GEOPHYSI('S, VOL,. 02, NO. 2 (MARC'II-AI'KIL 1097); I! $05-520, 12 FICiS., 4 'I'A14L.ES.
Parameters controlling sonic velocities in a mixed
carbonate-siliciclastics Permian shelf-margin
(upper San Andres formation, Last
Chance Canyon, New Mexico)
Jeroen A. M. Kenter*, F. F. Podladchikov*,
Marc Reinders*, Sjierk J. Van der ~ a a s t f ,
Bruce W. Fouke*, and Mark D. Sonnenfeld**
ABSTRACT
We have illeasured the acoustic properties ar~clmineralogic co~llpositionol48 rock specimcxls from mixed
carbonate-silicicl~icoutcrops of the Permian upper
Sa11 Anclres forillation in Last Chance Canyon, New
Mexico. The goals were: (I) identily and xnoelel the parameters controlling the sonic velocities; (2) assess the
influence of postburial diagenesis on the acoustic velocilies.
The variation in sonic velocity in thc 0 to 25 OO/ porosity
range is ~nrimarilycol~trolledby porosity, ancl secor~cllyby
the ratio of carbonate-siliciclastic material. Linear multivariate fitting resulted in a velocity-parosity-carbollate
content transform that accurately predicts sonic velocity
at cliffese~lteffective stresses. The slope of the velocityporosity transform steepens wit11 increasing carbonate
content, wl~ichmay be explained by the higher velocity of carbonate minerals, Another rcason may be the
property olcarbo~lateminerals to farm more perfect intercrystallinc boundaries that improve the trans~nission
INTRODUCTION
Tlle complex relationship between acoustic properties and
the texture, lllinerczlogiccomposition, ancE facies of sedimentary
rocks strongly influences the geologic interpretation of wireline
logs and seislliic reflection data. To connect seismic data wit11
geology, identifying and quantifying the parameters influencing
the porosity and acoustic velocity of the sedimentary rocks that
generate seismic reflections are essential. Second, to build and
test seismic lnodels of complex stratal reflection patterns, more
properlies oC acouslic waves sznd are less ser~silivcto
chax~gesin effective stress,
The velocity riltia V13/Vsis an excellent tool to discrilni~l~le
be twee~lpscdominantly calcitic litl~ologics(1%tio betweell 1.8 ancl 1.!IS) anrl prcdominacltly clolc~niitic
and quartz-rich lithologics (ratio hctwecn I .65 ancl
1.8). Garclner's cxperimcntal curve ovcrcstin~ales,i111d
Ihc velocity-porosity transforr~~s
by Wyllje and Rnymcr
unclercstimatc, tllc obscrvcd sonic vclocitics, 1>1-obahly
because lhcy do not account for variations in tcxturc,
carbanszte mineralogy, and porc gconzetry. Pctrogrnpllic
obscrvatic~t~s
show that pos~b~lrial
diagcncsis is minor
and docs 1101 secni to ~ignifica~~liy
affect laorasily, 'l'hcrcSorc, tllc O L I ~ C I ' Odata
~
S C can
~ bc rcgardccl i ~ as proxy for
the subsul-hcc analog.
These findings unclcrlir~ethe signilica n tly x~lorccam
plex acoustic bcllavior in rnixcd carbonate-siliciclestic
seclitncrltary rocks than in purc siliciclastics whcrc; rnineralogic co~npusitioncxplains most of thc obscrvcd rc?lationskips betwccn porosity am1 sonic vclocit y,
-
inSc)rr~lationis ncedecl on thc spatial dislribuliun of acouslic
propertics in outcrop atxalogs.
Siliciclastics traditionally I~avchcen lhc pl.i~risrytarget lor
rescarcl~an thc acoustic prcqserties as seditllcntary rocks, clur:
to their inzportancc in petroleum exploration. Extcnsivc work
over the last fewclecades has estal-~lisl~ccl
sornerelativcly simplc
relatiozls belwcen seisinic velocities ancl inzportant rock parametcrs such as porosity, densily, and clay contcnl in carbonatepoor siliciclastics (Wyllie ct al,, J956,l958; Gardncr ct al., 1974;
Raymer et nl., 1980;Klirnentos and McCann, 1990;Tasaya and
Manuscript received by the Editor July 5,1995; revised manuscript received February 26,3996.
"Department of Earth Sciences,Vsije Universiteit, De Boelelaan 1085,1081 I-IV Amsterdam, The Netherlands.
$Netherlands Institute of Marine Research (NIOZ), P,O, Box 59,2790 AB Den Burg,Texcl,Thc NethcrXands,
**Departmentof Geology and Geological Engineering, Colorado School of Mines, Golden, Colorclclo 80401,
@ 1997 Society of Exploration Geophysicists. All rights reserved.
Nur, 1952; Kowallis et al., 1984; Han et al., 1986). (liardncr*~
experimental equation and the empirical 'Ivyllic and Raymcr
solutions have been used widely for the prc;cliction of porosity,
density, and litl~ologyfroill acoustic velocity. However, they ignore important lithologic parameters such as x~zincrnlogiccornposition, textural position of clay iuld secondary minerals in
general, and pore size and shapc distribution. Vernik ancl Nus
(1992) developed a classification system that is basecl on both
the petropiiysical as well as coinpositional and textural parameters of silicicIastic sedimentary rocks. 'Illis model is basecl on
the grain mineralogy, amount of textural position ol' clay, and
load-bearing structure of the rock (clayey nlatrix versus granular framework). Vcrnik (1994) expanclecl and furtller refined
this model ancl proposed new linear compressional velocityporosity transfor~nsfor lour groups oEconso1iclatccl siliciclastic
rocks, ranging from clean arenites to sliales.
In contrast, little inlormation and fewer madels on the
acoustic behavior of carbonates are available in thc literature. Though scveral case studies lxave been publisl~ccl(c.g.,
Rafavich et al., 1984; Wilkelxs ct a]., 1984; Wang et al., 1991;
Anselrnetti and El~erli,1993; Ansclmetti, 1994; Biddle et al.,
1992; King et al., 1992; Kenter and Ivanov, 1995), the relacal
in cartion betwecn sedinlentary and p ~ t ~ o p l ~ y s iproperties
bonate rocks is still poorly clocumentcd (c.g., Bourbik et al.,
1987). Tl~esecarbolxate and mixed carbonate-siliciclaslic data
sets have in common a deviating behavior with regard to the
Wyllie and Raymer solutior~s,and Gardner's equation (see e.g.,
Ansellnetti and Eberli, 1993; Kenter and Ivanov, 19%).
I11 this paper, the cornbilled petsophysical and petl-ographical approaches of Vernik (1994) were applied to a study ol
the acoustic behavior of mixed carbonate-siliciclastics sedimentary rocks of the upper San Andres forillation in the
Guadalupe Mountains in New Mexico. We investigated and
modeled the relation between siliciclastic material (preclominantly fine- to medium sand-sized quartz grains and n~inor
autlzigenic clay), carbonate matter, and porosity at: cffcctivc
stresses up to 30 NPa. One important result is that, in contrast
to pure siliciclastics,the gradient of the porosity-vclocity transl'orn~ssteepens with increasing carbonate content. Sccond, wc
will doculnent that the effect oC postburial diagellctic oltcsations 011 the acoustic behavior is minimal, Tlxcrcforc, the clala
set could be used as an input for seismic nzoclelii~gcxpcrimcnts
and cornpared with the suhsurl'ace analog.
DATA SET
Earlier work by Sarg and Lehrnann (19863, 198617), I<cl.ans
et al. (1993), and Sonncnfelcl a ~ i dCross (1993) su~~~imarizcs
t l ~ cslratigrapl~icsetting, sequence slratigrapl~icconcepts, ancl
I~igherfrequency cyclicity in the Permian. San Anclrcs shelf margins in the Guadalupian Mountains, New Mexico (Figure l).
Recently, Stafleu ancl Sonnenfeld (1994) publisllcd a serics ul'
seismic nlodels that were basecl on detailerl stratigraplxic crosssections in the Last Chance Canyon, ancl clircctly cornpar4cd
with a multifold seismic line locatcd approxin~atcly50 km
nortl~westof Last Chance Canyon. We I~uvecollcctccl48 samples from two San Andres scrluenccs lhat arc exposed in this
canyon: sequences uSA3 and uSA4 (Figure 2). Both sccluenccs
represent a complete cycle, fall and risc, of sca lcvcl, and
have an estimated duration of appraxin~atcly0.5 to 1.0 nil lion
years (Sonnenfeld and Cross, 1993). Thc sarnplcs 11avc varying
FIG.1. Regional paleogeographic setting during the Late Guadalupian (Permian) after King (1948). Hatched
areas represent basins and dotted areas represent mountains.
Velocity in Mixed Carbonate-siliciclast'rcs
507
depositional environnlents ranging from restricted platlorm lo
low angle ramp, and rnineralogic composition, ranging Sro~n
pure limestone to dolomite and llearly pure quartz sandstones.
(P,-P,))wcre: 2-0, 5-2, 8-2, 12-3, 20-5, and 40-10 MPa. No
information i s present in the literature to whitt cffcctive stress,
or maximum burial belore removal ol overlying sediment, the
formation has been subjected but present subsurfi~ceanalogs
METEIODS
have an estinlaied 15 ta 30 MPa effective stress (750 to 2500 rn
burial), assuming llydropressured conditions.
Sonic velocities, density, and porosity
Following the rneasurenlent ol the acoustic velocitics, salUpon arrival in the laboratory, two 1,s-inch,diameter cylinuratccl sample nlnss and sample volume were measured asdrical samples were drilled from each sample using a watersuming the samples are pcrlectly cylindrical, Subscquerttly, tltc
cooled diamond-coring drill. Sample ends were ground flat
samples were dried for 72 hours at 7OUC,the dry mass of the
and parallel to within 0.001 inch. The samples were stored
sa~npleswas mcasurcd, and wet bulk and dry bulk density (p,,
under vacuum for a period of 72 hours and saturated with
and p i ) were calculated Rom the saturated and dry mass, and
(P -wave) and
de-aired demi-water. Ultrasonic co~lzpressioi~al
volumc, respeclivcly. Ron1 cach sample, two perpenclicular
shear wave (S-wave) velocities were llleasured as a function
thin sections, impregnated wit11 epoxy (blue clye), were preoP pressure. A single P-wave and two S-wave velocities were
pared, and subsamples werc takcn for the measurements ol
measured with a transducer arrangement (Verde ~ e o s c i e n c e ~ . grain density and carbonate content. Grain density (p,) was
Vermont, U.S.A.) that propagated the co~npressional(T/Tp)
and
calculated froni the mass ancl volume, measurecl with a helium
two independent and orthogonally polarized shear waves (Vsl
pycnometer, of 111c powder. Total porosity (4) was calculated
and VS2)along the core axis.
Irom thc grain density and dry density ( p , and 4,). Tablc I
T l ~ eexperimental procedure for obtaining the acaustic vesurn~narizesthe petropliysical mcasurcmcnts.
locities involves measuring the one-way traveltime along the
sample axis and dividing into the sanlple length, In the experQlmntitutive rllineral composition
iment, a source and receiver pair of like crystals are selected
X-ray diflractio~i(XRD) analysis of 13 reprcsentativc samthrough an ~iltrasonicsignal selector switch. Tile source crystal
ples was used to identiSy tlic dominuit minerals in the rock
is excited by a fast risetime electrical voltage pulse, producing a
specimens. An X-my spectrometer was uscd to analyze all spccbroadband ultrasonic pulse with frequencies between 300 and
imens and measure csl~centrnlionsin weight percentages CJC
800 k1-I~.The arrival time is picked when the signal exceeds
CaQ, KzO, MgO, A1203,nnd SiQ2 that arc prcsent in ilre dea threshold voltage equal to 3% of the overall peak-to-peak
tected group aS minerals. With tllc information ol' the X-rny
amplitude of the first tlzree half-cycles of the signal, Precision
clifSraclion analyses, the weight percentages of oxides were
o,f the measurement of the velocities in highly porous and rclused for conversion into thc cjuaiititativclnincral cornposiliolz
atively poorly consolidated low-velocity carbonates is within
of the specimens. In i~ddition,carbonate contcnt was measurcd
approximately 5%. The ultrasonic measurements were confor calibration with the X-ray fluorcsccncc (XRF) analysis. The
ducted at five to six effective stresses (PC)that ranged frorn 2
Sollowing is a description of thc analyticti1 methods ancl thc
to 30 MPa. Common values for confining and pore pressure
Scale very approximate
VE= 14:l
-
- - -
sequence boundary
maximum flooding surface
uSA1-5 upper San Andres sequences 1-5
I-m SA1-4 1 ower-middle San Adres sequences 1-4
FIG. 2. Schematic stratigraphic setting of Leonardian and Guadalupian strata along the northwestern shell of the Dclawarc Basin
after Stafleu and Sonnenfeld (1994). The box represents the area of the Last Chance Canyon cross-section where samples wcre
collected from sequences uSA3 and uSA4 of the San Andres I1 seismic-scale sequences defined by Sarg and Lelimann (1086a). Sec
text for discussion.
rationale bellind the conversion into a quantitative lnineralogic
~~tllpositian.
Carbonate content was measurccl using the adapted gas volurr~etr.icb'Scllcibler" mcthod. Wholc rock snrnplcs arc crusl~ed
~lnclapproxilnately 400 mg of the powcter is brought in contact
with hydrochloric acid. ?Xe subsecluent production of C02is a
mcizsure of t l ~ ecarbonate C C ) I I ~ C I Iof~ the sample. Accuracy of
the m e i l ~ o dis approxin~ately1 4 % .
XRD a~ialysiswas casricd out with CoKa, radiation and a
Pllilips PW1050/2S goniomcter, whicl~was equipped wit11 a
monochromator in the diffracted beam and an automatic variable divergence slit. Horizontally (with respect to the true vertical axis of the plugs) oriented discs were cut Srom a selection of
13 representative specimens. One side of the discs, wit11 a thickness of 4 mrn and a diameter of 35 mm, was polished to obtain
a snlootlz and flat surface. This lnelhod izliniz~~izes
both distortion of the mineral orientation as well as the surface rought~ess.
The flat discs allow the detection of the mincral co~~lpositian
as
well as a study ol' the preferred orientation of individut~lrlninerals within the rock texture. The speci~nenswcrc ~ncasured
Table '1. Slililmary of petrophysical classification and measurements.
V~J
vs,,,
Vs,ll
(30MPa)
(9MPa)
(30MPa)
VI>/Vs
VIJ / \/as
(kmls)
(kmls)
(kmls)
(9 MPa)
(30 MPa)
(glcc)
(glcc)
qhl
1.89
2.26
1.8S
1.77
4.00
2.73
0.18
2,42
2.93
1.87
1.83
5.36
2.71
0.06
2.73
2,85
2.66
1.69
2,68
4.48
2.56
2.74
0,15
2.47
2.30
2.17
1.82
1,79
4.11
2.78
0.14
2.53
3.27
1.84
1.83
6.00
3.19
0.08
2.7 1
2,86
2.32
1.89
1,75
4.05
1.93
0.14
1
2,44
2.68
2.98
3,lO
1.76
1.71
5.32
2.79
O,l(l
2
2.6 1
2.91
2.97
1.73
1.72
5.10
2
2.56
2.81
0.12
2.3 1
1.85
1.711
4.12
2.09
1
2.50
2.70
0.13
2.73
4.87
2.59
1.80
1.78
2.60
2.81
0.1 2
2
1.80
1.80
4.92
2.55
2.73
2.82
0.1'1
2
2.63
2.82
3.19
1.90
1.71
5.45
2,8'1
0.08
2
2.64
2.66
4.45
2.55
1.68
1,67
2
2.55
2.79
0.13
2.36
4.03
2.15
1 *75
1,71
2
2.50
2.72
0.13
258
2.65
4.63
1,75
1.77
2.78
0.16
2
2.5 1
5.03
2.85
2.97
1.72
1.70
2
2.54
2.76
0.12
4.82
2.67
2.81
1.72
1.70
2.54
2.84
0,15
2
2.40
4.26
2.35
1.70
1,78
2
2.44
2.83
0.21
5.24
2.85
3.06
1.78
1-71
0.1 1
3
2.68
2.88
2.00
2.18
1.72
1.74
3.78
2,711
0,19
1
2.37
2.20
1 .XO
1.75
2.00
3.84
2
2.49
2.73
0,54
6.09
2.96
3.36
2.03
1.81
2.74
0.09
5
2.76
2.52
2.77
2.44
2.25
6.24
2.79
0.02
3
2,76
4.63
2.54
1.77
1.77
2.6 1
0,23
3
2.43
239
3.35
3.40
1.75
1.73
5.87
3
2,68
2.85
0.08
2,99
5.28
1.81
1.77
2'73
0.09
2.89
3
2.61
4.80
2.66
2,70
1,8O
1,78
2
2.48
2.84
0,2J
2,93
2.99
5.24
1.76
J .75
0.16
3
2.59
2
6.14
2.77
3.23
2.11)
I .OO
5
2*69
2.7 1
0.02
557
2.98
3.00
2.70
0,03
1.87
1 ,XS
5
2.67
5.18
2.63
2.73
1.91
1.90
2.75
0.04
5
2,69
6.34
330
1.95
1.02
3.25
5
2.68
2'70
0.08
0.02
3
2,66
2.68
4.81
2.68
2.69
1,79
1.79
2.83
0.03
6,66
3.55
3.64
13 5
1.83
4
2,77
6.1 l
3.29
3.40
4
2.70
2.83
0.06
1.82
1,80
2.76
2.86
0.04
237
2.99
1.91
4
5.72
1,91
6.32
3.30
4
2,67
2,85
0.1IJ
3,44
I .HI)
1.H4
5
2.6 1
2.71
5,92
3.1 0
3.1 5
1,95
I ,X8
0.05
5.48
2.96
2.75)
0.07
3.01
4
2.65
1.82
1.82
2.76
2
2.5 1
4,lX
2.23
1.77
1.65
0.14
2.53
2
2.45
2.77
0.14
4,17
2.12
2.41
1.79
1.73
2
2.57
2.76
0.11
4.73
2.68
2,62
1.79
1.77
2
256
2.71
0.09
4.87
2.62
2,82
1,77
1,73
3
2,63
2.85
0.13
5,67
3.12
3.32
i $85
1.71
3
2.50
0.16
5.04
2.83
2,71
2.78
'1.82
1.81
2
2.48
2.72
4.24
2.33
0.15
1+73
2.48
1,71
2
2,74
2.83
0.05
5,20
2.89
2.93
1.73
1,77
2.80
0,11
5.68
3
2.64
3.33
3.17
1,679
1,79
Note, PI? class. = petrophysical-petrographical classification; 1= quartz-grain supported wit11 weight% quartz > SOo%; 2 = weight%
quartz: 10% iquartz 50%; 3 = dololnitic limestone with ratio dolomitelcsllcite > 1.5 and quartz% < 10%; 4 = mixcd dolomiticcalcitic limestone with quasee% < 10%; 5 = calciticlimestone with ratio dolomitclcalcite < 0.66 and quartz1%)< 10%; p, = saturated
hulk density; p, = grain density; =total fractional porosity; V p= compressional wave velocity; Vs,,, = mean shear wave vclocity;
Vp/ V s = velocity ratio; = effective stress, P,.
PP
class.
2
3
2
2
3
Ps
P,s
Velocity in Mixed Carbonate-siliciclastics
froin 3 to 46" 20 with a counting time of 4 s10.02" ut 50% relative humidity, The irradiated specirnct~lcx~gthwas 12 inm,
the receiving slit 0.2 nzm, and the antiscatter slit 0.5''. X-ray
diffraction patterns wcre digitally rccorded end corrected for
the Lorentz and polarization Siictor (McEwan et al., 1964) and
for the irradiated specimen volunle.
Major element data werc dctern~ineclan a Philips PW 1404
sequential X-ray spectrometer, ccluipped with an R11 ai~oclc
XR'F tube. Samples were dried ovcmigl~tat 11O0C,illld igniicd
at 1000°Cfor 30 minutes. Beads werc preparccl from the ignitccl
material by fluxing with a mixture o f 2 Li2B407-t 1 LiB02,
in a 1 4 dilution; melting tinze was 8 nzinutcs at 1OOO"C in
a Pt-Au crucible on a. higlz-frequency incluction coil. Matrix
corrections were calculated by the applicatiar~of thearctical
alpha coefficients. Separate sets of alpha coefficicrzts were ~isccl
Tor silicates and carbonates. Tlze system has bccn calihralecl
using well-analyzecl international rel'ere~zccsamples, silicatcs
as well as carbonates.
XRD patterns indicated the presence of the lollowing n~iizerals: dolomite, calcite, quartz, mica, kaalinite (possibly dickitc),
and potassium feldspar (Figure 3). Gypsum was dctcctccl at
very low levels in one sample and therefore ncglcctcd. T11c
relative concentrations of major e l e ~ ~ ~ c(MgO,
n t s AI2oR,CaO,
SO2, K 2 0 ) were convertecl to weight laerccntages of the minerals that were detectecl by XRD analysis. Thc conversio~lof
+
509
weiglzt pcrccntagcs oS mrijor clcrne~ztsinto wcight pcl.ccntngt:
of minerals was carried out in scvcral steps. First, the wciglzt'k
of dololnite was calculatcd by assuming that all the available
MgO is ]>art of tllc dolomite. Tlze11, the wcight"~ or calcite
was dctermiucd using tfzc rcrnclil~i~~g
weightO/~of CaO. Tlzc total carbonate coilccntration is the SUI~Iofdolomitc i d calcilc
weight pcrccntagcs, ass~uningthat thc contribution by gypsum
is minor. Fc)r the silicates, tllc conversion proccclul;.e is somewhat tzlorc complcx. The concentratio~iof mica is calculatcd
using literature vnlucs oT a MgO-rich mica, biotite (Weaver
and Pollard, 1973; Newman, 1987) and its weight gercenlagcs
of KzO (9.23%), A1203 (15.77%)),and Si02 (38.04%). Similarly, publishccl wcight pel-ccntages of A1203 (l9.64%)? SiOZ
((14,98%),and K 2 0 (7.33%) wcre used to calculate Ihc concentration ol' a relatively potassium-pool*feldspar, sincc tllere
is a "shortage" oS K2Q in the major element composition,
Comparison of thc peak lzeiglzls in lhc XRD patterns suggests an equal contribution by fcldspar and mica. Therefore,
the available ICzO was equally cliviclccl a~zrlassigncd to bath
feldspar and mica. The weight percentage of kaoli~zile(literature values of weight pcrccntagcs: A1203is 39,4% alzd SiOz
is 46.55%; Ncwman, 19517) was calculalcd from the remaining
weight(%of Alz03.Rnnlly, thc concentration of quartz was calculalecl by subtracting thc co~lccntrntionsoESi02 used by mica,
fcldspilr, and k ~ o l i ~ lfro111
i t ~ the tneasurcd weight% of SiOz. A
400-
300 -
a 200z
.4
b)
U
G
.n
g
loo-
'3
CJ
L..l
300-
200
-
100 -
FIG.3. XRD patterns showin the presence and relative mineral composition of two rock specimens, samples LCC06 (u per
diagram) and sample LCC20 Bower diagram). Key to identification of peaks: Q-quartz; F f e l d s p a r ; D-dolomite; C-ca cite;
K-kaolinite (possibly dickite); M-mica. See text for discussion.
P
sumnzary oT the quantitative nlineralogic composition of the
rock specimens is prese~ztcdin Table 2.
Petrography and geoche~~~ical
classification
Thin sections were stained (Dickson, 1965) and petrograpliically examined using plane-light and catl~oclolun~inesence
(Teclinosyl~Lun~inoscopeoperated at 10 to 12 kV) microscopy.
A Zeiss Ibas-20 Image A~zalysissystem was used to generate pore-grain rnaps of tlze most. pron~incntsediment textures.
The nzixed siliciclastic-carbol~atesa~zlplesanalyzed for their
acoustic properties in this study have been peirograpllically
(Dunham, 1962; Pettijol~nel al., 1973) and geochenzically
(Table 2) segregated according to tlicir coiltent of quartz, calcite, and dolomite.
Thc specimens are subdivided inlo groups that are discriminated by pctrograpl~icobservations on tcxtural I'actors (e.g.,
Vernik, 1994), and on the n~incralogiccon-iposition (Table 2).
These petropl~ysicalgroups and their associated clzaracteristics (Table 3) i~iclucle:(1) quartz graywackes (>SO% Si02)
with sutured quartz grain contacts (Figurc 4a); (2) quartzricli limcstones (wacke- to packstone texture; 10% > SiOz <
50%) also witlz some sutured gl-ain co~itactsand containing
Table 2. S~~~lllnary
of pelrophysical classification and quantitative mineralogy.
CaC03
PP (weight %) IC20
Doloniite Calcitc Carbonated' Mica
ICaolinite K-FcIclspar Quartz
Total'b'k
Sample class. Scheibler (weiglzt%) (weight%) (weight%) (weigllt%) (weiglltO/~)(weight%) (weiglit%) (weight%) (weight%)
Note: PP, class. = petropIiysical-petrogrc?.~~1~ical
classilication; 1= quartz-grain supported with weigIll% quartz > 50%;2 = wcigl~t%
quartz: 10% < quartz < 50%; 3 = clololnitic limestone witlz ratio dolomite/calcite r 1.5 and quartz% < 10%; 4 = mixed dalon~iliccalcitic limestone with quartz% < 10%; 5 = calcitic limestone with ratio dolornitelcalcite < 0.66 and quartz% < 10%; * = total of
dolomite and calcite measured througl~XRF analysis; *fsdc = total weigl~t% of calcuIatecl minerals; a = not analyzed,
Velocity in Mixed Carbonate-siliciclastics
of typical examples of ihc five pcirophysical groups. (a)
to packstone, group 2 (sample LCC44). (c) CLphotornicrograph
(d) CL photomicrograph of quilrlz-rich wake- to packstonc, group 2
3 (sample LCC45). (I) Doloinitizccl quartz-rich grainstonc, group 3
grainstone, group 4 (sample LCC38). (11) Dololnitized gminstonc, group
group 5 (sample LCQI).
rnoidic porosity lillccl with z o ~ ~ chriglit
d
catIit>dol~imincsc~~cc
(CL) coluin~~ar
calcite ccmcnts, ctchcd zo~ledclull CL hlocky
calcite ccments, and fibrous non-CL sj?clcotlzcm calcitc cements (Figures 4L7,4c, ulid 4d): (3) ~lolomitizcdpack- to grilinstones (CaMg(CQ3)21CaC03 =.1.5; Sic)? .;: 1 O'k) cxhi hi ting
dull zoncd CL dolomite ccnlcnls, dull zo~~ccf
CL blocky calcite cements, vug porosity, and lihrous xlon-CL spclcotl~cm
and nan-CL whisker ci~lcitcccmc~its(Figures 4c and 4C); (4)
partially dolomitized limcsloncs (pack- to gralnstune tcxturc;
0.66 > CaMg(C03)2JCaC03< 1.5; SiOz < 10%) with nonCL blocky calcite ccrllcnts iuld chalccdo~~y
ccnicnts (Figurcs
4g ancl 411); ancl (5) calcitic limcstoncs (wnckc- to packstone
texture; CaMg(C03)2/CaCOn< 0.h6; Si02 < 10%) contair~ing
dull zoncd CL clolomitc ccmcnts, multiplc nun-CL lo dull CL
zoncd blocky calcite ccnic~ltsin primary and fracture porosity,
dolor~-litclcacl~i~ig
ancl licn~atitc(Figurc 4i).
Thc quartz-grain supposted tcxturcs of g ~ o u p1 should I~ave
a cluartz pesccntagc highcs tlla~i65%).I-JOWCV~I*,
pctrtrgrriphic
obscrvatio~isof grain-supported cluz\rtz texlurcs correspond to
niinirnunl weight pel.cclitagcs ol'cluarlz oTSS'% t ~ n dl~iglzcr.Ohscrvations sllow that thcsc clunrtz grnywackcs llavc so1.n~10 10
15%) carbonate grains that are part oT the grain-suppostccl I'ahric. rrllere~or~,
the rock spccin~cnsolgroup I arc not cclmplelely
quartz-grtlin supportd.
Porc types clo ~ i ocouelatc
t
signilicantly lo tllc petr.ophysicnl
groups (Figure 5). Moldic poscs (Tro131250 1~171to m111 s i x ) are
FIG. 5. Digitized images of the porc structure of typical cxamplcs ol the Tour rnicrolacies groups. Porc~sityis clispl ayed
grains are black. (a) Sample LCCOL, group 2. (b) Sample LCC44, group 2. (c) Sample LCCO4, group 2. (d) Salnplc LCC
3. (e) Sample LClC46, group 3. (f) Sample LCC38, group 4. Sce text for discussion,
Velocity in Mixed Carbonate-siliciclastics
obsorvcd in both groups 2 and 3. They are smooth to irregular. have no specific orientation, and may be partially filled
513
only. Rock specimens with carbonate content higher than 90%
uniquely represent the higher velocity-porosity con~binations
in the diagrams. At lower carbonate contents, the relationship
between mineralogy and velocity-porosity is more complex,
and no single mineralogic parameter explains the remaining
variation.
When the rock specimens are discriminated for both mineralogy and textural properties, a significant separation between the petrophysical groups is observed (Figure 9). Overall,
carbonates with more than 90% carbonate content (groups 3,
by cement (Figures 5a, 5b, and 5c, and Figure 4c). Interparticlc porosity is mainly restricted to groops 1 and 3. Micropores
rongc in size between 10's of microns up to 250 pm, are irregular in shape, and may be partially filled with cement (Figures 4a
and Sd). Very large, mm-sized, highly irregular and unorientcd
pores arc observed in groups 3 and 4 (Figures 5e and5.f).Microporosity, smaller than 10 pm, is predominant in groups 1 and
2. In general, pore types are l-~igblyvariable and not restricted
to onc specific petrophysical group.
DISCUSSION
I
Sonic vclocity
p'lblc1 s~lmrnarizesthe measuremellts of the petrophysical
l~ropcrtiesof' the 48 specimens. Substantial scatter occurs in
thc velocity-porosity relationship (Figure 6a). A certain value
of comp.essiot~alvelocity correspollds to a range of about 15
porosity percent. The velocity-porosity transfarms in Wyllie et
al. ( 1958) Tar dolo~niticlimestones and sandstone, and Raymer
ct al. (1980) far sandstone, are only crude approximations ofthe
vnriation in the clata set. Most of the data exhibit significantly
higllcr sonic velocities than predicted by these empirical equations. 111cantrast, Gardner's (1974) experiinental equations for
lii~lcstoneand sandstone have parallel trends to the data set but
prcdict velocities that only explain the upper part of the data
(Rgurc 6b),
'rhe rock slsecitnens were measured at an effective stress
lcvel ol'30 MPa for two reasons: (1) at this effective stress the
~z~icrocracks
iizduccd by stress-relief should be closed (Vernik,
1994); (2) the closure of the rnicrocracks and general stiffening sl' the grain and crystal contacts reduces the local flow
cflcct on velocity dispersion that is known to offset laboratory data relative to the lower frequency field measurements
(Mavko and Jizba, 1991). Specimens with velocities lower tlian
i~ppraxirnately4.5 km/s have lower carbonate content and
higher porosity and are more sensitive to stress (Figure 7a).
Mosl critical in this relationship is the carbonate content (Figure 7h); at carbonate contents of less than approxin~ately
SO%),thc effect of increasing effective stress is nearly twice
of that at carbonate contents higher than 50%. This may imply thal as eff'ective stress increases from 2 to 30 MPa, difScrcnces in the data set may be sigtlificantly enhanced. Porosi ty is similarly correlated (reversed) with effective stress but
shnws considerably more scatter than the stress versus carbonate content plot, possibly d ~ l eto the variation in pore type
(Figure 7c).
Figure 8 shows cross plots of sonic velocity versus porosity discsiminated for carbonate-, quartz-, and mica content
- 6
I
5
zII
&4
Q.
Wyllie et al.; sandstone (5.5 kmls)
Raymer et al.; sandstone
$.
'
-
3
2
0
L
1.8
0.2
0.1
2
0.3
0.4
Fractional porosity (%)
2.2
2.4
Saturated bulk density (glcc)
0.5
2.6
0.6
2.8
FIG.6. Cross plots, undifferentiated for mineralogy,of sonic velocity versus porosity and density. (a) Cross plot of P-wave velocity versus porosity along with velocity-porosity transforms
by Wyllie et al. (1956) for calcite and quartz, and Raymer
et al. (1980). (b) Cross plot of velocity versus density along
with Gardner's solutions for limestone, sandstone, shale, and
Gardner's general equation. See text for discussion.
Table 3. Petrophysical classification.
Croup
Litl-Iology
%Si02
%Carbonate
1
quartz graywacke
quartz-rich limestone
dolomitic limestone
mixed do1ornit;ic-calciticlimestone
calcitic limestone
>SO
10 < SiOz < 50
<10
<10
el0
t50
>50
>90
>90
>90
2
3
4
5
Note: DoICa = ratio of weight% of CaMg(C03)2over CaC03.
Ratio
DolCa
>1.5
>1.5
>1.5
0.66 < Do/Ca c 1.5
~0.66
*uo!ssnDs!p roj 1x33 aas il!so.~odlelol pue ssalls aiyDajja Bu!sea~ou!jo u o ~ o u nsj se d~ 10aseamui ayl uaaMlaq uo!ieIaa (3) .quaquo3
alauoq~e3pue ssarls aa!l~ajjaBu~saar3u!30 u o ? p u q n se d~ 30 ascanu! at11 uaaMlaq u o y y a x (q) *ad'ssar~sa ~ y 3 a JO
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(%) ~ J ! S O Jll?UO!J3l?.Id
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p u ~~z?uoyssascIu~o~
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'sassarls say
PUB rCl?so.tod worj Ag~oola~
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nogeuuqu! aql 01 ill1uux..ru8rs ppe ~ o saop
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0 uaahlaq lll~so-~od
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sly3 IOJ 'saldrues arll JO U O I I ~ ~ I J ~ S ~ Vl~?qsdydos~ad
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a r u 'say!
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-3013h 1S3M013Y1 3 A U Y S;S>I~BMAB.I~pa1.1oddns uru~8-zjlenbarlJ
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~~oI~~
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u a ~ @ .suarulnacis rpr.~-zu~?nb
+
515
Velocity in Mixed Carbonate-siliciclastics
for 30 MPa are 1.5 and 1.8 kr~lls.Similar valucs Tor thc sl~earwave velocity are 0.4 and 1.5 knits (at 9 MPa), and 0.5 ancl
1.1km/s (at 30 MPa).
Figures IOc-d and 1lc-d are contour plots of the original
data and show vcry similar trcncls except for the low pcn*ositylow carbonate and 11igl1porosity-liigl~ carbonate corncrs where
data coverage is poor. Thosc cor~~crs
arc thc rcsult ol the lincar interpolation of the contouring function. Howevcr, calculations of compressional and shear-wave velocities have better
values in those corners (cf. Ver~iik,1994). Cross plots of measured vclocity versus calculated vclocity (Figures 10e ancl 101;
and Figures 1I e and 11S) along with histograms of t11c rcsicluals
(Figures 1Qga11cl1Oh, and Figures 11g nncl 1111) clearly demonstrate tllc good lit (R = 0.92-0.94) of the calculated velocityporosity-carbonate translorms.
The slope oS the velocity-porosity relation at coi~stantcarbonatc content, 3i Vli34, derived rroin ecluatiox~(1) is givcl~by
Frilctio~~ill
porosity,
@
0.20
0,IS
0.10
0.05
0
(11
0,25
1, Qunrtz-gmin supported; weight% Si02 > 50%
0 2, Weight% 10%< SiO2 < 50%
3, Dolomitic lirnestonc; rntio dolomitclcalcite~1,5; weight%SiO2 < 10%
4, Mixed dolomitic-calcitic limcslane; weight%SiO2 < 10%
a 5 , Cnlcitic lirncstone; ratio dolo~nitclcnlcitc<0,66; weight% SiO2 < 10%
()
The slope ollhe velocity-porosity trnnsl'ormsl'or low-carbunatc
contents, sandstones, is steeper than that lor high-carbanatc
contents since Cq 1x1s all opposite sign con~parcdto Cz (sec
Table 4).For compressional-wave vclocitics this gmdicnl is
.
. . ..
l
:
'
'
'
~
l
'
'
'
l
l
l
~
~
0 00% < cilrl~onntc< 1(30%
0 66%< rnrbonntc c 9OC
.
3 3 2 e carbonntc < 66%
0% <ccbonntc ~ 3 3 %
l
l
,
.
l
,
0
*
0
,(
0
~
1
-
h
'
'
I
I
.
I
I
'
1
1
1
4
(
~
0 0
o
1
0
3.5
O
$
0
0 0
0
8
'
8
-
s
l
t
2
~
1
'
l
'
~
l
l
l
'
l
l
l
A
l
~
~
~
'
'
~
'
l
0.10
-
~
--
Q
A
l
~
0.1 5
Fractional porosity (%)
0
*A
s
~
0.20
'
A
-
0
%b
A
l
-
~
A
0
0
l
1
'
0
A
0.05
r
-
OoOo
-
o
1
.
0
0
4
1
g
.
0
A
A
~
'
-
0
O
'
0
"o
0
0
0
A
"
0 0% < rhica < 2.0%
0 2.0%<mica c 6%
A 6.0% c micn
0
o0
-
I
l
%
A
A
0
i 'o8
-
I
~
.. .
.. ..
.
A
A
I
~
< qutlrv?.
a 20% .:qunrtr, < 40%
0 10%c quartz c 208
0 (1% c qunrlz c 10%
0
3 9
l
~
A 40%
-
0
0
0
.
0
A
~
A
-
@
A 0 0
~
O
.
A
0
A
l
•
1e
i .t
1 .
OOO*
A
'
a .
I,
0
*
~
A
A
1.t
-
'
PIC;.9. Cross plot of compressional velocity vcrsus porosity.
Data are discriminated accordi~lgto the petraphysical classilication. Sce tcxt for cliscussiox~.
~
~
l
025
Diagrams displaying the relationship between sonic velocity and weight percentages of dominant rni~lerals(a) Cross
'
~
'
'
' and
~ weight% of carbonate. (b) Cross plot
plot
of' sonic
velocity
of sonic velocity and weight% of quartz. (c) Cross plot of sonic
velocity and weight% of mica.
FIG. 8.
'
~
l
c
a)
b)
Pe = 9 MPa
Pe = 30 MPa
Fmctionnl porosity
Fractional porc~sily
dl
c)
Fractionni porosity
Fractional porosity
V p (measured; kmls)
V,, (mmwu'ecl;kmls)
14
14
12
12
2 I0
$1 0
s
rg
rt:
w
p"
6
%
4
4
2
2
?1,5
-1
-0.5
0
0.5
1
0
1.5
-1.5
-1
-0.5
0
0.5
1
1.5
Range
Range
FIG.10. Diagrams showing cross plots of carbonate content and porosity, and contours of compressional-wave velocity oS bcst-fit
linear surface. (a) and (b) are plots generated by the linear velocity-porosity-cavbo~latecontent tral~sllarmcalculated lor the data
at 9 and 30 MPa eflective stress, respectively. (c) and (d) are contour lots of the original data at 9 and 30 MPa cfictive stress. (c)
and (f) are cross plots oI measured velocity and predicted velocity.
and (h) show bistograrns of the residuals [or both eftcctiw
stresses. See text for discussion.
(gy
Velocity
In Mixed Carbonate-siliciclastics
b)
Pe = 30 MPa
Fractional porosity
Fractional porosity
dl)
c)
Fractional porosity
Range
Fractiotlal porosity
Rnngc
FIG.11. Diagrams showing cross plots of carbonate content and porosity, and contours of sonic velocity of best-fit linear surhce.
(a) and (b) are plots generated by the linear velocity- porosity-carbonate content translorrn calculated for the data at 9 and 30 MPa
effective stress, respectively. (c) and (d) are contour lots of the original data at 9 and 30 MPa effective stress. (e) and (I) arc cross
plots ol measured velocity and predicted velocity. and (11) show histograms of the residuals tor both efIective stresses. Sce text
for discussion.
(gp
'
9.7 at low-carbnn;ilc corrtcrzt, silrlclsio~~es,
n l ~ c l7.1 (linl/s) ;it
high-carbonate contcnt and cSfecti\/e strcsscs oS C) MI'it. For
slictlr-wave velocities the clifScrc~iccbctwccn the two gradients is larger (4.6 vcrs~~s
2.6 (kmis) I ) . Thc dii'fcrcncc is dccreasing with increasing cfkctivc S ~ ~ Clit
S S-70: MPit the v:llucs
are 1.0 for compressional ancl 2.A (km/s)- Sclr s11car.-wave velocities, The value lor the sandstoilcs i1t 30 Ml'n, 8.2 (k~l~ls)'-',
is sligbtly higher than that reportcd by Vcr~lik(1994), about 6.9
to 8.0 (km/s)-l for arcnitcs. This is possibly clue to the presence
or clay in his claln sci ( L Ilo~ IS%). Tl2c rclaljvcly lower slopc
for com~ressional-wavevelocity. ahout 7.1 to 7.2 (kmls)-I at
9-30 MPa, is common for pure carbonates and has becn doc-
c:c,ntcilt
Figurc 12 plots V p / versus porosity and carbo~~atc
and sllows tlzat (1) predominnnlly calcitic limcslot~cswith less
than 10% quartz (I/P > 5 km/s and V p / V , y 3 1.8) could he
clistixlguislied Sr.oin quartz-rich limcstorles (Vll < 5.5 k ~ ~ iancl
ls
VII/ Vs < 1 4 , and (2) dolollzitic limestones with less Illan 10%
quartz could not be separated from qua~.tz-sichlimestones t111rl
s l ~ a wan overlap wit11the calcitic limestomcs,This ratio 1 ~been
s
lilted by a siinple linear relationship that alrcady rcsultccl in a
high, correlatio~~
coeMcient (Ii = 0.97) for both r) ancl30 MPa:
Vp/ Vs = 1.81 - 0.394 f 0.03 a,, -I- O.IXw,.,
Tor P,, = C)MPa,
V[J/ Vs = 1.73 - 0.23@t 0.06 a,, -t- 0.1 8 w,. ,
A rough esti~l~ate
ol' the prcssurc ciepcncicnce of coltlprcssional a ~ i dsllcar-wave velocities can he clcrived fro111 CqUiltion (1 ) by subtracting thc cocfficicnts nt c) MPa Src)111thosc at
30 MPa ancl diviclilzg thc results by thc difference jn the elfcctivc stress:
.k
for Po = NMPa,
wl~crcu,,ancl cu,, arc the fractions of dolamite tuld calcilc matcrial, rcspectivcly. Thc cocfflcicnl rcspotlsiblc for t l ~ ccffccl a[
1.05
for P,,= 9 MPa,
8Vey/8P= 3.75 -J- 6.804 -J- 3 . 3 6 ~- 9.55$~,
,
(4)
(3)
for P,,= 30 MPa,
--.
I .o
%
&'
ni
wlzcre i3Vp/i)Pis i l l kmlslMPa. Sincc the clata sets usccl for
rn
iij
Q 1-8
the lilting are only at two pressure Icvels, this rough estil~iatc
is equivalent to a Icast-square Iitting of the cnti~.csct oS men'.t,
surcments assuming linear dcpcnclcnce an prcssurc. The li11c;ir
irrQ. 1.75
,
clclscndcnce ol' vclocily on prcssurc hetwccn 9 and 30 MPa is
conl-irmcd by thc diagram in Figure: 73, in which the di~taset
I -7
inclucles velocity measurenlcrlts at ilztcrn~cdialcPI-CSSLI~CS.11.is
clear from cquatio~~
(3) that tlic influcilcc of effective stress on
I. ....
65
the velocity is stronger in the law-carbonate domnin, 2nd vir-iu0.0
~ l l yabsci~tat Izigh-carbonateconlctnts. This is p~-obi~ldy
a cumbilled effcct of the closurc of'n~lcrocracksand the projncrtics
of carbonate diagcnelic Iablics. Cnrhonatc rni~~crnls
arc more
b)
susceptible to changing ~actropl~ysic:\l
conditions tlzat result in
a bcttcr lit of cryst~11contacts. 'Tbgcthcr with the higher matrix
ve1ocii.y or carbonate minerals (6.5 to 6 8 km/s; Carr~~icl~acl,
I989), this may explain the carlicr docun~cntecl tcndcncy 01'
carbonates to llavc highcs sonic velocities at highcr porosities
Cl~ansiliciclastics, ancl to bc less scnsitivc to variations in ~ f f c c live stress, For futurc synthetic scistnic nioclcling of' lhc Last
Cllance Canyon cross-section it is important that thc subsurPace a~ialsgbe at a clcpt11 of anIy sorzle 100's of mclcrs wliich
is ccluivalent to an effective stress cjf a fcw MPa's. The overall
acoustic behavior of the data set will subseclucl~tlycliangc with
i~~creasing
depth of the s ~ ~ b s u r h target.
cc
v)
Velocity ratio
The ratio of comprcs~ional-waveancl shear-wave velocity
(VP/V S )is often regarded as a tool for indicating pore fluid and
porosity (Robertson, 2 987), to clifferentiate between litl~alogies (Tatham, 1982; Dumenica, 1984; Wilkens ct al., 1984;
Castagrla et al,, 19851, or to determine the r~~inel-alogic
composition (Eastwood and Ca~tagna,1983;Rakvich al.7 1984).
Data presented in Rahvich ct al. (1984) show a clear separation between dolomitic and calcitic limestones.
0.05
0'2
0,o
0,10
0.15
0.20
025
0.4
0 .o
0,H
I .ll
J:ractional p o ~ o " i ~ y
Fraction:rl carbonnte content
a I , CJUII~M-gri~in
suljl~ortcti;wcigtlVl1 sic)^ r 5 0 ' ~ .
0 2. Wciglai% 10% '/o Si02 < 50%
4 3, Dolorr~ilicli~ncslono;raLio dolon~ilclci~lcilc
.s 1 .5; w c i g l r ~ Sit)?
~ l ~ ..z lUi:4,
EJ
LA
Mixcd tlolomitic-calcitic [irllcstollc; w c i ~ [ ~ Ni(.l?
t O c IO~XI
s.filcilic limchlonc; mtio dolrr~nitclciilc~t~
< 0.0h; ~ C ~ ~ I SI ILU~e~AIc~rg
61,
FIG.12. Cross plots of'the vclocity ratio Va/ Vs versus porosiiy
),( ,d
conlcnt (b)
cffcciivc stress 30
Data are discriminated according to ihc pct~.ophysic;llclassincation. See text lor discussion.
519
Velocity in Mixed Carbonate-siliciclastics
dolomite content is significantly snialler than that rcprescnting porosity and calcite content, This explains tlie possibility
of identifying calcitic linlestones and high porosity using thc
V p/ VSdiagrams and the difficulty in the separation of dololnitic
linlestoiics from the sandstones.
Witllin the presented mixed siliciclastics-carbonate data set,
tlie velocity ratio is a powerful tool to discriminate betwcen
lithalogies that are predominated by calcite and quartz. Predominantly dolonlitic lithologics can be distinguisl~edSrom calcitic limestones but not from siliceous lithalogies.
Mineralogic composition
Several techniques have been used in the literature to determine the quantitative mineralogy ol sedilncntary rocks. These
n~ethodsvary from point counti~igmineral grains in thin sections (e.g., Rafavicll et al., 1984; Vernik and Nur, 1992; Vernik,
1994), to quantitative X-ray diffraction of powdcred saniples
(Shams-Kanshir, 1994); in many studies, no method is rcferenced at all. Especially, the quantification of grains wit11 significant textural different characteristics that affect the fabric ancl
grain-to-grain contacts, like cletrital and autlligenic clay minesals, is 01 critical inlportance to the elastic properties of the rock.
Point counting has severe limitations when determining pcrcentages of mineral grains that have dimensions smaller than
30 Irn, such as clay minerals, Second, tlle extrapolation from
2-D observalions of usually oriented particles will1 high aspect
ratios to 3-Dunits is unreliable, as shown by experiments with
image analysis of porosity. The quantitative phase analysis of
unknown materials by XRD is practically impossible (Wilson,
1987). Especially, analyzing clay minerals, the great variability
in diffracting powder and preparation techniques renders measureinellts semiquantitative (McEwan et al., 3 961). Therefore,
in this study we used XRD to identify tlze dominant mineral
groups and, e.g., the peak heights of mica and feldspar to dctermine tlieir relative contribution to the host rock, XRF analysis
were then used to convert to a quantitative rnilzeralogic composition. The accuracy of the total carbonate content (dolomite
ancl calcitc) calculated this way is close to that determined using
the Scheibler technique (see Table 2), and its accuracy is prohably higher than i 1 . 5 weight%. The accuracy of tlze noncilrbonate mineral composition is estimated at k3 weight% which
renders some of tlie very low contributions by mica doubtSul.
Only two specimens, LCC37 and LCC46, havc relatively low
accuracy that is probably related to the presence olunidcntiliecl
minerals (see Table 2). However, the main paramctcr affecting
the acoustic behavior of the specimens is the total carbonate
content,
Ditligenetic influences
The presence 01. absence of quartz grains is a first-order control on the acoustic behavior of the siliciclastic and carbonate
samples analyzed in this study. However, a complex array of
second-order diagenetic controls have created tlie present-day
porosity and permeability of each lithology, and therefore further influence the acoustic behavior within the two main groups
determined by the presence and absence of quartz.
The quartz graywackes and quartz-rich wackestones (groups
1 and 2) exhibit sutured grain contacts formed as a result of
pressure solution during burial (Pettijohn et al., 1973).In addition, biomoldic porosity is partially occluded by columnar and
blocky calcite cements, which postdate the pressure solution
and ere thcrcin a producl of burial diagencsis. The nondolomitizcd, partially dolomitized, and complctcly tlolomitixctl limestones (groups 3, 4, ancl 5) exhibit various diogcnctic cflccts.
Extensive dolomite cemeiitntion, and blocky cnlcitc ccmcntation, as wcll as some chalcedol~yccmeiltation has occluded
much of the primary porosity. ThereEorc, present-day porosity
in these Eacics is primarily the result of later stagc dissolution
Corming vuggy porosity. TLlc rcsult is that lithologics with significantly dilkrent primary depositional porosities now have
similar porosilies and acoustic bel~avior.
The late-stage diagenesis in all live lithologic categories includcs dissolution vugs, some fracturing, and the precipitatiol~
of nonluminescent fibrous and whisker calcites that suggest
precipitation from oxygenated ~netcoricneas-surface groundwaters (Esteban and Klappa, 1983; Verrechia and Verecchia,
N94). All of tliese features may have been Eornied since the
platfor111 margin was exhumed and subaerially exposcd. However, late-stage leatures are minimal in the thin sections studicd
and may not havc significantly allected porosity.
CONCLUSIONS
Meas~irenlentsof the pctropllysical properties, quantitative
minernlogic composition, observations on the texturc, and rhc
subsequent subdivision into petropkysictll groups oS 48 rock
specinlens horn n forn~ationof Permian limestones, dolornitcs,
ancl quartz salidstones have in~partanti~liplicationslor the
~inclerstanclingof the acoustic behavior of mixed carbanatcsiliciclastic rocks.
Comnlon velocity-porosity transforms by Wyllie et al. (2958)
and Raymer et al. (1980) only impcrlcclly cxplaiil thc observed relationship between velocity and porosity in 111e data
set. Ciardner's experimental curves overestinlate the velocitydensity relation present in the mixed carbonate-siliciclastic
data.
Linear lnullivariate analysis rcsulted in velocity-porc~sitycarbonate coizient transforms that accurately predict sonic velocity, comprcssiollal velocity, and sheas-wavc velocities withili
thc mixed carbonate-siliciclastic data set, The predoniina~lt
control on sonic vclocity is by porosity and, sccundly, by carbonate contcnt. The infiucllcc of carbonate contcnl is larger
in the shear wave-porosity-carboniltc col~tenttranshrn. Tlle
slope oC the porosity-velocity transforms, within tbe 0 to 25%)
porosity domain, steepens with increasing carbonate conlent.
Third, the velocity ratio V r / Vs is a powerful tool lo separate
prcdominently calcitic lithologics (ratio between 1.8 and 1.95)
lrani dolomitic and quartz-suppo~*tcd
sedimcntary rocks (~aatia
between 1.65 and I.$),
These findings suggest a more cornplicatcd relationshi11 for
mixed carborlate-siliciclastics than earlier documcntcd for ~L~UI'L:
siliciclastics. Rcasons far this may be tl~chigher mineral velocity and the property of carbonate minerals lo form more pcrfcct
intercrystalline boundaries.
Finally, postburial diagenesis is minor and does not seem
to l~avealCectec1 the acoustic properties, Tlzerefore, the documented acoustic parameters can be regarded as describing the
true subsurfslce petrophysical behavior of the rocks.
ACKNOWLEDGMENTS
We wish to tliank Nanda Rave-Koot and Volker Wiederl~old
for their assistance in preparing the rock plugs. Marianne
Kentcr et al.
520
Broekema and Ed Verdurrncn are acknowledged lor XRF
analyses. Roe1 van Elzas learned the tsaclcs of measuring
sonic velocities, and his contribution is greatly appreciated.
The ultrasonic equipment was engineered by Carl Coyner and
benefitted from rapid and successful electronic first aid by
Johan de Eange. Wolfgang Sclllagcr and Christian Lehr are
acknowledged for their reviews of an earlier version of this
manuscript. The authors would also like to thank Rick Sarg and
an anonymous reviewer for suggestions on how to improve the
m,muscript, Funding to tile first author was provided tllrougll
the Industrial Associates Program aE Wolfgang Schlager. This
is N.S.G. publication number 950509.
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