Interest in natural gas hydrates
has grown in recent years with the recognition that gas-hydrate bearing
sediments are wide spread in certain marine and permafrost settings and that the
volume of natural gas trapped in hydrate form is enormous. Reflection seismic
data are extensively used to map and interpret the gas hydrate bearing zones and
to compute the volume of gas hydrate and free gas. However the structure and
the seismic characteristics of permafrost in the near surface poses a strong
problem in processing the seismic data to obtain high resolution images of the
deeper gas-hydrate zone. A seismic data processing and interpretation study is
proposed to process the 3D seismic survey recorded in 2002 on Richards Island,
Mackenzie River Delta of Canadian Arctic. Accurate refraction statics from a
previous study of seismic tomography modeled velocity structure of the
permafrost will be applied to correct for the uneven permafrost zone. The
processed data will be interpreted to identify gas hydrate zones and to
estimate the free gas beneath the hydrate layer.
Seismic Tomography
A 3D seismic survey (Mallik
3D), covering 126 km2 (Figure 1) was conducted in 2002 to map the
shallower (1000 m) gas-hydrate zone and deeper hydrocarbon reservoirs. The data
was recorded on Richards Island, Mackenzie River Delta of Canadian Arctic, over
a lake-covered, marine-inundated, permafrost terrain. The acquisition was
optimized for deeper conventional hydrocarbons and thus has limited shallow
spatial resolution, fold, and frequency (60 Hz maximum). Shallow data gaps reach
700 ms while irregular permafrost ice creates complex static solutions, degraded
velocity control, and energy transmission losses. Four wells (Mallik 5L-38 and
three industry exploration wells Imperial Mallik J- 37, P-59 and A-06; Figure
1) drilled at the survey location indicate a thickness of approximately 500 m
for the permafrost zone.
Lakes cover approximately 25% of
the surface area of Richards Island, Northwest Territories. Many of the lakes
are deeper than the thickness of winter ice and have taliks that penetrate
permafrost. The warmer temperatures beneath deep lakes and water channels affect
ground conditions and in particular, they modify physical properties of
sediments that are relevant to propagation of seismic waves. Sediments with ice
in the pore space are stiffer and characterized by higher seismic velocities
whereas unfrozen sediments have lower velocities.
Typical P-wave velocities of unfrozen and unconsolidated sediments are near 1650
m/s whereas fully frozen ice-bonded sediments have a P-wave velocity higher than
2400 m/s. These velocities are only approximate as many other factors such as
composition, density, porosity, water saturation and pressure affect P-wave
velocity of sediments. The presence of unfrozen water at the bottom of the lake
and unfrozen soil underneath would result in a significantly reduced seismic
velocity in the sediments compared to the high velocities in ice-bonded
permafrost. The unfrozen
areas also attenuate more severely seismic waves propagating through them.
Areas with lower velocity delay
seismic waves propagating down to and up from deeper reflective geological
structures. These delays, because they occur at shallow depths, must be taken
into account during data processing to produce the most accurate images of
deeper geological structures and quantitative information about the velocity
distribution of the permafrost is required to estimate proper static
corrections. Due to this, stacked data of the Mallik 3D indicate data
contamination which might potentially confuse gas-hydrate interpretation. These
include: 1) energy reverberations generated from impedance at air/ground,
top/base ice-bonded permafrost, and 2) amplitude and frequency degraded zones
(wash-outs) caused by lake-controlled near-surface conditions creating signal
attenuation.
The upper 2 seconds of the 3D
seismic volume that constraints the gas-hydrate zone is currently available for
developing new methods to process reflection seismic data to reduce the effects
of permafrost in the shallow subsurface. A recent 3-D travel-time seismic
tomography study resulted in robust seismic velocities (Figure 2). This map
reveals a heterogeneous distribution of velocities mostly related to thermal
variations within the permafrost and correlates well with surface lake locations
(Figure 2).
Figure 1. Location map of Mallik
3D-Seismic data shown by the green outline. Mallik 5L-38 and three
industry exploration wells, Imperial Mallik J- 37, P-59 and A-06, are shown on
the map. A horizontal slice of tomographic velocity for the region bounded by
the green line is shown in Figure 2.
Figure 2. A 3D
travel-time tomography algorithm was used to produce a map of the permafrost
velocity structure The 3D velocity map clearly reveals an heterogeneous
distribution of velocities at a depth of 150 meters, mostly related to thermal
variations within the permafrost.
Figure
3. Isometric plot of the permafrost
velocity structure along with surface expression of lakes.
