Closure in stratigraphic
traps is created by stratigraphic, lithologic
or hydrodynamic variations rather than
by folding and faulting. Most stratigraphic
traps are subtle, in that detection is
problematical and lateral closure is difficult
to prove. Stratigraphic trapping mechanisms
may combine with open structural noses
and small anticlinal closures to produce
subtle combination traps, which are also
difficult to detect and evaluate. Most
large stratigraphic/subtle traps were discovered
either by accident or through geologic
reasoning that proved to be incorrect.
Explorationists have made limited efforts
to deliberately search for stratigraphic/subtle
traps because it is relatively easy to
find structural traps with present-day
tools and ideas, and the concept for structural
plays is more acceptable to management.
Historically, more than 80% of the stratigraphic/subtle
traps discovered to date are found in North
America, which surely reflects the fact
that much more drilling has taken place
in North
America than elsewhere. The rest of the
world has been looking for structural
traps and
must eventually begin to uncover the numerous
fields in stratigraphic/subtle traps that
will probably contain a large proportion
of future reserve replacement. These traps,
which represent one of the last domains
of hydrocarbon exploration, will pose
significant
challenges for geologists and geophysicists.
Scope of Project
The Stratigraphic/Subtle Traps
E&P
TREATISE
presents systematic analyses and comparisons
of existing fields with a view toward
gaining a better understanding of the
geologic processes responsible for the
creation of stratigraphic/subtle traps
and the timing of trap formation relative
to hydrocarbon charge at the prospect
scale. This improved geologic understanding,
together with recent advances in seismic
modeling and attribute analysis, better
data acquisition and processing, and
sequence stratigraphic concepts, will
lead to reduced exploration risk.
Most basins contain the basic requirements
for the creation of stratigraphic/subtle
traps. These requirements include
depositional and diagenetic changes,
unconformities
with resulting truncated beds, buried
erosional or constructive surfaces,
and variations
in reservoir fluids. Foreland, intracratonic,
passive margin and rift basins are
the most
common settings for the development
of stratigraphic/subtle traps. Because
of
constant recurrence of
the same depositional and diagenetic
processes and tectonic movements
that produce stratigraphic/subtle
traps, petroleum basins throughout
the world probably contain many more
of these
traps
than have been discovered so far.
Hidden trends and features may occur
in association with unconformities
at undrilled
depths
in productive trends and in relatively
unexplored regions. Although seismic
control and interpretation
can assist in locating such trends
and features, geophysics is not
the full
answer. Successful
identification and delineation
of stratigraphic/subtle traps require
the integration of a
wide variety of exploration techniques
with
knowledge
of analog fields and detailed study
of stratigraphy, paleogeomorphology,
paleogeography,
paleostructure,
paleontology and palynology. Analog
fields are particularly useful
for
the implementation
of new technology by highlighting
and illustrating how a play concept
exhibits
itself within
the available data.
The E&P
TREATISE on
Exploration for Stratigraphic/Subtle
Traps is now available digitally on
CD-ROM and in Web-deliverable format.
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