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The AAPG/Datapages Combined Publications Database
Houston Geological Society Bulletin
Abstract
Abstract: Predicting
Structural
Trap Geometries
in Overthrust Belts
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By
ARCO Exploration and
Production Technology, Plano, TX
Trap-forming
structures in
overthrust belts are
commonly characterized
by structural
complexity and poor
seismic data quality.
Therefore, seismic
data is usually insufficient
to constrain
the location and size
of potential
structural
traps. Geologic and
geophysical data, when combined with
structural
modeling and balanced
cross
section techniques can improve
structural
interpretations and thereby reduce trap risk
for prospective structures.
Predictive models for interpreting structural
styles must incorporate key geometric
and kinematic characteristics observed in
well-constrained surface and subsurface
structures. Mechanical contrasts of key
lithotectonic units play an important role in determining the
type and geometry of fold-fault
relationships, such as fault-bend,
fault-propagation, and detachment folds.
Lithotectonic packages with strong competency
contrasts, such as thick carbonate
units encased within thin shale units, result
in fault-bend folds and duplexes. Examples
of this
structural
style are found in the
Sawtooth Range in Montana, the Canadian
Foothills and the Southern Appalachians.
Moderate to low competency contrasts,
such as interlayered, thin-bedded sandstones
characterized by high flexural slip
efficiency, result in fault-propagation folds.
Examples of these styles are found in the
Polish and Romanian Carpathians. Overthrust
belts characterized by relatively
competent packages overlying weak shales
or salt typically result in disharmonic,
detachment folds with related accommodation
faults. Examples of this style are found
in the Dinaride, Zagros, and Jura fold belts.
Delineation of deep anticlinal traps typically involves the extrapolation of surface
and near-surface geometries to deeper
horizons. Trap risks are related to changes
in the geometry, location, and size of structures
with depth. The relationship between
the geometries of deep targets and near-surface
horizons are dependent on the
structural
model,
structural
disconformities
or changes in
structural
style. Therefore, a
good understanding of the mechanical
stratigraphy and its influence on
structural
geometry is critical in delineating poorly-imaged
traps.
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