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Fracture Patterns in Atoll-like Carbonate Platforms: Insights from Latemar Platform (North Italy)

Boro, Herman 1; Bertotti, Giovanni 1; Hardebol, Nico J.1; Agar, Susan M.2; Amour, Frederic 3; Christ, Nicolas 4; Luthi, Stefan M.5; VD Vaart, Jeroen 1; Ewonde, Kenneth 1
1 Tectonic & Structural Geology, VU University Amsterdam, Amsterdam, Netherlands.
2 ExxonMobil Upstream Research Company, Houston, TX.
3 University of Potsdam, Potsdam, Germany.
4 Ruhr University Bochum, Bochum, Germany.
5 Delft University of Technology, Delft, Netherlands.

Carbonate platforms form major reservoirs worldwide and their physical properties are commonly controlled by multiscale fracture networks. Predicting fracture patterns in these platforms is important for flow simulation since many of the fractures are sub-seismic and borehole imaging can only provide information limited to the wellbore. Prediction is particularly difficult in atoll-like platforms which comprise domains with different sedimentological and mechanical characteristics separated by non-horizontal boundaries. The outcrop studies complement subsurface data and provide insights to the geologic controls on fracture populations.

We have chosen the Middle Triassic Latemar platform (Dolomites, Italy) as an analog for steep carbonate platforms. The Latemar has a wide platform interior, dominated by lagoon facies and passing laterally to coarse-grained and poorly organized slopes. The reefal margin is nearly absent as a consequence of the collapsed of the margin and platform interior blocks into the adjacent basins. The whole platform is intensively fractured at various scales. We have devoted particular attention to changes of fracture patterns across the stratigraphy. Data acquisition adopted a method developed by VU Amsterdam and TU Delft. In this method, an outcrop photo is imaged in GIS application to define an internal scale. Fractures are then directly recorded on the screen together with their properties, i.e. orientation, length, filling, etc. Automatic processing provides detailed cm-scale information on the changes in fracture spacing or density. Using our method we were able to collect >1100 of fractures data from different facies within 9 days spread over 20 stations.

Fractures in Latemar exhibit two different sets which strike NNW-SSE and ENE-WSW and cut both platform interior and slope facies. There are significant differences in fracture patterns. In the platform interior, spacing distances are around 0.3-1m but significant variations are related to the presence of mechanical units. Units with more grainstones have higher fracture densities. The fracture heights vary from tens of cm (bed-confined) up to 20m. Slope facies, characterized by massif steeply dipping pack-grainstones and breccias, reveal wide fracture spacing, around 2-2.5 m, with fractures heights ranging from 3m up to 20m. In subsurface reservoir, such variables will show strong fracture related fluid flow inside the platform interior compared to slope facies.

 

AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009