PETROPHYSICAL CONSTRAINTS ON DEFORMATION STYLES
IN AZTEC SANDSTONE
Eric Flodin1, Manika Prasad2 and Atilla Aydin1
Rock Fracture Project, GES Dept., Stanford University
2
SRB Project, Geophysics Dept., Stanford University
email: [email protected]
1
Abstract
Introduction
Knowledge about the pore and confining pressure dependencies of the compressional wave velocity (VP) and
quality factor (QP) in reservoir rocks is an important requisite for relating these laboratory measurements to other
physical properties of rocks and for interpreting seismic measurements in terms of subsurface petrophysical
parameters.
Presence of cracks and their morphology and texture are known to significantly alter seismic velocities
in rocks. Various theoretical models exist to predict seis-
River
Mud
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Las Vegas, NV
167
Virgin
NV
to I-15
Mou
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to St. George,
UT
Overton
North
169
2 miles
Muddy
Mountains
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Me
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(O
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Valley of Fire
State Park
La
The Jurassic Aztec sandstone of southern Nevada, a
multi-colored eolian quartz arenite with large scale
cross-strata, is characterized as having two distinct
zones of deformation style, hereafter distinguished the
Lower and Upper units. With regard to small displacement deformation features, the Lower unit has deformed
predominantly by opening mode fractures, whereas the
Upper unit has deformed predominantly by deformation band microfaulting.
In an attempt to distinguish these two units beyond
their differing deformation styles, we present rock physics data obtained from 35 samples of Upper and Lower
unit Aztec sandstone. Porosity measurements were made
with a helium porosimeter at ambient conditions. Permeability measurements were made using a gas probe
permeameter. P- and S-wave velocities were measured
by pulse transmission at increasing pressure steps up to
a maximum pressure of 60 MPa. Attenuation was calculated with the spectral ratio method using aluminum
as reference. By monitoring length changes we estimated
porosity changes with each pressure step.
The Upper and Lower units show two distinct trends
with regard to velocity-pressure relationships: The
Lower unit shows a rapid increase in velocity with increasing pressure in contrast to the Upper unit’s slight
velocity increase with increasing pressure. Both units
level off after ca. 20 MPa, although the terminal velocities of the Lower unit samples are generally higher than
that of the Upper unit samples. For example, at 60 MPa
all Lower unit samples have Vp values at or greater than
4 km/s, whereas Upper unit samples have Vp values
generally less than 3.8 km/s.
Figure 1. Geographic location of the study area.
mic velocities in rocks with varying densities and types
of cracks (e.g. Hudson 1981). Stress dependence of elastic moduli is also known to depend on crack types and
densities. Experimental studies on attenuation and velocity have shown the effect of pressure on crack closure and layering. For AVO analyses, Adriansyah and
McMechan (1998) have shown that intrinsic and scattering attenuation can greatly affect AVO.
Main results can be summarized as follows.
! Lower P- and S- wave velocities
! Higher attenuation and dispersion
! Larger pressure dependence of velocity and
attenuation are observed in the presence of
cracks in a rock (Lo et al., 1986; Prasad and
Manghnani, 1996; Teng, 1998).
We use the above observations along with measured
petrophysical data (ultrasonic velocity, porosity, permeability, etc.) to detect variations in host rock properties
that coincide with variations in deformation styles. The
rock, the Aztec sandstone of southern Nevada (see location map, Figure 1), is a multi-colored eolian quartz
arenite that is characterized as having two distinct domains of deformation style:
• Lower Domain: Dominated by
opening mode fractures
• Upper Domain: Dominated by
deformation band faults
Stanford Rock Fracture Project Vol. 11, 2000
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Al
Au
Al
Al
K
Au
Am
WIllowTank
Thrust
Au
Al
39
36°29'
T
K
34
Au
American Cordilleran miogeocline to the west. In the
Sevier-age Summit-Willow Tank thrust sheet, the Aztec
sandstone defines the upper plate and is found overlying synorogenic conglomerates and sandstones of the
Baseline Formation. Two generations of compressional
shortening are assigned to this period: first generation
east-directed vergence and second generation east-northeast-directed vergence (Bohannon, 1983; Carpenter and
Carpenter, 1994). The next major deformational event
occurred in Miocene time with the onset of Basin and
Range extension; during this period, the most prominent style of deformation in the Valley of Fire was strikeslip faulting with a lesser amount of oblique-normal slip
(Myers, 1999).
Cover
Cretaceous
Upper Aztec
Middle Aztec
Lower Aztec
Triassic
Syncline axis
1 km
Fault
N
Am
Au
Au
Am
K
34
Al
Water Pocket Fault
Am
36
33
Am
28
Am
Mesa
Baseline
Al
ult
Tank Fa
Mouse’s
36°26'
Fault
Al
T
114°32'30"
T
114°30'
Figure 2. Geologic setting of the Aztec
sandstone. The upper and lower Domains are
also characterized by different colors: Upper
Domain has orange and beige units, whereas
Lower domain consists of red unit.
In this paper, we first introduce the geologic setting
of the study area and provide a brief review of the deformation mechanisms at work. We then discuss how
these structures are heterogeneously distributed in the
Aztec sandstone of the Valley of Fire State Park, southern Nevada. Our experimental procedure follows. We
conclude with the presentation of our results that show
a distinct petrophysical differences between the two
sample suites.
Deformation Mechanisms
This paper focuses on contrasting the relative abundance
of two distinct deformation mechanisms known to operate in porous sandstone. The first is deformation band
faulting (Figure 3a). This mechanism results from shear
strain localization along narrow tabular zones (1 mm –
1 cm in width; up to 100 m in length); deformation as-
(a)
a
a’
Geological Setting
The Jurassic Aztec sandstone is an eolian sandstone time
correlated to the Navajo sandstone of the Colorado Plateau. Compositionally, the Aztec is a feldspathic quartz
arenite composed mostly of rounded to well-rounded
quartz sand. Grain size ranges from 100 to 1000 µm.
The most dominant sedimentary structures are tabularplanar and wedge-planar cross-strata (Marzolf, 1983).
The Aztec sandstone in the Valley of Fire region
has been subjected to at least two major deformational
events following its stable, sub-aerial deposition in Jurassic time (Figure 2). According to Bohannon (1983),
during the Cretaceous and early-Tertiary(?) Sevier orogeny, the Aztec sandstone was over-ridden by the Muddy
Mountain thrust sheet; a thrust sheet consisting primarily of Paleozoic carbonate rocks derived from the North
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Stanford Rock Fracture Project Vol. 11, 2000
(b)
Figure 3. a. Schematic drawing of deformation
band fault. b. Schematic drawing of a Mode I
fracture (adapted from Pollard and Aydin,
1988)
Lower
Domain
Upper
Domain
100
% deformation bands
transitional
Deformation bands as percent
of total structures present
transitional
80
60
40
upper
Aztec
lower
Aztec
20
-1500
-1000
-500
0
500
1000
Distance (m) relative to Lower/Middle contact
Figure 4. Left: Contact between Lower (dark) and Upper (light) domains in Aztec sandstone.
Right: Scan-line data collected in the vicinity of photograph to the left. Plot shows relative
abundance of deformation bands with respect to all structures present at a given locality.
71
33
60
58
35
74
61a
73
72
57
Upper
Domain
56
54/55
70
66
69
68
67
23
65
Lower
Domain
64
63
20
36
Figure 5. Sample localities and structural domains in Aztec sandstone. Numbers indicate
sample localities. White dotted line demarcates the approximate domain boundaries. Image
is from a high-altitude aerial photograph. Scalebar in lower-left of image is 2 km.
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3
4
2.5
Vs (km/s)
Vp (km/s)
5
3
2
0
concretion lower
transitional upper
20
40
60
confining pressure (MPa)
2
1.5
0
20
40
60
confining pressure (MPa)
Figure 6. Velocity - pressure relationships for all samples.
concretion
lower
transitional
upper
4.5
4
3.5
3
0
3
Vs (km/s) at 40 MPa
Vp (km/s) at 40 MPa
5
2.8
2.6
2.4
2.2
2
10
20
porosity (%)
0
30
10
20
porosity (%)
30
2.7
concretion
transitional
lower
upper
30
porosity (%)
bulk density (g/cm3)
Figure 7. Velocity- porosity relationships for all samples.
2.3
1.9
20
10
1.5
0
Figure 8. Contrasting bulk density and porosity.
bulk modulus (GPa)
Poisson’s Ratio
0.3
0.2
0.1
30
25
20
15
10
5
0
Figure 9. Contrasting Poisson’s ratio and bulk modulus at 40 MPa confining pressure.
Vp - up
Vs - up
4
3
2
1
0
5
Vp - down
Vs - down
velocity (km/s)
velocity (km/s)
5
4
3
2
20
40
60
confining pressure (MPa)
1
0
20
40
60
confining pressure (MPa)
Figure 10. Pressure dependence of velocity in two
typical Upper and Lower Domain samples.
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Stanford Rock Fracture Project Vol. 11, 2000
sociated with this mechanism includes pore volume loss and cataclasis (Aydin, 1978; Jamison and
Stearns, 1982). These features have been shown
to reduce host rock permeability by up to three
orders of magnitude, providing a substantial barrier to fluid flow in what otherwise may be thought
of as a high permeability sandstone (Antonellini
and Aydin, 1994).
The second deformation mechanism is the
opening joint (or mode I crack) (Figure 3b). This
deformation feature begins in response to a localized tensile stress about a microscopic imperfection (e.g. pore space or grain boundary). Further
growth continues as long as the localized stress at
the joint tip exceeds the tensile strength of the rock
(for thorough review, see Pollard and Aydin, 1988).
Distribution of Deformation Features
Taylor (1997) made short scan-lines to record
the relative abundance of tectonic structures with
respect to the various alteration units found in the
Aztec sandstone. Results of his study (Figure 4)
show that deformation bands are more abundant
in the Upper Domain Aztec Sandstone. A transitional zone, coincident with the contact between
the red and white alteration unit, shows no apparent dominance with regard to deformation features. Taylor (1997) further attributed the varying
deformation styles to mechanical variations induced by chemical alteration.
Experimental Procedure
Samples of Aztec sandstone were collected
at 27 localities within the Valley of Fire State Park.
They were chosen from Lower domain (red), Upper Domain (beige), and the Transition Zone between the two (see Figure 5 for sample location).
Additionally, two well cemented concretion
samples were collected for analysis. The concretions, which are considered to be a sub-set of the
lower Domain, are very densely fractured. Table
1 summarizes the sample suite with respect to
domain and deformation style.
Sample Preparation
Cylindrical core samples with 25 mm diameter and 20-30 mm length were prepared with their
faces parallel to within 100 µm. Bulk and grain
densities and porosity were measured at ambient
conditions, after drying overnight at 50°C, using
a Helium porosimeter. The microstructure of the
samples was examined under an optical microscope. Permeability was measured using a steadystate
gas
probe
permeameter
(or
minipermeameter).
Ultrasonic Experimental Setup
The pulse transmission technique (Birch,
1960) was used for P- and S-wave velocity (VP,
VS, respectively) measurements. The experimental setup consists of a digital Tektronix (Model
TDS 420A) Oscilloscope and a Velonex (Model
345) pulse generator. The sample was jacketed
with rubber tubing to isolate it from the confining
pressure medium. PZT-crystals mounted on steel
endplates were used to generate P- and S-waves.
The principal frequency was about 1 MHz for P
and 700 MHz for S-waves. A high viscosity bonding medium (Panamterics SWC) was used to bond
the endplates to the sample. A pore fluid inlet in
each endplate allowed passage of pore fluids
through the sample. In this report, however, only
results of VP and VS from room dry measurements
will be discussed.
The experimental configuration allowed simultaneous measurements of P- and S-waves at
various pressures up to 60 MPa. The pressure limits were defined by an estimated maximum depth
of burial (4 kilometers; Bohannon, 1983) of the
samples. Length change in the sample as a function of pressure was monitored using three linear
potentiometers. Porosity changes were estimated
from these changes in length at each pressure step.
Table 1. List of samples collected from Aztec sandstone.
#
1
2
3
3a
Domain
Upper
Transition
Lower
Concretion
Deformation Type
Predominantly deformation band faulting
No apparent dominant deformation mechanism
Predominantly open mode fractures
Densely fractured
# samples
12
6
9
2
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Traveltime was measured after digitizing each
trace with 1024 points at a time sweep of 5 µs,
thus allowing a time resolution of about 5 ns or
about 0.2% error in velocity. Actual error in velocity measurement is estimated to be around 1%
due to operator error in picking first arrival. The
system delay time was measured by taking headto-head time at 2 MPa. The travel-time calibration was confirmed by measuring an aluminum
cylinder at different pressures.
Results
Figure 6 shows the effect of pressure on Pand S- wave velocity for all samples. All samples
show a general increase in velocity with pressure.
However, the Upper domain samples show markedly lower velocities compared to that of the
Lower domain. In Figure 7, velocity at 40 MPa is
plotted as a function of porosity. At similar porosity, the two domains can be separated by lower
velocity in the Upper Domain.
The main results from this petrophysical study
are summarized in Figures 8 and 9. A summary of
sample bulk density and porosity is respectively
shown in Figure 8a and b; the sample bulk modulus and Poison’s Ratio are shown respectively in
Figure 9. Results from this study reveal mechanical contrast between the two structural domains
in the Aztec sandstone (Table 2).
Pressure dependence of velocity is plotted in
Figure 10 (left and right) for typical Lower and
Upper domain samples, respectively. In the Lower
Domain, initial zero pressure velocities are lower
than for Upper domain. However, their rate of
change of velocity with pressure is larger so that
their terminal velocity is higher. The different behavior of velocity gives an indication of the type
of cracks with pressure in a sample. A steep increase in Vp at low pressures is indicative of the
closing of microcracks as large aspect ratio pores
(Lo et al., 1986; Wepfer and Christensen, 1990;
Prasad et al, 1994).
Acknowledgments
All laboratory data was collected in the Stanford
Rock Physics laboratory. Jeff Chapin and Nick
Davatzes are thanked for assisting in the field. This
research was supported by a grant from the US
Department of Energy – Basic Energy Sciences
(to A. Aydin and D.D. Pollard).
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Stanford Rock Fracture Project Vol. 11, 2000
Table 2. Summary of petrophysical data
Property
Porosity
Bulk density
Bulk modulus
Shear modulus
Permeability
Comparison between domains
Lower Domain < Upper Domain
Lower Domain > Upper Domain
Lower Domain > Upper Domain
Lower Domain > Upper Domain
Lower Domain < Upper Domain
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