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Abstract
AAPG Bulletin, V. 84, No. 3 (March 2000), P. 314-344.
A Natural Analog for a Fractured and Faulted Reservoir in
Dolomite: Triassic Sella Group, Northern Italy1Marco Antonellini2 and Pauline
N. Mollema3
©Copyright 2000. The American Association of Petroleum Geologists. All rights
reserved.
1Manuscript received February 13, 1998; revised manuscript received June 28,
1999; final acceptance September 3, 1999.
2Terra Porosa, Independent Consultants, 11839 Cedar Pass Drive, Houston, Texas
77077; e-mail: [email protected]
3Baker Atlas, Geoscience, 10205 Westheimer, Houston, Texas 77042; e-mail: [email protected]
This work was supported by the extramural research agreement WC/57416 with Shell
International Exploration and Production, B. V. Research and Technical Services.
Cartography used in the field was provided by the technical office of the city of Canazei
(Italy) and aerial photographs by the Istituto Geografico Militare Italiano. We would like
to thank Robert Humphreys for a review of an earlier version of this manuscript and Neil
Hurley, Stephen Laubach, Allan Smith, Denise Gerard, and an anonymous reviewer for
constructive comments. We thank also Robin Leinster for assistance in the field and in the
lab. ABSTRACT
We have used outcrops of dolomite exposed in the Triassic Sella Group of the Italian
Central Dolomites as an analog for subsurface low-porosity faulted and fractured dolomite
reservoirs. The Sella Group was mildly deformed at shallow burial depth (21,000 m) in a
tectonic strike-slip regime during the Eocene-Miocene Alpine compression that caused the
formation of joints and strike-slip faults. Because the matrix porosity in the dolomites
is low (<5%) and poorly connected, joints and faults are essential to connect vugs and
to provide permeability. Field observations of the Sella Group explain why many dolomite
reservoirs and aquifers in strike-slip/compressive tectonic regimes are intensely jointed
when they are mildly deformed. In this type of tectonic regime, in fact, pervasive
jointing over a wide area accommodates small strains and is strictly associated with the
formation of strike-slip faults. Our observations allow us to recognize different kinds of
fault architectures that correspond to different stages of fault development. In addition,
theoretical models and microscopic observations were used to estimate the petrophysical
properties of the faulted and jointed dolomite. Small-offset faults (offsets up to 30 mm),
characterized by en echelon arrays of joints and pockets or seams of breccia up to 10 mm
wide, form areas of high permeability (100-3000 md) due to the presence of many joints and
high-porosity breccia. Faults with 1-10 m offsets, characterized by a breccia zone (1-2 m
in width) and associated with high joint density in the wall-rock, contain high-porosity
(10%) breccia and represent areas of preferred fluid flow. Large-offset faults with
offsets more than 10 m contain a wide zone of low-porosity (<1%) breccia and form
potential permeability barriers. The areas adjacent to the intermediate- and large-offset
faults have high permeability (100-3000 md) because of high joint densities. An important
implication of the way faults develop in dolomite is the consistent relationship between
the orientation of joints and faults: the fault's strikes differ 15-35° from the strikes
of the pervasive joint systems. Joint density also increases four to five times in the
proximity of the faults. Such relationships can be used to predict the distribution and
orientation of joints and faults in subsurface dolomite reservoirs.
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