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Fracturing of Flat-Topped Carbonate Platforms: Implications for Platform-Scale Flow

G. Bertotti¹^,² and H. Boro²
¹Department of Geotechnology TUDelft, The Netherlands
²VU University Amsterdam, The Netherlands

Because of their heterogeneous structure and their not-so-simple evolution, flat-topped carbonate platforms are expected to display significant lateral changes in the pattern of distributed fracturing. This bears consequences on the path followed by hydrocarbons during the charging stage (migration) and on production schemes during exploitation. Flat-top carbonate platforms are characterized by i) a strong 3D (sub-circular) geometry, and ii) the lateral juxtaposition of first order sedimentological domains with different mechanical properties separated by not-horizontal and not-vertical contact surfaces. The direction and magnitude of stresses and, consequently, the distribution, orientation and type of fractures are predicted to be non-obvious. In addition, rocks of the platform will experience substantial diagenetic changes during subsidence from deposition to maximum burial depth resulting in mechanical properties changing through time.

Combining Finite Element numerical modeling and data from outcropping analogs we map changes in the patterns of fractures affecting flat-topped carbonate platforms from deposition to maximum burial depth thereby providing tools for predicting fractures in buried reservoirs. Numerical models, are inspired by the Latemar platform (Dolomites, N Italy) a nicely exposed, 3*5km wide, 800m thick atoll-like platform we have investigated in the last years and from which we have derived the data on fracture networks discussed in the second part of this contribution.

Despite their intrinsic simplifications, finite element modeling provides unique insight in the magnitude and distribution of the volume of fractured rocks in a carbonate platform descending from sea-level to maximum burial depth. These studies show that significant fracturing occurs at shallow depths, however, affecting only <30% of the platform. The orientation of fractures is strongly dependent on the tectonic scenarios and will be perpendicular/parallel to the main facies boundaries only in the (unlikely) case that gravity is the dominating stress. More commonly, fractures will tend to be aligned along the direction of maximum principle stress. Knowledge of fracture geometries is key for improving predictions in reservoirs hosted by atoll-like carbonate platforms.

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