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Seismic
Evidence of Vertical Fluid Migration Through Faults, Applications of Chimney and
Fault Detection.
By
Roar Heggland
Statoil ASA, N-4035 Norway
Summary
Columnar
disturbances in seismic
data
are frequently interpreted as free gas in the
sediments, either as small accumulations captured in the shales, or as upward
migrating free gas. These features, usually termed
seismic
chimneys, or gas
chimneys, have been observed to tie in to features associated with gas seepage,
like pockmarks and carbonate build-ups, and to shallow gas accumulations and
faults. Recent studies have revealed that chimneys can represent a link to
deeper hydrocarbon accumulations.
To improve the consistency in the mapping of chimneys, a method for semi-automated detection of chimneys was developed. This method has also been adapted for fault detection.
Gas seepage features and chimneys have been observed at different stratigraphic levels, which indicates that gas seepage is not a continuous process, but takes place during limited periods on the geological time scale.
Due
to capillary resistance, vertical migration of hydrocarbons in shales has to
happen through faults or fractures. Examples from 3D seismic
data
show that most
chimneys are located at faults or fractures.
Figure Captions
Figure
1. Standard exploration 3D
seismic
data
before and after chimney detection.
Figure
2. Visualization of 3D
seismic
data
, Gulf of Mexico. Mapped horizons (brown,
green, blue), high amplitude clusters (red), detected salt diapir (light blue)
and detected chimneys (yellow).
Figure
3. Visualization of 3D
seismic
data
, Gulf of Mexico. Chimneys (yellow) are
located at faults visible as low amplitude features (dark) on the amplitude map
in display.
Introduction
Since
the late 80’s, exploration 3D seismic
data
have proved to be very useful for
shallow gas and geohazards evaluations for E&P drilling sites (Heggland et
al., 1996).
As part of this work, indicators of fluid flow, like gas chimneys, pockmarks, possible carbonate build-ups, as well as mud volcanoes and diapirs are mapped. Studies of these shallow features have also been focused on whether they can give information about deeper hydrocarbon accumulations.
Method
The
mapping of gas chimneys from seismic
data
can be time consuming and difficult,
because of their diffuse character and often weak appearance. To improve the
identification and mapping of gas chimneys, a method was developed to detect gas
chimneys in
seismic
data
(Meldahl et al., 1998, 1999, Heggland et al., 1999).
The method with examples and
interpretation
is also described in Heggland et
al., 2000, and Meldahl et al., 2001.
The
method makes use of multi trace and multi attribute calculations and a neural
network (de Groot, P., 1999). The neural network is trained at example locations
on different attributes to recognize a chimney. In addition, the neural network
is trained at example locations not representing a chimney. The network is
finally applied on the total 3D volume to make a classification of the data
into
“chimney” and “non chimney”. The output is a 3D probability cube, giving
high values for chimneys and low values in the surrounding volume, see the
example in Figure 1. In a similar way, the method has been applied to fault
detection (Meldahl et al., 2001, Tingdahl et al., 2001).
To highlight features associated with gas seepage, various attribute maps have been applied, like “edge detection” and rms amplitude. The “edge detection” maps highlight the geometry of the horizons, where features like pockmarks, carbonate build-ups, mud volcanoes and diapirs can be visualized. Azimuth and dip maps are good alternatives for the study of the shape of a surface. The rms amplitude maps can highlight hydrocarbon accumulations and sand deposits (high amplitudes), as well as faults (low amplitudes).
Results
The
semi-automated detection of seismic
chimneys has been applied to several 3D
seismic
data
sets from the Norwegian shelf and the Gulf of Mexico (Heggland et
al., 2000), see Figure 2.
Seismic
chimneys frequently tie in with pockmarks, carbonate mounds and mud volcanoes,
as well as with amplitude anomalies indicating shallow gas accumulations. As
such, the mapping of chimneys has significance in geohazards
interpretation
.
Features associated with gas seepage, and chimneys, appear at different stratigraphic levels, indicating that vertical fluid migration occurs within limited periods of time (Heggland, 1997, 1998).
Possible
hydrocarbon migration systems have been visualized by the seismic
detection
method in areas of proven oil and gas fields. The chimneys indicate fluid and/or
gas migration from a source rock into a reservoir, and between a reservoir and
the seabed. In the same manner,
interpretation
of detected chimneys are used in
the ranking of prospects.
The semi-automated detection of chimneys has made it possible to make consistent comparisons between chimneys in areas with discoveries and in areas with dry wells. Preliminary results have showed a difference in the density of chimneys. Discovery wells and oil and gas fields are located in areas with a high density of chimneys, whereas dry wells are located in areas with a low density of chimneys, or no chimneys at all.
Vertical
migration of hydrocarbons in shales has to happen through faults or fractures,
due to capillary resistance in the shales. Detection of chimneys in 3D seismic
data
shows that, in most cases, chimneys are located at faults or fractures (see
Figure 3).
In near surface sediments, and where no faults or fractures are visible, chimneys are still present and they seem to occupy a much larger space than at deeper levels, where the chimneys are located at faults. Above a certain level, where the sediments are less consolidated, the capillary resistance may be small enough to allow for vertical hydrocarbon migration. Another explanation could be that by an upward movement of gas saturated water, gas can be released when the pressure drops. For other papers on application of chimney detection, see Aminzadeh et al., 2001.
Conclusions
Results
of detection of chimneys in 3D seismic
data
, show that chimneys tie in to
features associated with gas seepage, shallow gas accumulations and deeper
hydrocarbon reservoirs.
The appearance of seepage related features and chimneys at different stratigraphic levels, indicate that vertical fluid migration takes place within limited periods of time.
High concentrations of chimneys are observed in areas where discovery wells and oil and gas fields are present. Low concentrations of chimneys are observed in areas with dry wells.
The appearance of chimneys at locations of faults and fractures, strongly indicates that vertical fluid migration through shales takes place through faults and fractures.
Acknowledgements
Statoil
ASA is acknowledged for the use of their data
and the permission to publish this
paper.
De Groot-Bril Earth Sciences BV (dGB) is acknowledged for the performance of the chimney and fault detection.
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