High-Resolution 3D Modeling of Architecture and Properties of Base-of-Slope Carbonates: The Upper Cretaceous Outcrops of the Gargano Promontory (South-East Italy) - Relation with Flow Units

Hairabian, Alex¹, Borgomano, Jean¹, and Nardon, Sergio²
¹Aix-Marseille University, C.E.R.E.G.E. - Centre Saint-Charles, Marseille cedex 3, France
²ENI S.p.A. Exploration & Production Division, San Donato Milanese, Italy

A reservoir model should accurately replicate features affecting fluid storage, distribution and flow. To accurately characterize the reservoir, one must first addresses the geological variability (or heterogeneity) of the system. In that, carbonate reservoirs present special challenges since their generation involves a more complex suite of processes than many other sediment types. The fact that carbonate sediments are largely of biogenic origin and are easily altered by diagenesis, creates particular characteristics. Where in siliciclastic systems, input source and hydrodynamic conditions play the most important role; in carbonates, the biologic factory is highly receptive to environmental changes. As a result, the architecture of carbonate systems can be highly variable and their internal heterogeneity, in terms of lithological and petrophysic properties, is even more difficult to predict. The scale at which are expressed these heterogeneities in carbonate reservoir is not resolvable using current subsurface data. Wells are often widely spaced and seismic imaging may resolve large-scale architecture of the depositional system but has a limited resolution. For many years, the study of outcrop has permitted direct observation of reservoir analogues on a large range of scales and bridged the resolution gap between seismic and well data. However, a lack of fully reconstructed outcrop models that could be realistically and quantitatively compared to their subsurface analogues persists. This is mainly due to the difficulty of capturing the 2D, and moreover the 3D geometry of outcrops in an accurate and efficient manner. Technological improvements over the past two decades have changed the rules. Digital tools such as real time kinematic global positioning systems (RTK GPS), photogrammetry and/or light detection and ranging (LIDAR) allow connecting all the geologic observations and measurements within a realistic geographic spatial position into the numerical realm. The integration of field observations on virtual outcrops (with their spatial attributes) is used to construct digital outcrop models (DOM’s; Bellian et al., 2005). Through this process, complete information on the sedimentary systems is made accessible numerically to reconstruct a relevant and coherent 3D model. This ‘virtual laboratory’ can then host different types of physical experimental models: static/dynamic reservoir simulation model or synthetic seismic model.

The purpose of the study is to build a numerical 3D model of a reservoir analogue. The model must take into account the different scales at which the heterogeneities occur within the system: (1) the large-scale (km’s) sedimentary architecture, regional unconformities, main faults, morphology of the basin; (2) the medium scale (km to m’s) which refers to the spatial organization of the sedimentary bodies, their facies and 3D geometry; (3) up to a smaller scale (cm to mm’s) defined by the microfacies, pore type and quantification of porosity and permeability on plugs. The studied outcrops are located in the Gargano peninsula (South-East Italy) and consist in a thick interval (1000 m) of Cretaceous base-of-slope carbonate which are analogues to the Aquila Field (Italy) and the Pozza Rica Fields (Mexico). The sedimentary succession has been deposited in a deep-water environment by sediment gravity flows interbedded with pelagic mudstones (Borgomano, 2000). The area presents exceptionally well-exposed outcrops forming a succession of N/S elongated crests (over more than 15 km) and bordered to the south by high cliffs delimiting the coastal plain of the Adriatic Sea. This configuration allows a detailed study of stratal architecture, lithologies, and sedimentary processes responsible for these deposits. Detailed 3D mapping has been performed using a high-resolution digital elevation model (D.E.M.) derived from heliported LIDAR survey and completed with photogrammetry. The set of aerial orthophotos draped over the DEM was used to generate virtual outcrops over 100 km² with a resolution varying from 5 m, for the photogrammetry, to ~1 m for the zones scanned with the LIDAR.

The study lead to the distinction of two main types of carbonate gravitary systems (or reservoir systems): aprons and fans (or fan-like lobe complexes). Aprons are thick prisms of sediments (several hundred meters thick and tens of kilometers long) that originate parallel to and along the platform margin. As described by Schlager and Chermark (1979) and Mullins et al. (1984) in the Bahamas, aprons are fed by a multitude of small submarine canyons or gullies that cut across the upper slope and act as a line source for the down-slope transportation and distribution of granular sediment. The fans correspond to spatially disconnected lobe complexes (few meters to more than one hundred meters thick and 1 to 5 kilometers of extension) fed by few canyons or gullies with act as point sources. Aprons as fans may be dominated either by bioclastic sands (including grains transported from the outer shelf) or by breccias (including cemented limestone blocks eroded from the shelf edge). Reservoir potential in breccia dominated systems is moderate to nil. In contrast, bioclastic systems show a high reservoir potential. For these reasons, this study focuses on the bioclastic systems.

Bioclastic aprons are characterized by massive sedimentary bodies (decametric sand sheets of kilometric lateral extension) and rare intercalations of pelagic mud. The main sedimentary processes on these aprons are grain flows and turbidity currents. These reservoir rocks, dominated by grainstones and rudstones, are characterized by high primary and secondary porosity (intergranular, skeletal, moldic and microporosity). Although small scale (metric to decametric) variations of the porosity and permeability are highly significant; at larger scales (hectometric to kilometric), these properties can be considered as homogeneous. Flow units may then be identical to reservoir units.

Bioclastic fan-like lobe complexes are, in contrast, much more spatially organized. For example, sandy lobes (few meter thick and 10’s to 100’s meters of extension) can be stacked vertically (100 meter thick aggradational packages) along the axes of few gullies which cut across the slope. As the coarser material is deposited in these topographic lows, the finer sands and lime mud form small levee deposits on each side. In more distal parts, the deposits occur as alternating sheets of fine sands and pelagic mud. Thus, specific sedimentary geometries and facies exist (sandy lobes, sandy channel-fills, muddy levees, sheets,…) whose location shows a good correlation with the topography and the position in the base-of-slope environment. Sandy lobes show good porosity and permeability related to a dominant intergranular pore type in very well sorted material. Levee deposits and pelagic mud blankets show variable porosity values (microporosity) and low permeability values. Even if muddy deposits can have a good potential for fluid storage, sandy deposits (lobes or sheets) have the best potential for fluid storage and/or flow. Spatial variations of petrophysical properties within a single sandy body are low. Vertical connectivity between the lobes is high along gullies axes while lateral connectivity is more critical when compensational stacking occur in more distal parts. Thus, by improving the detection and 3D modeling of subtle pre-existing morphologies (gullies, fault scarps), it is possible to predict the position and the geometry of the sandy bodies which can be considered as the best flow units in these carbonate fan systems.

References

Bellian, J.A., Kerans, C. and Jenette, D.C., 2005. Digital outcrop models: applications of terrestrial scanning lidar technology in stratigraphic modeling. J. Sed. Res., 75, p. 166-176.
Borgomano, J.R.F., 2000. The Upper Cretaceous carbonates of the Gargano-Murge region, southern Italy: A model of platform-to-basin transition. AAPG Bull., 84, p.1561-1588.
Mullins, H.T., Heath, K.C., Van Buren, H.M. and Newton, C.R., 1984. Anatomy of modern open-ocean carbonate slope: Northern Little Bahama Bank. Sedimentology, 31, p. 141-168.
Schlager, W. and Chermark, A., 1979. Sediment facies of platform-basin transition, Tongue of the Ocean, Bahamas. Soc. Econ. Paleontol. Mineral. Spec. Publ., 27, p. 193-208.

AAPG Search and Discovery Article #120034©2012 AAPG Hedberg Conference Fundamental Controls on Flow in Carbonates, Saint-Cyr Sur Mer, Provence, France, July 8-13, 2012