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| Application of the seismic
lithology processing technology for gas hydrate recognition
WU Zhi-qiang, CHEN Jian-wen, GONG
Jian-ming
Qingdao Institute of Marine Geology, Qingdao 266071,China
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The seismic lithology processing technique is a
special technology that can be used to acquire the reservoir geophysical
character using the seismic data. It integrated AVO inversion, impedance
inversion, travel time inversion and physical character picking
etc. When the loose sediment layer is filled with gas hydrate, the
physical character can be improved obviously. The seismic compressional
wave and shear wave can sensitively indicate the gas hydrate bearing
layer which make the seismic lithology processing technology feasible
to recognize gas hydrate.
To recognize the gas hydrate-bearing layer, we must built relations
between the velocity (compressional wave and shear wave) and porosity,
concentration of gas hydrate-bearing layer by studying the physical
character. Using forward modelling method, we study the various
seismic responses to different styles of gas hydrate bearing layers.
Based on the studies mentioned above, we develop the gas hydrate
recognition models, which can indicate the occurrence of gas hydrate.
The analysis results of physical properties gas hydrate bearing
layer and the forward modelling results of various kinds of the
gas hydrate bearing layers indicate that: 1) with the increase of
concentration of gas hydrate, the velocity of the compressional
wave and shear wave increase while Poisson’s ratio will decrease.
2) BSR’s amplitude is related closely with the thickness, porosity
and concentration of the gas hydrate-bearing layer and free gas
bearing layer, and changes with the variety of the layer parameters.
We have built the gas hydrate recognition models using seismic data
based on the simulation results.
At present, direct evidences of the existing of the gas hydrate
has not been found in the East China Sea, but there are many geology,
geophysical and geochemistry indications showing that gas hydrate
may be distributed in the East China Sea. For the purpose of gas
hydrate recognition, we processed the seismic data with “3-H (high
resolution, high signal/noise ratio and high true) ” method, AVO
inversion, trace integration, main parameters processing and impedance
inversion etc. With the BSR’s recognition models, we predicted two
kinds of gas hydrate perspective targets.
The BSR characters in the perspective areas are as fellows: In
the “3-H” seismic section, a very strong amplitude lies between
300 m and 1000 m under the seafloor on the seismic profile, and
is parallel to the seafloor, oblique to bedding planes, with the
presence of high amplitude strong reflectors, reverse polarity and
sometimes amplitude blanking above them.
The amplitude-versus-offset (AVO) analyses that determine the
change in reflection amplitude as a function of receiver offset
or reflection angle，the change in amplitude and waveform for the
hydrate BSR as a function of offset distance. At far offsets the
amplitude increases rapidly and there is a phase shift and the hydrate
BSR has high Poisson’s ratio.
In the velocity inversion section, the hydrate-bearing layer has
high seismic velocity (from 1850 m/s to 2200 m/s) and the layer
above hydrate stabilized zone has smaller velocity. While the free
gas blow the HSZ (hydrate stabilized zone) has the smallest velocity
(about 1600 m/s).
In the trace integrated section, the hydrate BSR shows strong
wave impedance, while free gas bearing layer with weak wave impedance.
In the main amplitude section, the hydrate BSR is of very strong
amplitude.
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