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Gas chimneys – indicating fractured cap rocks
By
Helge Løseth, Lars Wensaas, and Børge Arntsen
Statoil, N-7005 Norway
Seismic
interpreters often observe anomalies that cannot be explained by primary
depositional processes or processing artifacts. Gas chimneys are among such
anomalies. Several years of research has led to a refined understanding of
seismic
anomalies related to hydrocarbon leakage processes. The aim of this
paper is to present well and
seismic
data related to gas chimneys and to argue
that a fractured cap rock model can explain the observations. The term “gas
chimney” is an interpretive term claiming that gas is causing the noise
observed on
seismic
data. The link between gas chimneys and noise zones is
clearly shown on un-scaled
seismic
data (Figure 1). Here, the primary
reflections are very weak or absent. When the
seismic
signals are gained (Figure
2), the zone with weak primary reflections appears as a noise zone.
Wells drilled inside noise zones have:
• higher pore fluid pressure,
• higher mud gas readings,
• higher mud gas wetness,
• more hydrocarbon shows,
• lower velocities, and
• higher temperatures
than wells drilled outside (Figure 3). Similar observations have been made in several well pairs inside and outside noise zones. Based on these observations we believe that gas is one important cause of the noise and that it is appropriate to interpret such noise zones as gas chimneys.
Gas chimneys are observed in low-permeable cap rock shales. We believe that fractures must exist in the low-permeable cap rocks in order to explain the observed oil shows within the gas chimneys. Such fractures can also explain the other observations related to gas chimneys.
Tectonic induced processes can create swarms of fractures; e.g., above salt or clay diapirs. Several gas chimneys exist in tectonic fractured cap rocks above salt structures in the Central Graben. In the northern North Sea, gas chimneys are located in cap rocks that show no or few signs of tectonic activity. These gas chimneys are located above high-pressure Jurassic fault blocks. High-pressure fluids in the Jurassic sandstone reservoirs are interpreted to have hydrofractured the cap rock. Well observations from gas chimneys inside tectonic fractured cap rocks above salt domes in the Central Graben are similar to those above high-pressure fields in the northern North Sea (Figure 3).
In
order to improve our understanding of gas chimneys we tried to mimic the
observed seismic
noise by
seismic
modelling. Several scenarios were modelled.
The best match was obtained when gas was distributed as wavelength-sized
patches. The low-velocity gas patches must be irregularly distributed in the
low-permeable shales. We suggest that fractures, both hydro-fractures and
tectonic fractures, distribute the gas into irregular patches within the cap
rock.
Based
on the observations, interpretations, and modeling
, we propose that gas chimneys
indicate zone of fractured cap rock. If this model is correct, it has several
implications to the oil industry. For example, gas chimneys should be
interpreted as hydrocarbon migration pathways, and the extent and location of
the migration route can be mapped directly from
seismic
data.
Figure Captions
Figure
1. Un-scaled
seismic
section illustrating that the primary
seismic
signals are
very weak or absent above the Gullfaks South Field.
Figure
2. Scaled
seismic
section illustrating the noisy zone above the Gullfaks South
Field. The noise zone is interpreted as a gas chimney because it is believed
that gas in sediments is causing the noise. Mud gas parameters from wells (in
projected positions) inside and outside the noise zone is illustrated in Figure
3.