprons with lobes and channels. The main sedimentary processes on these aprons are mass flows and turbidity currents. The volume of the resedimented bioclasts largely exceeds the volume of missing outer shelf, indicating that carbonate production and sediment bypass are more critical for this stratigraphic response than carbonate erosion. These reservoir rocks dominated by grainstones and rudstones are characterised by high primary and secondary porosity (intergranular, skeletal, moldic, microporosity) and large sedimentary bodies. The lowstands, controlled by tectonic deformations, are characterized by base-of-slope breccias, conglomerates and blocks eroded from the shelf edge. This coarse carbonate material was deposited as debris flows or high-low density turbidity currents. Reservoir quality in these rocks is nil to moderate. It can be concluded that the origin and the nature of the resedimented carbonate material is more critical for the reservoir quality than the gravitary depositional processes. Skeletal aragonite-dominated grainstone-rudstones, sourced directly by a granular carbonate factory (rudist), have better reservoir potential than breccia and lithoclastic grainstone-rudstone, sourced by the erosion of ancient well cemented and compacted carbonate platform.


AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California