GEOPHYSICAL WELL-LOG ANALYSIS IN
CHARACTERIZING THE HYDROLOGY OF
CRYSTALLINE ROCKS OF THE CANADIAN SHIELD
by
F .L. Paillet
Borehole Geophysics Research Project
Water Resources Division
U. S. Geological Survey
Denver, Colorado 80225
ABSTRACT
A full suite of geophysical logs, including nuclear, electric, acoustic transit-time, acoustic waveform, and acoustic televiewer logs, and high-resolution flowmeter measurements have been used to investigate the lithologic and hydrologic properties of three
igneous plutons located on the southern margin of the Canadian shield. Geophysical
logs were used to identify lithologic boundaries, determine the properties of unfractured granitic or gabbroic rocks, interpret and calibrate the results of surface geophysical surveys, and characterize permeable fracture zones that could serve as conduits
for fluid migration. Nuclear and acoustic transit-time logs provided good quantitative correlation with changes in lithology. Electric logs yielded consistent qualitative
correlations, with lower resistivities associated with more mafic lithologies. Lithologic
contacts indentified on logs generally confirmed the results of surface electromagnetic,
seismic, and gravity surveys. All major fracture zones intersected by boreholes were
clearly indicated by the geophysical logs. Electric, epithermal-neutron, and acoustic
transit-time logs gave the most consistent indications of fracturing, but the lithologic
responses associated with some thin mafic intrusions were difficult to distinguish from
possible fractures, and some steeply-dipping fractures were not indicated by conventional acoustic transit-time logs. Electric and neutron log response is attributed to
the effect of clay minearl alteration products in the vicinity of fractures. This alteration may be indirectly related to permebaility, but no direct relationship between
resistivity or neutron attenuation and permeability appears to exist. Tube-wave attenuation determined from acoustic waveform logs was related to the transmissivity
of equivalent infinite, plane fractures; these results agree qualitatively, and possibly
quantitatively with packer isolation and injection tests if the combined effects of differing scales of investigation and borehole enlargements in fracture zones are taken into
account. Tube-wave attenuation in waveform logs also compares well with the permeability distributions determined from tube-wave generation in vertical seismic profiles.
142
Paillet
Comparison of conventional geophysical logs, acoustic televiewer images of the borehole wall, and fracture frequency distributions measured on core samples indicates
that many fractures are completely sealed and have no effect on log response, whereas
many more apparently sealed fractures have been slightly opened during drilling, and
do provide some log response. High resolution flowmeter meaurements of natural flow
in boreholes and comparison of packer isolation tests with log data indicate that a
relatively few individual fractures often provide a large proportion of fracture zone
transmissivity in the immediate vicinity of the borehole, and that the orientation of
these fractures may not coincide with fracture zone orientation. These results indicate
that the scale problem in relating borehole logs to regional configuration of fracture
flow systems may be the most important consideration in the appplication of geophysical well logging to the characterization of ground water flow in crystalline rock
bodies.
Well-log Analysis of the Canadian Shield
143
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a:
w
IW
::;:
6
,"/
:i
I-
fu 1000
o
15ooo!-------!2~--L----"~4::.L-"-'-----'---:':6--:.......---;8~----I:..L-:1;';:0?"---'----:-1
~2----""-:'14
HORIZONTAL DISTANCE, IN KILOM ETERS
EXPLANATION
III
UNCONSOLIDATED SURFICIAL
DEPOSITS OR GLACIAL DRIFT
~
UNDIFFERENTIATED SCHIST
U
GRANITE
I~I
XENOLITH IN GRANITE
1:'::;1 ALTERED GRANITE
@%1 PEGMATITIC INTRUSION IN GRANITE
Figure 1: Idealized cross-section of fractured granitic intrusion in the Canadian shield,
illustrating major fracture zones, depth of weathering, contact with country rock,
and xenoliths; individual and isolated fractures not shown.
Paillet
144
SUB HORIZONTAL
FRACTURE
ZONE {
r::;;j OPEN
t::I FRACTURE
q
E:J
m
I:.::.J
n
t::.::J
":~~~~4~~~~r
bl
~
S
NUCLEAR '1---1SOURCE
INFILLING
MINERAL
ALTERED
GRANITE
CLOSED
FRACTURE
2
D
I
I
3
I
METERS
Figure 2: Illustration of typical fracture zone showing alteration of granite, infilling
minerals, fracture interconnections, damage to fractures at the borehole wall, and
contribution of fracture zone to geophysical sampling volume.
Well-log Analysis of the Canadian Shield
EPITHERMAL
NEUTRON. IN
COUNTS PER
SINGLE-POINT
o
r-~Ri'E;;S!!;IS:,T:,A:,N:,;C,:,:E:;:::l
INCREASING
RESISTANCE
SECOND
3000 5000
ACOUSTIC·
TRANSIT TIME,
IN MICROSECONDS
PER METER
o 400
145
CORE FRACTURES PEA METER
a
TOTAL
50
OPEN
100
0
30
F
~
\i{~
200t-
EXPANDED
RESISTANCE
LOG SHOWING_
INDIVIDUAL
FRACTURE
ANOMALIES
.
-
w 400t-
~a:
::>
<J)
o
z
:'i
~
-
m600
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I-
<J)
w
I;j
::E
-
t--
;;:;
~.
w
a 800
.
.
1000
-
-----...J
1200 LL
1-,
~~
,,
Figure 3: Comparison of geophysical logs to incidence of open and sealed fracture in
orientated core for borehole with nearly complete core recovery.
146
PaiIlet
130 r
_S_I_N_G_L_E_P_O_IN_T_R_E_S_IS_T_A_N_C_Er-J
r
INCREASING ___
RESISTANCE
140
w 150
TUBE WAVE
AMPLITUDE,
IN PERCENT
N_E_U_T_R_O_N_LO_G
_ _----:;=--, 0 50 100
UNCORRECTED
RESISTANCE LOG
"
o
<>:
LL
a:
:0
en
0160
Z
L_-
:5
5:
9
w 170
CD
en
a:
w
fW
:;; 180
~
I
f-
a.
w
SHIFTED AND
FILTERED
RESISTANCE
LOG
0190
200
210 L-
-L.-----l
L
---'c......J L-,----I.----.J
Figure 4: Differential single point and epithermal neutron (uncompensated) logs compared to fracture incidence indicated on televiewer log.
TELEVIEWER
Well-log Analysis of the Canadian Shield
TELEVIEWER LOG
100
147
CORE FRACTURE DATA
EXPLANATION OF CORE DESCRIPTION
105
- - PERMEABLE OR POSSIBLY PERMEABLE
FRACTURE
..-...- CLOSED FRACTURE, OPENED DURING
DRILLING
.•....
UJ
"
"'"
u.
•
0:
::>
'"
'"-'
;;:
110
CLOSED FRACTURE
TELEVIEWER FRACTURE
CORRESPONDING TO ONE OR MORE
PERMEABLE CORE FRACTURES
D
Z
0
-'
UJ
III
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0:
UJ
IUJ
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~
115
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lll.
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~
D
'"Ie I
..-'
·"-V
~
120
POSSIBLE
•
HYDAAULlC--"""
FRACTURE
125
'-'
_t>
Figure 5: Comparison of acoustic televiewer log with orientated core fracture data
(plotted in televiewer log format).
Paillet
148
TUBE WAVE
AMPLITUDE
FROM ACOUSTIC
FULLWA VEFORM
DATA
ACOUSTIC
TELEVIEWER
DATA
ACOUSTIC WAVEFORMS
TUBE WAVE
AMPLITUDE DEFICIT
150r-------.
3:
o
...l
w
mw
(flo
a:«
A
w"-
f-a:
W::>
:::;(fl
-'
B
2°
_2
.«
1
I
I...l
f-
I
1-·<----->.-1
D..
W
°
B
ATTENUA TION
160~---'---...J...J
a
PERCENT
25
50
PERCENT
100
a
200 400 600 800 1000
TIME, IN MICROSECONDS
100r-----~---__,
f2
w
OW
::>0
f-a:
_w
...lD..
D..
:::;~
« .
w2
>Q
B' 55 PERCENT
- - EQUALS 0.210
MILLIMETERS
«f-
3:;3
w2
mw
::>ff-f-
12 PERCENT EQUALS
0.087 MILLIMETERS
« aa'.£---;:-'".-----:;"-;-----:~-"'""--='
0.2
0.4
0.6
0.8
1.0
EQUIVALENT SINGLE FRACTURE
APERA TURE, IN MILLIMETERS
Figure 6: Estimating fracture permeability using tube-wave attenuation.
149
Well-log Analysis of the Canadian Shield
CORE FRACTURES PER METER
EQUIVALENT SINGLE FRACTURE
APERA TURE, IN MILLIMETERS
o
o. ,
0.2
0.3
0.4
o
TOTAL
50
OPEN
100
0
30
200
400
w
()
'"u.
a:
=>
UJ
0
Z
'"-"
;: 600
a
-"
w
'"a:
UJ
w
IW
::;
~
rI-
c. 800
w
0
1000
1200
b==~~;;;;2.J'---_---l_ _J
Figure 7: Estimated hydraulic aperture of equivalent single infinite fracture from tubewave attenuation compared to frequency of open and closed fractures in core (waveforms obtained with 34 kHz source).
150
Paillet
ACOUSTIC TUBE WAVE ATTENUATION
INDIVIDUAL
FRACTURE
RESPONSE
INTER VAL SUM OF
TRANSMISSIVITIES
PACKER TESTS
EQUIVALENT SINGLE FRACTURE APERATURE. IN MILLIMETERS
0.2 0.4 0.6
0
0.2 0.4 0.6 0.8 0 0.02 0.04 0.06
o
NO DATA
TUBE WAVES GENERATED
IN VERTICAL SEISMIC
PROFILES
VSP TUBE WAVE
AMPLITUDE. IN PERCENT
OF MAXIMUM VALUE
0
25
50
75
100
,
,
,
NO DATA
w
~ 100
-
U-
0:
:::J
rfJ
o
Z
<t:
...J
3:
o
...J
w
m
rfJ
0:
W
tu 200
-
I-
::;:
~
I
fa..
w
LOWER
FRACTURE
ZONE
o
~"""""""""''''''''''''''''''
300L----'---l...-...J
I
I
Figure 8: Comparison of fracture permeability given in equivalent single fracture aperture with packer tests and VSP tube wave data (waveform logs obtained with
34 kHz source).
I
Well-log Analysis of the Canadian Shield
SOUTH
NORTH
200
--'
UJ
>
UJ
--'
«
UJ
(f)
UJ
>
0
!!l
«
(f)
fI
w 100
fW
:2
z
Z
0
i=
«
100 METERS
>
w
--'
w
0
151
_
UNCONSOLIDATED SURFICIAL
DRIFT
k,t,]
MAJOR FRACTURE ZONE
~
HIGH PERMEABILITY CORE
OF FRACTURE ZONE
Figure 9: Geologic cross-section showing variability of fracture zones.
152
Paillet
180
100 MICROSECONDS
PER METER
175 L..-.
Figure 10: Acoustic logs indicating the extent of alteration and fractures along three
adjacent boreholes penetrating the same fracture zone; there are approximately
50 m between boreholes in the plane of Figure 9.
--.J
153
Well-log Analysis of the Canadian Shield
APPROXIMATE LOCATION OF
CROSS SECTION IN FIGURE 13
/'
INTERPRETATION OF TOP OF LOWER
FRACTURE ZONE IN METERS
ELEVATION ABOVE SEA LEVEL
_90"- SEISMIC
!
N
200 METERS
I .
80.
LOG
I
Figure 11: Comparison of the top of the lowermost fracture zone in Figure 9 as interpretted form surface seismic reflections and geophysical logs.
154
Paillet
TELEV!EWER LOG INTERPRETATION
720
r"
.!
)SREAKOUTS
"
730
ACOUSTIC TELEVIEWER LOG
DARK STRIP WITH NO DATA CAUSED BY
DECENTRALIZATION OF ACOUSTIC TELEVIEWER
IN SMALL-DIAMETER BOREHOLE
I
w
U
""u.
"
"'o
z
""
::J
740
~
"
o
BREAKOUTS
~
W
<D
""'....w
w
"
~
~
0
750
0W
o
760
BREAKOUTS
"
770
Figure 12: Televiewer log indicating stress concentration and reorientation wsithin
fracture zone because borehole wall breakouts are limited to a single deep fracture
zone; fracture distribution along the complete borehole is indicated in Figure 7.