GEOPHYSICS,
VOL.
50. NO.
I (JANUARY
1985);
P. 3743,
15 FIGS.,
1 TABLE
Evaluation of direct hydrocarbon indicators through comparison of
compressional- and shear-wave seismic data: a case study of the
Myrnam gas field, Alberta
Ross Alan Ensley*
INTRODUCTION
ABSTRACT
A recent paper (Ensley, 1984) documented a new method of
evaluating “bright spots” or other direct hydrocarbon indicators (DHIs). The technique involves the qualitative comparison
of compressional (P) wave and shear (S) wavei seismic data. In
practice, such a comparison offers a viable means of evaluating
DHIs previously observed on P-wave data. Ensley (1984) describes the theory behind this technique, and demonstrates its
feasibility with model data and a case history. This paper
presents an interpretation of P- and SH-wave seismic data from
the Myrnam held, Alberta as a second case history.
Shear waves differ from compressional waves in that
their velocity is not significantly affected by changes in
the fluid content of a rock. Because of this relationship,
a gas-related compressional-wave “bright spot” or
direct hydrocarbon indicator will have no comparable
shear-wave anomaly. In contrast, a lithology-related
compressional-wave anomaly will have a corresponding
shear-wave anomaly. Thus, it is possible to use shearwave seismic data to evaluate compressional-wave
direct hydrocarbon indicators. This case study presents
data from Myrnam, Alberta which exhibit the relationship between compressional- and shear-wave seismic
data over a gas reservoir and a low-velocity coal.
‘S-wave is here understood to include both horizontally polarized
shear waves (SH-waves) and vertically polarized shear waves (SVwaves). All of the S-wave seismic data discussed within this report are
SH-wave data and will be referred to as such when appropriate.
S WAVE
P WAVE
+
IMPEDANCE
CONTRAST
+
SV WAVE
IMPULSE
IMPULSE
+
+++++++++++
FIG. 1. Direction of energy propagation and particle motion for compressional (P) waves, horizontal shear (SH) waves, and vertical
shear (SF) waves.
Manuscript received by the Editor June 4, 1984; revised manuscript received August 1, 1984.
*Exxon Production Research Company, P. 0. Box 2189, Houston, TX 77001.
0 1985 Society of Exploration Geophysicists. All rights reserved.
37
Ensley
38
I
8ooo
SHALE
VS = 5200
SHALE
1
0
VP = 10,400
;‘,:
24f:;$i
FTiSEC
FTiSEC
7500 FT’SEC
2000’
A.
AMPLITUDE
ANOMALY
PHASE
CHANGE
FIG. 2. Seismic models showing the differences between the P- and S-wave response to a gas-filled reservoir. The models are 2-D
ray-tracing models and the velocities used are representative of moderately compacted sediments. (a) Depth model of a gas
reservoir; (b) P-wave synthetic section showing a DHI characterized by a high-amplitude reflection along the top of the reservoir
and a phase change at the edge of the reservoir; (c) S-wave synthetic section showing no anomalous response to the gas reservoir.
+
R
t
IOF
I
:
+
FIG. 3. Location man of Mvrnam field.
Shear waves differ from compressional waves in both the
direction of particle motion relative to the direction of wave
propagation (Figure I) and in the rock properties which control
the wave velocity. A P-wave is an elastic wave in which the
particle motion is parallel to the direction of wave propagation.
In contrast, an S-wave is an elastic wave in which the particle
motion is perpendicular to the direction of wave propagation.
Because of this difference between P- and S-waves, the velocities of the two are functions of different rock properties.
Consideration of the elastic properties which control the
velocity of P- and S-waves in a rock indicates that P-waves are
sensitive to the type of pore fluid present within a rock while
S-waves are only slightly affected by changes in fluid type.
Thus, if the presence of gas within a reservoir rock gives rise to
an anomalous seismic expression on P-wave data, a DHI, there
should be no comparable expression on S-wave data. This
relationship is illustrated by the hypothetical model shown in
Figure 2. A P-wave anomaly generated by a lithological feature, however, should have a corresponding S-wave anomaly.
One consequence of this relationship is that it is possible to
evaluate the potential of P-wave DHIs through a comparison
of P- and S-wave seismic data recorded over a prospect. This
concept is more thoroughly discussedby Ensley (1984).
P
The application of SH-wave seismic data for evaluation OI
Direct
Hydrocarbon
39
indicators
SILTSTONE
GAS SAND
COAL
Table 1. Map and litbology symbols.
MAP SYMBOLS
0
DRY WELL
0
GAS WELL
.
\
OIL WELL
-----
GEOLOGICAL
CROSS SECTION
SEISMIC
LINE
WITH
COUNTY
LlNF
2201
---
RESERVOIR
LIMITS
,DASHEDWHERE
ESTIMATEDI
”
SHOTPOINTS
FAVLT
iDASHED
WHERE
ESTIMATED1
\
During 1979, Hudson’s Bay Oil and Gas Company, acting as
contractor for itself and several other companies, recorded
seismic data at the Myrnam field in Canada to test the appli-
cation of SH-wave data for gas detection. The Myrnam field is
located 90 miles east of Edmonton, Alberta. P- and SH-wave
seismic data were recorded along two lines (Figure 3). Sections
from line T56-4.5 will be used for examples in this report, but
the second line shows the same relationship between P- and
SH-wave data. Table 1 is a key to the symbols used in this
paper.
The Myrnam field produces gas from the Cretaceous Colony
formation. The reservoir contains a maximum net pay of 22 ft
within the study area and the gas/water contact is at a depth of
1 759 ft below ground level. The exact geometry and trapping
FIG. 4. Well logs from the zone of interest in the Duvernay
11-26 well.
FIG. 5. Well logs from the zone of interest in the Duvernay
5-28 well.
DHIs was previously documented with a case study of P- and
SH-wave data from the Putah Sink field of central California
(Ensley, 1983, 1984). Robertson (1983) reviewed the interpretation of data from the Putah Sink field and one other site in
central California.
CASE STUDY
40
En&y
Direct
Hydrocarbon
Indicators
41
Dhct
Hydrocarbon Indkators
B
6
8
44
Ensley
.o
E
.3
8
d
Direct Hydrocarbon Indicators
45
46
Ensley
Dlrecl Hydrocarbon Indlcntor8
47
FIG. 12. Detailed plot of the synthetic seismogram from the
Duvernay 1l-26 well. The pulse used is a zero-phase wavelet
with a peak frequency of 30 Hz.
FIG. 15. Detailed plot of the synthetic seismogram from the
Duvernay 5-28 well. The pulse used is a zero-phase wavelet
with a peak frequency of 30 Hz.
mechanisms of the reservoir are unknown because of limited
well control. In addition to the gas reservoir, the two lines also
cross known coal deposits within the Colony formation. The
coal beds range in thickness from 12 to 20 ft and are at
approximately the same stratigraphic level as the gas reservoir.
Well logs from the Duvernay 11-26 well show that the gas
sand has a slower velocity than the underlying water sand and
the overlying shale (Figure 4). This results in a low impedance
for the gas sand relative to the surrounding rocks. Logs from
the Duvernay 5-28 well show that the coal has an impedance
similar to the gas sand and is also surrounded by rocks of
higher impedance (Figure 5).
The quality of the seismic data recorded at the Myrnam field
is good and both the P- and SH-wave sections show comparable reflection continuity (Figures 6 and 7). This enabled the
two sections to be correlated on the basis of reflection character
(Figures 8 and 9).
In the primary zone of interest, over the gas reservoir, the
P-wave data exhibit a DHI but there is no comparable SHwave expression (Figures 10 and 11). The P-wave DHI is
characterized by an amplitude anomaly and an abrupt change
in amplitude laterally. The SH-wave data, though, show only
low-amplitude continuous reflections. The dark trace on Figure
10 represents a synthetic seismogram from the Duvernay 1l-26
well. A detailed plot of the synthetic seismogram confirms that
the P-wave DHI is caused by the gas (Figure 12).
In the second zone of interest, over the coal deposit, both the
P- and SH-wave data exhibit similar anomalies (Figures 13 and
14). The P-wave DHI is characterized by an amplitude anomaly and an abrupt change in amplitude laterally, much like the
DHI over the gas reservoir. The SH-wave data exhibit a similar
expression. A detailed plot of the synthetic seismogram from
the Duvernay 5-28 well demonstrates the P-wave DHI is
caused by the coal bed and is a false DHI (Figure 15).
CONCLUSIONS
This case history demonstrates that although a gas-related
DHI and a false, lithology-related DHI may have similar appearances on P-wave seismic data, it is possible to distinguish
between them through a comparison of P- and SH-wave data.
This conclusion, in conjunction with similar results previously
published (Ensley, 1983, 1984; Robertson, 1983) strongly suggests that the comparison of P- and SH-wave seismic data is a
viable method for evaluating P-wave DHIs.
ACKNOWLEDGMENTS
I would like to thank Exxon Production Research Company
for the opportunity to publish this paper. I would also like to
acknowledge Don Hartman, Paul Tarantolo, and Gordon
Weisser who were instrumental in organizing the shear-wave
project at Exxon.
REFERENCES
Ensley, R. A.. 1983. Direct hydrocarbon detection with P- and SHwave seismic data (abs.): 53rd Ann. Inter. SEG Mtg., Abs. with
Biographies, 349351.
~
1984. Comparison of P- and S-wave seismic data: A new
method for detecting gas reservoirs: Geophysics, v. 49, p. 142&1431.
Robertson, J. D., 1983, Bright spot validation using comparative
P-wave and S-wave seismic sections (abs.): 53rd Ann. Inter. SEG
Mtg., Abs. with Biographies, 355-356.