Figure Captions
Figure 1. Gas chimney (in yellow) overlaid on reservoir structure.
Figure 2. An example of a fault cube with the horizontal and vertical
slices tied to an original seismic section.
Figure 3. Creation of a meta- attribute .
Figure 4. Distinguishing gas-charged versus non-charged fault segments.
Figure 5. Comparison of a horizontal slice of a chimney cube (A) and
fault cube (B).
Figure 6. Active chimneys (in yellow) along faults that form the
trapping mechanism of the gas reservoir.
Chimney cubes (Figure 1) and fault cubes (Figure 2) are used to map
areas where the seismic detects anomalous patterns of amplitude and
similarity in combination with other attributes like dip variance and
curvature. They help determine where hydrocarbons originated, how they
migrated into a prospect and where they leaked, creating shallow gas
(and sometimes mud volcanoes, or pockmarks) at the sea floor.
Current applications of chimney and fault cubes include:
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Unraveling a basin’s
migration history.
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Distinguishing between
charged and non-charged prospects.
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Distinguishing between
sealing versus non-sealing faults.
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Determining vertical
migration of gas.
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Identifying potential for
overpressure.
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Detecting shallow hazards.
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Predicting hydrocarbon
phase and charge efficiency, especially in multiphase petroleum
systems.
Computers can be trained to search through data volumes looking for
seismic objects, using carefully designed criteria “meta-attributes,”
which are an aggregation of a number of seismic attributes where the
interpreter’s insight is combined with the power of a trained neural
network to detect a particular seismic anomaly.
As shown in Figure 3, a multitude of attributes from known or suspected
chimneys (or faults) are used as input to a neural network. Training of
the neural network using interpreter’s insight renders the
“meta- attribute ” suitable for detection of a given seismic body, like
gas chimneys or fault patterns.
Figure 1 shows a typical gas chimney in yellow overlaid on a deep salt
structure with deep and shallow reservoir units. It highlights the
migration pathway of hydrocarbon from deep structures into shallower
reservoirs and into near surface gas pockets.
Gas clouds and gas chimneys have often been considered as a source of
seismic noise that degrades the quality of seismic reflection events.
Much effort has been devoted to filter out the impact of gas clouds and
provide interpretable sections by imaging through them. Our main focus,
however, is to highlight such events and establish a link between
chimney characteristics (occurrence, type and extent) and geologic
concepts critical for successful exploration. For example, mapping the
location and origination/termination points of gas chimneys helps the:
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Understanding of deep
petroleum migration processes.
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Distinguishing between
charged and non-charged fault segments.
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Detecting sealing versus
leaking faults.
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Distinguishing oil-prone
versus gas-prone prospects.
Sometimes it is difficult to pinpoint deep migration pathways on a
conventional seismic line – but chimney cubes can highlight subtle
features like vertical gas migration in the geopressured sections of the
Gulf of Mexico. This helps substantiate predictions of geochemists and
geologists that vertical migration is an important process in charging
Tertiary reservoirs in the Gulf of Mexico and in other similar basins
around the world. Chimney and fault volumes improve the understanding of
the petroleum system and identify the role faults play in the migration
of hydrocarbons into the reservoir.
In Figure 4 we have overlaid the chimney halo (in orange) on top of the
seismic section. Note that the two structures on opposite sides of the
fault have similar seismic response but very different charge
probability. The structure on the right side of the fault has no chimney
halo associated with it, and thus is less likely to be charged. In
general, structures with some associated strain possess preferential
charging potential. Of course, we have to keep in mind that excessive
strain would be a major leak risk, so chimney analysis should be used in
conjunction with other tools which predict stress/strain regimes.
Combining fault and gas chimney data can be a powerful tool in detecting
hydrocarbon migration pathways. Figure 5 shows their use in determining
sealing versus leaking faults. While all the mapped faults are
highlighted in Figure 5b, the subset of the faults that are likely to be
leaking show up in the chimney volume of Figure 5a. This information can
then be integrated with other regional information to assess probability
for hydrocarbon charge and seal.
Many fields in the Gulf of Mexico and other basins demonstrate that the
fault systems associated with gas chimneys have been major charging
pathways for the reservoirs. Figure 6 shows active chimneys (in yellow),
both large (e.g., one at the intersection of the two lines) and small
(along selected fault blocks) that are considered to be leaking.
Presence of chimney-like behavior along faults can indicate evidence of
vertical hydrocarbon movement.
In multi-phase petroleum systems, where both oil and gas are migrating
into a trap, the structures that vent the gas (either through faulting
or fractures) will be more oil-prone. Processing can detect the weak
signal associated with venting. This approach has been used to
successfully predict hydrocarbon phase in a number of basins in West
Africa, the North Sea, GOM and the Far East.
Based on worldwide case histories from gas prone basins, chimney and
fault cube analysis is a proven tool to make geologic predictions. This
includes:
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Relating surface seeps to
subsurface structures and reservoirs.
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Understanding the
hydrocarbon history model.
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Ranking prospects.
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Detecting reservoir leakage
and spill points.
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Assisting in identifying
potential over-pressured zones and shallow gas drilling hazards.
Assessing
the sea floor stability for platform design and drilling.
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