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PSImproving Predictions on Mechanic Stratigraphy of Buried Sedimentary Successions: Lessons and Workflow from Outcrop Analogs*
Giovanni Bertotti1, Herman Boro1, Nico J. Hardebol1, and Stefan M. Luthi2
Search and Discovery Article #40460 (2009)
Posted November 10, 2009
*Adapted from poster presentation at AAPG Convention, Denver, Colorado, June 7-10, 2009
1Tectonics/Structural Geology, VU University Amsterdam, Amsterdam, Netherlands ([email protected])
2Geotechnology, TU Delft, Delft, Netherlands
Contrary to what is 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 1) the quantitative description of fracture patterns in vertical outcrops spread over a reservoir-scale region, 2) the definition of mechanical units, and 3) 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 turbiditic, 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.
DigiFract and the case studies we have presented have been developed during the last 5 years in the frame of a productive collaboration between the VU University in Amsterdam and the Delft University of Technologies.
At different stages and in different manners our work was generously supported by StatoilHydro, ExxonMobil, and Total.
This work would not have been possible without the contribution of Bachelor, Master and PhD students J. van Koppen, T. Beek, J. van de Vaart, M. van der Most, K. Ewonde, G. Strijker, and J. Klaver. Expenses of their fieldwork have been partly covered by VU University.
Bertotti, G., N. Hardebol, J. Taal-van Koppen, and S.M. Luthi, 2007, Toward a quantitative definition of mechanical units: New techniques and results from an outcropping deep-water turbidite succession (Tanqua-Karoo Basin, South Africa): AAPG Bulletin, v. 91/8, p. 1085-1098.
Emmerich, A., V. Zamparelli, T. Berchtsaedt, and R. Zuehlke, 2005, The reefal margin and slope of a Middle Triassic carbonate platform: the Latemar (Dolomites, Italy): Facies, v. 50, p. 573-614.
Hodgson, D.M., S.S. Flint, D. Hodgetts, N.J. Drinkwater, E.P. Johannessen, and S.M. Luthi, 2006, Stratigraphic evolution of fine-grained submarine fan systems, Tanqua depocenter, Karoo Basin, South Africa: Journal Sedimentary Research, v. 76, p. 20-40.
Salvini, F. and F. Storti, 2004, Active-hinge-folding-related deformation and its role in hydrocarbon exploration and development – Insights from HCA modeling, in K.R. McClay, Thrust Tectonics and Hydrocarbon Systems: AAPG Memoir 82, p. 453-472.
Taal-van Koppen, J.K.J., 2008, A multi-scale case study of natural fracture systems in outcrops and boreholes with applications to reservoir modelling: Ph.D. thesis, Delft University of Technology, 157 p.