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The AAPG/Datapages Combined Publications Database
Houston Geological Society Bulletin
Abstract
Abstract: Subsalt Type Archetype Classification: A Diagnostic Tool for
Predicting and Prioritizing Gulf of Mexico Subsalt Traps
By
BP, Houston, Texas
Many Gulf of Mexico subsalt traps remain poorly imaged on
even the best depth-migrated seismic datasets, necessitating
the use of geologic models to help guide prospect evaluations.
We introduce a subsalt trap
classification scheme to address a
long-standing industry need for a
comprehensive and practical
method of characterizing subsalt
traps according to their structural
merits. Designed for exploration
applications, the classification helps
interpreters recognize and, in the
case of ambiguous seismic data, infer
the presence of key
trap
attributes
that improve or diminish subsalt
prospectivity. This
trap
assessment
tool is based on the following tenets:
- The full spectrum of Gulf of Mexico subsalt structural styles
can be effectively described by a finite number of
trap
archetypes, each connoting a particular set of
trap
risk factors.
- Ribbon truncation closures and upwardly flexed subsalt stratal
crests increase
trap
risk, whereas downwardly flexed and inverted stratal crests generally improve
trap
viability.
- Within Gulf of Mexico multi-tiered salt systems, subsalt
trap
geometries manifest the kinematic linkage (or lack of linkage) between targeted strata and underlying autochthonous and allochthonous salt bodies.
- Although deep salt tectonic sequences can be complex and varied, their net effect on subsalt stratal geometry can be described by four constituent deformation modes: rotation, counter-rotation, upward flexures, and downward flexures.
- Subsalt stratal deformation modes are, in turn, impacted by the
underlying salt root type; autochthonous roots, fore-ramping
allochthonous roots, and back-ramping allochthonous roots
each impose a distinctive suite of stratal motions and flexural
styles on overlying subsalt strata.
These three root types define genetic
subsalt
trap
families that exhibit characteristic ranges in geometry and prospectivity.
- Traps formed against sutured salt
base highs are a fourth
trap
family, often remaining kinematically unlinked to deep salt roots and thus preserving their pre-suture stratal truncation patterns.
The four trap
families are qualitatively ranked for overall
trap
risk and play value, a ranking that is affirmed by Gulf of Mexico
subsalt drilling results. Contractional, extensional, and passive
subsalt anticlines occur almost exclusively above deep
autochthonous salt, and traps of the top-ranked autochthon
rooted play family have yielded the largest subsalt discoveries to
date. Although subsalt traps underlain by back-ramping
allochthonous salt roots lack anticlinal closures, they often
exhibit inverted, flat-crested sigmoid folds and may present the
best play opportunities updip of the autochthon rooted subsalt
trends. The family of subsalt traps underlain by fore-ramping
allochthonous roots is relegated to a third-place ranking because
of the generic risk of upwardly flexed
trap
crests, although
specific variations (e.g., piggyback sills with subsalt inversions)
may remain highly prospective. Lastly, sub-suture traps often
retain their pre-suture stratal synclines, forming bi-lateral ribbon
truncation closures. These high-risk traps remain problematic
for the industry.
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