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Improving Predictions from Mechanic Stratigraphy of Buried Sedimentary Successions: Lessons from Outcrop Analogs

Bertotti, Giovanni 1; Boro, Herman 1; Hardebol, Nico J.1; Luthi, Stefan M.2
1 Tectonics/Structural Geology, VU University Amsterdam, Amsterdam, Netherlands.
2 Geotechnology, TUDelft, Delft, Netherlands.

Differently from what often assumed, fractures such as joints are not always confined to single sedimentary beds. In contrast, they can continue across a group of beds and/or start/end within layers. The concept of mechanical units has been developed to define a group of layers which displays a homogeneous fracture pattern. Presently, however, there are no theoretical tools to predict which parts of a sedimentary succession will behave as a mechanical unit. It is thus unknown which sedimentological or mechanical properties concur in defining mechanical units. Outcropping successions provide relevant information complementary to borehole data.

A “backward” work flow is proposed which includes i) the quantitative description of fracture patterns in vertical outcrops spread over a reservoir-scale region, ii) the definition of mechanical units, and iii) the establishment of correlations between mechanical units and observables. Such a work flow is made possible by a recently developed acquisition and processing protocol which produces an accurate and assumption-free description of fracture patterns across the stratigraphy. The image of the outcrop, corrected for distortion, is imported in a GIS environment thereby acquiring an internal scale. Fractures are traced directly on the screen of a laptop and attributes such as orientation and morphology, are associated to them. An automated processing routine extracts from the corresponding shape files changes across the stratigraphy of fracture characteristics at detailed scales of <2cm.

Results from three case studies are presented. In Permian deep water sandstones of the Karoo Basin, m-thick beds, typically turbiditc, show widespread intra-bed changes in fracture density, directly proportional to grain size. Dm- and thinner scale beds tend to form mechanical units unless intercalated in shaly layers. In central Morocco, a gently folded, 30m thick package of Devonian carbonate sands in a finer-grained succession behaves as a single mechanical unit with higher fracture densities at the top and at the bottom. In the atoll-like Triassic Latemar platform (Dolomites, Italy) fractures in the platform interior are organized in 10s of meters thick corridors cutting the entire stratigraphy and smaller, dm-spaced fractures affecting packages of beds where spacing is influenced by lithology. Fracture sets preserve their orientation entering the platform slope but their spacing and height increase substantially.

 

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