American Oil and Gas Reporter - April 2016 - 42

SpecialReport: GOM Field Development
existing 3-D seismic data near the well
location and the well geometry. The expectation of relatively flat structural dip
near the (straight) well and the subsequent
modeling results led to selecting a rigsource/zero-offset (ZVSP) receiver geometry.
A three-component, three-level vertical
seismic imaging tool was used to acquire
the data in both open and cased hole,
with 50-foot receiver intervals using rigid
interconnects for safer operation in the
open-hole section. The seismic source
was a six-gun, 1,500-cubic inch array

operating at 3,000 psi and deployed from
the starboard rig crane to a depth of 13
feet below sea level. Data acquisition of
151 VSP and check shot levels was completed in 12 hours from the start to the
end of logging.
Data processing followed a standard
ZVSP processing sequence, beginning
with shot editing, stacking, time picking,
wave field separation, deconvolution, and
finally corridor stack display. The final
VSP processing results were consistent
with the prejob modeling and provided a
good tie with surface seismic data, which

FIGURE 3A
Borehole Image and Dips Showing
Chaotic Nature of Mass-Transport Deposit Beds

indicated a generally flat structural dip.
However, when the borehole imaging
data were interpreted, highly dipping
events were identified, and the question
was raised as to why the VSP was indicating flat horizons. Conversely, if the
internal stratigraphic dip had been significantly higher, what would the VSP
data have looked like?
To answer that question, a 2-D model
with higher structural dip was created,
the ray trace modeling was run again,
and the synthetic travel times (in color)
were overlain with the actual upgoing
VSP wave field data in two-way time, as
illustrated in Figure 2. The synthetic VSP
traces from the high-dip model do not
conform to the actual VSP results recorded
in the well.
This demonstrates that the VSP was
responding to the regional structural dip,
which is closer in resolution to that seen
in the surface seismic data. The highly
dipping events recorded in the borehole
images are beyond the ability of surface
seismic or even higher-resolution VSP
data to resolve.
Microresistivity Imaging
Borehole microresistivity images acquired in the second Marmalard well
made it possible to conduct an advanced
geologic interpretation in the wellbore.
Unlike what was shown in surface 3-D
or borehole seismic data, the true dips
observed in the borehole microresistivity
images were not low dips and were not
consistent. The bottom of the sequence
starts with high structural dips that flatten
when sand enters the system. Immediately
after the sand deposition, mass-transport
deposits are seen. Invariably in the well,
low dips are seen with the onset of sand
deposition and chaotic dips of the masstransport deposit again after that.
Figure 3A shows a section of the borehole microresistivity images, showing the
chaotic nature of the beds observed in
the mass transport deposit and the organized nature of dips in the sands. From
left to right, the tracks are the gamma
ray curve, depth track, static image, true
dips, dynamic image, and dip azimuth.
The chaotic dips are observed in the
lower part of the figure, and are interpreted
to represent slump deposits within a masstransport deposit. With the onset of sand
deposition overlying the mass transport
deposit, the chaotic higher dips are reduced
in magnitude within the sand. After the
sand has been deposited, chaotic dips are

42 THE AMERICAN OIL & GAS REPORTER
American Oil and Gas Reporter - April 2016

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