Numerical Outcrop Geology of Carbonate Sedimentary Systems: Impact on Carbonate Reservoir Models?

Jean Borgomano, F. Fournier, A. Hairabian, P. Léonide, J-P. Masse, R. Richet, R. Toullec, and S. Viseur
University of Aix-Marseille, CEREGE, Marseille, France

New technologies and industrial needs can be a powerful engine for scientific progress. The recent evolutions in outcrop numerical geology are the results of converging developments in computer sciences and geophysics, and growing demand of the industry for improving the management and the production of hydrocarbon reservoirs.

The intrinsic complexity and the poor seismic imaging of carbonates have stimulated, during the last decade, numerous numerical outcrop geological surveys on reservoir analogues with the principal objective to improve and validate subsurface reservoir models, and a secondary objective to develop efficient numerical geological methods. Given the costs (human resources, time, computers, software, technologies,…) of these methodological developments and the potential high impact of these outcrop studies on hydrocarbon reservoir management, it is critical to assess their outcome for establishing future research strategies and educational program in geosciences. In the presentation we propose to evaluate the impact of numerical outcrop geological studies on carbonate reservoir models and discuss research strategies on outcrop carbonate geology.

Since 2004, several carbonate outcrop numerical models have been realized within the framework of collaborations between the industry and the academic carbonate group in Marseilles (ex “LGSRC”). All these projects target cretaceous carbonate platforms. Outcrop numerical geology includes the set of technologies and geological methods that allow the realization of complex Digital Outcrop Models (DOM), including surfaces and volumes, of geological objects and their associated properties. The general objective of these studies is to realize 3D high resolution stratigraphic and property models of pertinent carbonate systems, on the basis of high resolution Digital Elevation Model (DEM) acquired with LIDAR (aerial & terrestrial), outcrop photographs, geological mapping and sampling, and geomodeling with gOcad. Studied outcrops have been selected on the basis of their great lateral continuity at relevant scales. High resolution DEM, obtained from LIDAR, and photographs provide the continuous support for mapping in great details the stratigraphic architectures and the facies. Obtained DOM mimics closely subsurface carbonate reservoir models. The main difference between subsurface and outcrop models are the input data: well and seismic versus outcrop and LIDAR.

In the presented case studies, the lateral-vertical extend of the outcrops exceed generally the model dimensions, which minimizes the interpolation or stochastic extrapolation between punctual data. On the contrary, subsurface reservoir models are strongly based on interpolation between distant well data, within a stratigraphic framework obtained from seismic and deterministic well correlation. The outcrop models can therefore provide realistic stratigraphical-sedimentary frameworks at relevant subsurface scales, inter-well or seismic. The challenge is basically to ensure that the outcrop model improve the relevant subsurface reservoir analogue. On the basis of the presented outcrop models, which correspond to a variety of carbonate sedimentary systems and reservoirs, we are evaluating the potential improvement on reservoir models.

Four carbonate outcrop models are presented, three different types of Cretaceous platforms and one Cretaceous slope and base-of-slope system. For each outcrop models, we describe the key geological elements and we discuss the main potential contribution to analogue reservoir models:

• Despite high resolution and continuity of numerical outcrop data, "soft" information such as sedimentological-stratigraphical concepts, trend maps of properties or stratigraphic correlation methods (knowledge in general) are most critical for digital outcrop modeling. This strongly suggests that knowledge would be even more critical for subsurface carbonate models.
  
• Geomorphologic surfaces or 3D sedimentary profiles, extracted from the DOM, are fundamental data for constraining the facies-property distribution, and are more significant than the density or location of stratigraphical sections. Seismic-interpreted sedimentary surfaces (or surfacic seismic-attributes) are critical data for reservoir models.
  
• Sequence stratigraphical correlation of distant stratigraphic sections is valid in case of well established sedimentary profiles and good relationship between stratigraphic sequences and properties, but it is not a valid method when the sedimentary system, the facies or the properties are weakly related to stratigraphic sequences.
  
• Facies trends or facies associations in Cretaceous platforms are characterized by long range (1-5 km) stratigraphical structures, whereas the distribution of facies on shorter range seems to be more random, especially in the outer shelf domain with rudist-coral patches or bioclastic lenses, lobes or channels.
  
• Object-based modeling are applicable to slope and base-of slope systems, but sequence stratigraphic correlation is not adapted for modeling properties in such environments. Typically, lobes, lens and breccias can be distributed randomly in a given stratigraphic units. Geomorphological surfaces are the key to constrain such facies distribution, not the sequence boundaries, which are not easy to define in this case.

Digital outcrop models of carbonate systems demonstrate the importance of geomorphologic surfaces for modeling the sedimentary facies and property distribution in 3D. Such surfaces do not correspond necessarily to sequence boundaries or any other sort of stratigraphical surfaces, which can be interpreted from seismic markers or well correlation.

Carbonate digital outcrop models are characterized by a strong stratigraphical component at all scales, they are undoubting "stratified". This is result of the long range stratigraphical organization of the facies trends and associations. But the short range facies distribution is weakly correlated spatially and appears more random.

Spatial and genetic relationship between stratigraphic architecture and sedimentary facies distribution is therefore one of the key parameter for carbonate reservoir modeling from subsurface data. Diagenetic transformations, not studied here, control the ultimate distribution of reservoir properties, with variable degree of conformity to the strata and the sedimentary facies.

Property and flow modeling of carbonate reservoirs are based on surfaces modeled from seismic and stratigraphical well correlation. The significance of the seismic reflectors and stratigraphical well correlation in terms of sedimentary facies and property distribution is one of the main uncertainties of carbonate reservoir models (static and dynamic). Realizing synthetic seismic and flow simulations from the carbonate DOM should help to assess this uncertainty. This hydrodynamic-geophysical dual forward modeling of carbonate DOM is the next research phase that should converge with the dual hydrodynamic-geophysical inversion of subsurface seismic.

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