C~NnAH lx

JOURNAL OF EXPLORATION GEOPYSlc* 26, NO.5 1 8 2 iDECEMBER 1990,. P. 94.103

EFFECTS OF LITHOLOGY, POROSITY AND SHALINESS P AND SWAVE VELOCITIES FROM SONIC LOGS

ON

SUSAN L.M.

MILLER

AND ROBERT R. STEWART~

ABSTRACT Full-waveform soniclogsfromfour wells in Ihe MedicineRiver field of Alberta are analyred far relationships betweenP-wave velocity(V,). S-wave velocity (KS). VP/V, lihology, shaliness and and identifies mnistone, porosity. V,IVsin conjunchm VP with effectively limestone andshale lithologiesin the sampled intervals. increases VP quasi-linearly with V.7 sandstone limestone. Average in and VpiVs values I .h" for sandstone 1.89for limestone f"""d. In forof a"* are mationrwith mixedcarbonate,clastic lithologies Nodegg, silicate (6x Shunda Detrital), V, increases approximately and linearly with Vs. by The VP/h ratiosin the mixedlirhologies arehounded the VpiVs Ya,"es tile componenr or litholagies. I" tile san*s,ones considered here,b&7 v, an* vi decrease as porosityincreases but K dependence porosityis very weak.The on V,d%ratio decrcsses purusity increases. An increase shale as in COntent lowersVPand"r but increases "DIVX. Pomity hasa greater influence on velocitythanshalincss by about orderof magnitude. an Both VPandVr decrease porosityincreases the limeslone data as in but the correlationis poorberween VP/Kandporosity.The linear regression intercepts from the limestone velocity versusporosity plots accurately predictcalcite matrix velocities.In the Nordegg. Shunda Detrital Fornations m increase in porosity is accompaand nied by a decrease bothPi andS-Wave in velocities. VpiV,trend No is observed either theNordegg the Shun&,hut VpM decreases in or asporosityincreases theDetritalFormation. in

geometry, pore fluid, bulk density, effective stress, depth of burial, type and degree of cementation and degree and orientation complex of fracturing (McCormack et al., interaction of these and other factors 1985). The complicates

the task of inverting seismic velocities to obtain petrophysical information. In order to understand how rock properties influence velocity, researchers have employed a variety of approaches such as core analysis, seismic and well log interpretation and numerical modelling (e.g., Kuster and ToksGz, 1974; Gregory, 1977; Eastwood and Castagna, 1983; McCormack et al., 1984). In the well bore various logging tools provide a number but in the available.

of measurements which describe the subsurface, seismic realm VP and VS are the main descriptors S-wave well logs response to known of shear seismic multicomponent are crucial in tying geology and guiding sections. seismic

observed elastic the interpretation is to invert informa-

Ultimately, the goal data for petrophysical

tion. This paper considers four wells in the Medicine River oil field of central Alberta. The objective of this study is to analyze full-waveform basin and search for lithology, examine sonic logs in the western Canadian trends which provide information on

porosity and pore fluid. The approach has been to several of the factors which have been studied by workers data. and determine if trends are present in

INTRODUCTION
Recording shear waves during seismic acquisition as well as compressional and well logging provides waves addi-

previous these field

tional information about the subsurface (Nations, 1974; Gregory, 1977; Tatbarn, 1982; Robertson, 1987). Deciphering the litbologic information inherent in seismic elastic-wave velocities requires an understanding of the relationship between geology and velocity. To this end, we are interested in studying bore. Seismic factors the elastic-wave velocities rock velocity response by numerous at the well geologic pore

REVIEW
Work that has been with done to date suggests pore geometry Gregory, 1977; that S-wave informadata in conjunction P-wave data can provide

tion on lithology, porosity, among other things (e.g., Domenico, 1984). Compressional lithology indicator seismic because

and pore fluid, Tatham, 1982;

are affected matrix

including

mineralogy,

porosity,

velocity alone is not a good of the overlap in Vp for various

Manuscript received by the Editor July 1.5. 1990: revised manuscript received October 16, 1990. 1Depanment of Geology and Geophysics. The University of Calgary, Calgary, Alberta T2N IN4 We are grateful to David Byler. Manager of khnical Services a* Suncar Inc., for the donation of well log and lithology infommtion for this study. We would also like to acknowledge helpful suggestions from Dr. John Hapkim and Dr. Terry Gordon of the Universiry Of Calgary and Douglas Bayd of Hallihurton Lagging Services. This work was supponed by the Consonium for Research in Elastic Wave Exploration Seismology (CREWES) Project at the university Of Calgary.

94

EFFECTS OF LITHOLOGY, PRSlTY AND SHALINESS ON P- AND S-WAVEELOC,TIES

rock types. The additional information provided by shear carbonates and sand/shale sequences (McCormack

9s et al., 1987) et al.,

velocity can reduce the ambiguity involved in interpretation. Pickett (1963) demonstrated the potential of VB/Vs as a lithology indicator through his laboratory research. Using core measurements, limestone, 1.8 for and 1.6 for clean erally he determined VplVs values of 1.9 for dolomite, I.1 for calcareous sandstone sandstone. Subsequent research has gen-

1984; Anno, 1985; Garotta et al., 1985; Robertson, and well logging studies of elastic silicates (Castagna

1985). The increase in V,iV, with shaliness has been used in seismic field studies to outline sandstone channels encased 1985). in shales (McCormack (1983) et al., 1984; examined Garotta et al.,

V,dVs values V,A4 ratios

confirmed these values and has also indicated that in mixed lithologies vary linearly between the of the end members (Nations, 1974; Kithas,

Eastwood

and Castagna

full-waveform

sonic logs and observed porosity in an Appalachian

constant l/,/l/~ with increasing limestone and increasing V,/V,

1976; Eastwood and Castagna, 1983; Rafavich et al., 1984; Wilkens, 1984: Castagna et al., 1985). Various approaches have been taken to analyze the effect of porosity on velocity. These include the time-average equatransused equation (Wyllie et al., 1956). the Pickett empirical tion (Pickett, 1963) and the transit-time-to-porosity form of Raymer et al. (1980). Domenico (1984) Picketts data to demonstrate that I& in sandstones times more sensitive to variations in porosity sandstones or Vs in limestones. VP in limestone to be the least sensitive porosity indicator. The model of Kuster and Tokssz indicates

with increasing porosity in the Frio Formation sandstones and shales. VpiV, is sensitive to gas in most elastics and will often show a marked Gregory, 1977; 1983; Ensley, decrease Tatham, 1984, 1985; of carbonate in its presence 1982; Eastwood (Kithas, 1976; and Castagna,

Vp/V.r response

McCormack et al., 1985). The rocks to gas is variable, a disgeometry. with elonin

is 2 to 5 than V, in was found that pore-

crepancy which may be attributable to pore V,dV, reduction has been observed in carbonates gate pores carbonates gas effect penetration (Anna, 1985; Robertson, 1987) with rounder pores (Georgi may not be observed on well does not exceed the invaded

and absent

et al., 1989). The logs if the depth of zone.

aspect ratio has a strong influence on how Vp and VA respond to porosity (Kuster and ToksBr, 1974; Toksi% et al., 1976). The actual Vp/Vs ratio appears to be independent of pore geometry unless the aspect ratio is low, less than about 0.01 to 0.05 (Minear, 1982; Tatham, 1982; Eastwood and Castagna, 1983). Modelling suggests that for small-aspect ratio pores such as cracks, Vp/Vs will increase as porosity increases. Robertson (1987) used this model to interpret carbonate porosity from seismic data and correlated an increase in VpiVs with an increase in porosity due to elongate POES. A number of workers have included a clay term in empirical linear regression equations developed from core-analysis data (Tosaya and Nur, 1982; Castagna et al., 1985; Han et al., 1986; King et al., 1988; Eberhart-Phillips et al., 1989). When both porosity and clay effects were studied, porosity was shown to be the dominant effect by a factor of about 3 or 4 (Tosaya 1988). Minear and Nur, 1982; Han et al., 1986; King et al.,

STUDY AREA The Medicine River field (Figure I) which produces Cretaceous, formations chart is an oil field in central Alberta from a number of zones in rocks. The ages and in the stratigraphic examined in this

Jurassic and Mississippian of interest are indicated 2. The locations

in Figure

of the wells

paper are 9.5.39.3W5, 9.I-39-3W5, 15.18.39.3W5 and 9-13.39.4W5. These are development wells drilled by Suncor Inc. between 1987 and 1989 which are or have been oil producers. The 9-7 and 9.13 wells produce out of the Nordegg Formation. the 15.18 well produces from an interval in the Basal Quartz (Ellerslie) and the 9-5 well produces oil from both the Basal Quartz and the Pekisko Formations. fluid filling It is difficult to accurately determine the porein some of the zones as there are no drill stem for any of these wells. sampled in this study are the Basal

(I 982) examined

the importance

of clay on veloc-

test data available The sandstones

ties using the Kuster-ToksGz model. Results suggested that dispersed clay has a negligible effect on velocity: however, laminated and structural shale have a similar and significant effect in reducing velocities. Since clay tends to lower the shear modulus of the rock matrix, K decreases more than VP, resulting in an overall increase in V&S. Tosaya and Nur (1982) concluded that neither clay mineralogy nor location of clay grains were significant factors in the

Quartz sandstone (9.15, 15.18) and the Glauconitic sandstone (all four wells). Watkins (1966) describes the Basal Quartz sorted, in this field as a very fine to fine-grained, wellsubangular, quartritic sandstone. The Glauconitic well-sorted, angular to subanwith siliceous cementation

sandstone is a fine-grained, gular, quartzose sandstone (Watkins, 1966).

P-wave response
Because

to clay content. shear velocity is thought

The only limestone sampled in this study is from the Pekisko with data from the 9-5 and 15.18 wells. The well site geologists cuttings log identifies Formation as a slightly dolomitic, slightly the Pekisko argillaceous,

to be more

sensitive

than compressional 1984) and to clay either result (Han

velocity both to porosity (Domenico, content (Minear, 1982), an increase in result in an increase in V,AG. This in core studies of elastic silicates et al., 1988), seismic surveys over

component should has been observed et al., 1986: King

crypto to microcrystalline limestone with pinpoint porosity. The shale points are from the Femie shale only, with data fmm the 9-7, 9.13 and 15.18 wells. The Femie shale is medium to dark grey, platy, tissile, cakareous and micaceous.

s. L.M. MILLER and R. R. STEWART

metres through the zones of interest. Readings from the

5 n z : :

tool may be suspect in regions of horehole wave curve cannot he used in formations wave velocity mud as there is slower than the P-wave will be no S-wave refraction.

washout. in which

The Sthe S-

velocity of the In these wells,

ALBERTA

this occurs in some shales and in all coals and is indicated by warning flags on the log and either off-scale or straightline S-wave transit times. Those portions of the log where the elastic transit times were judged to be reliable were examined for zones which either represent a particular lithology or are of exploration interest. Intervals were chosen which could be clearly identified using well log curves (e.g.. gamma-ray curve) and geological infomution (e.g., well site geologists cuttings report). . EDMONTOI Only intervals which were a minimum of five mctrcs thick were selected for sampling. Date points were [not taken from the top and base of the formations where the well log curves were deflecting rapidly. The well log curves were digit&d with readings recorded every metre. Transit times were used to calculate VP, Vv

. PEACE RWER

. RED OEER

and !I,,/\/,,~. V,, and Vs are directly

used in seismic

processing

and conventional rock characterization; thus, WC prefer to USC V,dK instead olintroducing the related Poissons ratio. The gamma-ray curve was used to compute the gammaray index (G) as follows: c; = GRI~,,-GL,, GR,~dNm, (1)

STUDY
MEDICINE

AREA
RIVER
where

FIELD

GR is the gamma-ray response in GAPI units and CR,,,,, and GR,,,,,, values are based on the interpreted sand
lines. Various curves are available which can he

and shale Fig. 1. Location map showing the Medicine River oil field of central Alberta. Several mixed lithologies were also chosen for analysis. The Nordegg Formation is of Lower Jurassic age and is described by Ter Berg (1966) as a sandstonc consisting of medium-sorted, fine-to-medium which is cemented by dolomitic the 9.1 and Y-13 wells, Shunda is sampled from well site geologists limestone and shale. Shunda as dolomite grained quartz and chert limestone. It is sampled in The to the

used to convert the gamma-ray index to clay content, each of which will give significantly different estimates (Heslop, 1974). Heslop observed a linear correlation between gammaray response and clay volume as determined by x-raydiffraction data from core samples. Kukal and Hill (1986) confirmed the linear relationship and noted that most sheles contain about 60 percent clay. Based on their analysis. clay volume could be calculated from the expression above by multiplying G by 0.60. Since we do not have available the relationship between shale and clay content for this area, we have chosen to use a simple linear relationship between gamma-ray deflection and shale content. In this study we have used the gamma-ray index (x 100) as a measure of percentage shaliness. Porosities were calculated using neutron-density crossplots and, when the data were available, hulk density and photoelectric absorption crossplots. Porosity values were corrcctcd for shale content in the sandstone data. Gammaray readings formations are most likely due to shale content in these (J. Hopkins, 1990, pen. comm.) and were of shale volume. Neutron values lrom surrounding the shale point on the cross-

both of which are oil-filled. the 9-S well and, according

cuttings log, consists of intcrhedded The core-analysis report describes the with interbedded shale. The Detrital

refers to rock detritus on top of the Mississippian unconformity and consists mainly of dolomite in the 9-13 well from which the data is taken. Based on the available the pore fluids present formations production data and the well logs, in the Jurassic and Mississippian

are oil and water.

METHODS
The availability of S-wave data from these well logs was limited by several factors. The full-waveform sonic logs analysed in this study were only run over several hundred

therefore used as a mcasure porosity and density porosity shales were used tc~ determine

plot. This point was used to crcnte a scale from 0 to 100 percrnt shale so that the required correction for a given percentage of shale could be determined. The neutron-

EFFECTS OF LrrHDLOGY, POROSrrY AND SHALLNESSON P- AND S-WAVE ELOCrrlES

97

AGE - - I I I !I E : 2 , ii I 5 f5 : 5 I I I I- - j ij 1 5 I -) - - 5 I E j 1 I g I- - -

FORMATION

T
LEGEND
SH

I,,,

GLAUCONITIC

DOL, SH, SS M Conformity Unconformity

(afler ERCB, 1976)

Fig. 2. Stratigraphic chart of the ages and formations dolomite (DOL) and complex mixtures af these. of inter& The lithologies studied are shale (SH), sandstone (SS), limestOne (LS).

density porosity values were then shifted toward the clean lithology lines by the corresponding correction factor. Porosity values from the Nordegg and Dehital Formations were not corrected for shaliness as the radioactivity in these units may he due to rock fragments rather than shale (I. Hopkins, 1990, pus. comm.). The uncorxcted crossplot porosities agreed with other porosity measurements as described below. Although the Shunda has shale interbeds, uncorrected crossplot porosity values tracked core measurements in the vicinity closely. Neutron-density crossplot porosity values were compared to porosity data from two other sources to check their relia-

bility. The values for all formations (within 1 or 2 porosity percentage

are the same as or close units) to those deter-

mined by a computer-processed interpretation which incorporates a suite of environmentally corrected log cwves. Core reports from the 9-5 and 15.18 wells were examined although core porosity values were not available for the exact depths studied in this analysis. Neutron-density crossplot porosities from nearby units were generally within l or 2 porosity percentage At two depths the FDC porosity than the crossplot corresponded most closely units of available core porosities. values were closer to the core values, but overall the crossplot to core measurements. Porosity

98
values were not taken in shales obtaining meaningful values. The effect of pressure on velocity

s. L.M. MILLER an* R. R. STEWART

due to the difficulty has been examined of in and VS are consistent with the observations of other investi-

gators. Shale does not show a strong linear correlation between V, and VT (I = 0.29) but has an average Vp/Vs ratio of I .X9. VPIVS is quite variable for shales; however, the average value observed here falls within the range used by Minear (1982) and is comparable to the value of I.936 used by Eastwood and Castagna (1983) for modelling. The data have been replotted in Figure 4 to demonstrate the effectiveness of using Vp/Vs and VP to differentiate the pure lithologies in this data set. Although the Vp values for sandstone and shale compressional velocities about 4200 m/s and sandstone and limestone about 5200 m/s, the lithology types the addition of V&4 values. Several complex lithologies Nordegg, Shunda and Detrital overlap overlap at at by

laboratory work (Pickett, 1963; Gregory, 1977: Domenico, 1984; Han et al., 1986; King, 1988; Eherhart-Phillips et ill., 1989). Velocities generally increase rapidly initially but stabilire at higher pressures. The formations examined in this paper are all at depths between 2OCO and 22uO metres, giving an effective stress of about 25 MPa (3600 psi). At this depth, pressure effects have levelled off and should he similar for all the formations. Analysis involved crossplotting of velocity, slowness, velocity ratio, porosity and percentage shale for the selected units in each of the four wells. These plots were examined for trends. We have used single and multivariate linear regression analysis various parameters. to assess the relationship between the In regression analysis the independent

are differentiated

are also examined. The are mixtures of carbonates and limestone 1.75, 1.76 and

variable is assumed to be error-free. This is not the case for porosity or shale data obtained from well logs: however, the analysis is conventionally held to be a valid means of studying velocity dependence on these variables sometimes adjustment differed using reference. depths. (Troutman from All wiredepths and Williams, 1987). Well cuttings log depths line log depths and required geologic referred

and clestics and plot between the sandstone end members with average VplK values of

1.76, respectively (Figure 5). Clearly, complex lithologies can cuuse ambiguities in the interpretation of velocity data. The mixrd lithologies show approximately of the Nordegg, linear relationships Drtrital and Shunda between Vp und V.7 0.91, and 0.92,

a suitable

with correlation coefficients respectively (Figure 6).

of 0.84,

marker, such as coal, for to in this paper are sonic-log

Lithology Figure

effects 3 is a plot of Vp vs VT for sandstone, limestone

0 ss
q

and shale for all four wells. The superimposed lines have slopes of I .9 and 1.6, the conventional Vp/K ratios for limestone and sandstone, respectively. The sandstone data points are scattered around the V,/Vs value of I.6 and have an average VpIK ratio of I .60. The limestone data have an average CD/!/, ratio of 1.89. Within the range of data reprerelated coeffilinearly sented here, VP appears to be approximately to VT for both sandstone and limestone. Correlation

SH LS

%Moo
VP (mls) Fig. 4. v&Q vs v,, for sandstone, limestone and shale. The data from Figure 3 are replotted to demonstrate the separation of

cients (r) are 0.8X and 0.89, respectively. The I/,/V, values for both lithologies and the good correlations between VP

2~1 2.0
1.9

6
D

1.8
1.7 1.5 1.5 l.4 3000 4000 W 5000 (m/s) 6000

0 ss
q SH

2 >

0 Nord
emal Shunda

LS

1
7000

Fig. 3. VP vs Vi for sandstone (SS), limestone (LS) and shale (SH) lithologies. The data are from full-waveform sonic logs from the Medicine River field. Lines of constant Ifp/Vr are superimposed on the data points.

Fig. 5. vplV, vs vJ, for complex lithologies. The Nordegg, Shunda and Detrital are formations with mixed carbonateiclastic silicate lithologies. The superimposed Outlines from Figure 4 show that the V,dvs ratios for mixed rock types plot between the ratios of the component lithologies.

EFFECTS OF LITHOLCZY,

POROSITY AND SHALINESS ON P- AND S-WAVE VELOCITIES

99

The functional relationship between and Vs for different rock types is still open to question. We have considered the VP/V, ratio, which is an average value, and the linear correlation between VD and VS. The significance slope and intercept values obtained from regression of the analy-

VD

best fit was obtained by plotting velocity, rather than slow ness, as the dependent variable. Multivariate linear regrcssion with linear terms produced the following relationships:

sis and how they relate to physical rock properties is a subject for further investigation. Ikwuakor (1988) suggests that the slopes and intercept values derived from linear velocity relationships are better indicators of lithology than VplVs and may also contain other geologic information. However, for small data sets with inherent uncertainty slope in the meavalsurements, accurate ues may be difficult and repeatable to obtain. and intercept

V, (km/s) = 5.30 7.120 - 0.44~ !A (km/s) = 3.16 - 2.620 - 0.38~ V/As = I .68 0.99~ + 0.056~
where B = frdctional K = fractional I = correlation These relationships porosity, shale, and coefficient. are calculated

i-=0.51,

(2)
(3)

r = 0.32, and r = 0.66,

(4)

over

a porosity

range

of 0.04 to 0.14 and a shale range from 0.01 to 0.44. The standard error is about 5 percent for V, and ttr and 2 percent for V,lV,. The standard error of the porosity coefficients is almost 40 percent in the expressions for Vp and V,,lL and 56 percent in the equation for V.v. The large uncertainty in the coefficients suggests that these expressions are more useful for describing trends in the data rather than predicting values. shale coefficient is sometimes itself. However, so that the effect The intercepts The standard error in the as high as the coefficient

Porosity and shale effects

The highest obtained when correlation coefficients for these data are velocity (rather than slowness or traveltime against porosity. Results for the sandin Figure 7. P-wave velocity shows a

difference) is plotted stone data are shown weak value

dependence on porosity. The extracted Vp intercept indicates a sandstone matrix velocity of 5028 m/s lower than the range of 5486 to 5944 m/s given

somewhat

the magnitude of the coefficient is small, on predicted velocities is also small. in equations (2) and (3) are similar to et al. et al.

by Gregory (1977). An unexpected observation is in the S-wave velocity, which shows a very weak dependence on porosity. This differs from the previous studies cited which found S-wave velocities in sandstones to be highly sensitive to variations in porosity. Residual analysis on both data sets suggests that there may be a higher order dependence. However, the number of data points is too limited to warrant extensive statistical manipulation. ity of V, results in an overall decrease The greater sensitivin VplVs (Figure X), a

those obtained by Tosaya and Nur (1982). Castagna (1985). Han et al. (19X6) and Eberhart-Phillips

(1989). The coefficient for the porosity term in equation (2) is also similar in magnitude to those quoted by these workers, but the porosity coefficient in equation (3) is 40 to 60 percent lower, emphasizing again the lack of !A sensitivity to porosity in these data. We see this effect in equation (4), which shows that V,d!A will increase as porosity decreases. These equations also differ from those of the investigators referred to above in that the shale coefficient is lower than the porosity coefficient by about an order of magnitude rather somewhat of velocity than by a factor of 3 or 4. Although porosity is dependent on shale content and shale content indicate in these data, plots a minor effect from

result which also differs from research previously cited. The large scatter and relatively low correlation coefficient of 0.65 indicate caution in interpreting these results. In an attempt to improve regression variables. the fit to the data we have used with porosity and shale fracThe data suggests that porosmultivariate linear tion as independent

ity is weakly dependent on shale content, so that the results should again be viewed with caution. Addition of a shale term improves the correlation somewhat and also increases the intercepts, or predicted sandstone matrix velocities. The

shale. If we use clay fraction rather than shale fraction (where shale is assumed to be composed of 60 percent clay), the coefficient for K increases by about 67 percent in each of these equations.

Fig. 5. V, vs KY for complex lithologies. Approximately linear relationships exist between VP and v,, for the Nordegg, Shunda and Detrital Formations.

Fig. 7. VjJ and Vr vs percent porosity for sandstone. Data are from the Glauconitic and Basal Quartz sandstones. VP shows a weak dependence on porosity in sandstone, decreasing by 15 to 20% as porosity increases from 4 to 14%. V,Tis fairly insensitive to porosity variations.

100 The relative magnitudes that Vs is more sensitive

5. L.M. MLLER of the shale coefficients to the addition of shale indicate than &I.

and R. K. STEWART porous, velocity oil-saturated and a lower 9-5 well has both a lower than the shalier, P-wave tighter,

V,d!A ratio

This results in an apparent increase in Vp/V.t with increasing shaliness (Figure 9). This trend is consistent with the observations of the other researchers cited. Eberhart-Phillips et al. (1989) found that using the square root of clay content improved the fit as it accounted for the large change in velocity observed when only a small amount of clay was present. Use of the square root term here improved the correlation contain too much scatter marginally: however, the data to support a more complex term. of a particin Vp/V.s The Basal

water-saturated interval in the 15.18 well. This separation could be due to a differcnce in porosity, shale, pore fluid, or some combination thereof. The production GOR (gas/oil ratio) for the 9-5 well is about 150 which is quite high for this formation. This suggests that gas might be a factor in reducing Plots indicate

VD and, thus, VIA4 in this well.

of velocity that both vs porosity for the Pekisko

V, and I4 are dependent

limestone on porosity

The data were also analyzed for the response ular formation from well to well. Variations response to changes within the same formation pore fluid should or fxies. in porosity,

(Figure 11). The intercepts represent limestone matrix velocities and are very close to the V, (6259 m/s) and V, (3243 m/s) values for calcite

be attributable

V,Z/VAratio for the limestone

quoted by Domenico (1984). The matrix is about I .9 as predicted

Quartz sandstone is sampled from the 9-5 well (2144 to 214X m) and the 15-1X well (2170 to 2174 m). The portion of the Basal Quartz sampled in the 9-S well averages about 13 percent the 15.18 sampled porosity, 4 percent shale and is oil-saturated. well, the Lower Basal Quartz section which averages about 6 percent porosity, 35 percent In is

but shows little change as porosity increases (Figure 12). This is consistent with findings by Eastwood and Castagna (1983). It is also the response predicted by Kuster-Toksbz modelling in a saturated limestone in which the majority of pores are round, i.e.. have high-aspect ratios (Robertson, 1987). This has pinpoint is likely porosity to be the cast with (secondary porosity the Pekisko with voids which <l/l6

shale and is water-saturated. wells separate distinctly (Figure 10). The siimpled

The data points from the two on a crossplot of VplVs vs Vp interval from the relatively clean,

mm). Velocity and porosity data from the Nordegg, Shun& and Detrital Formations are plotted in Figures I3 through 15, respectively. In each case, both Vp and V, show a dependence on porosity. Both Vp and VS show a similar decrease as porosity increases in the Nordegg Formation.

.=1 1~70 1~65 /1___1 1.60 1% 1.Y)

0 0 0 0 0 0 000 08 0 0 0 8 o.

In the case of the Shunda, V, appears to be slightly more sensitive than VS to the rise in porosity, but the porosity range is very limited. In the Detrital Formation, P-wave velocity decreases more rapidly than S-wave velocity for an overall decrease in V,dVs. This decrease is consistent with the observations for the sandstones but differs from the previously cited results by a number of other researchers.

:: ; >

y = 1.71 1.208-a

R = 0.65

1.45 .,.,.,.,.,.,. 2 4 6

8 10 12 % Porosity

14

I 16

Fig. 8. ViiVs v* percent poro*ity for sandstone. The data are scattered but the velocity ratio shows an apparent tendency to decrease as porosity increases in sand*tone.

CNCLUSI0NS Data that obtained from full-waveform discriminate sonic Average logs between indicate sand-

Vn with

l~y successfully

stone, limestone

and shale lithologies.

Vd!A ratios

1
::::i

m 1.65 c -;I 0 0 a 0 1.60 0 2 1.55 me 8 Oc. 00 o 00 00 0 o 1.50 y=1.55 +*me-3x=0.55 R 1.45 0 10 20 30 40 %SHALE
Fig. 9. V,dVf vs percent shale for sandstone. ,JV., may exhibit a slight trend of increasing V~IC,Ias the shale content Of the sandstone increases.

... . 0
. l

. . .

1.451 4200 4400 4600 4800 5000 5200 :

vp (m/r)

Fig. 10. VP,, vs p for the Basal C)uartz sandstone. The data points from the 9-5 well, which averages about 13% porosity, 4% shale and is oil-saturated. are separated lrom the IS-18 well which averages about 6% porosity, 35% shale and is water-saturated.

EFFECTS OF LtTHclLOGY, POROSfTY AND SHALINESS ON P- AND SWAVE ELOCITIES are 1.60 for sandstones and 1.89 for limestone and shale. Complex lithologies have V&T ratios which plot between the values ambiguities of their component in interpretation. rock types. This

101

Vp is approximately

may cause linearly

correlated with Vs in the lithologies studied here. Seismic velocities in sandstone are affected by variations in porosity and sheliness, with porosity having a stronger effect. Multivariate linear regression results in three relationships which describe the trends observed in the data. These relationships indicate that Vp decreases as porosity is relatively insensitive to increases. I/S in these sandstones changes in porosity, showing only a slight reduction. As a result, the V,l!A ratio decreases with increasing porosity. This observation differs from that quoted by several other investigators, who observed an increase in VDIVT with rising porosity. Porosity has a greater influence on velocity than shaliness by suggest Vs and agrees about an order of magnitude, but our correlations that an increase in shale content will lower V, and cause the Vp04 ratio to rise. The increase in V,d!A with observations by other workers; however. in

response predicted by Kuster-Toksoz modelling for limestones in which the pores tend to be round, which is probably the case for the pinpoint porosity of the Pekisko. The linear regression intercepts from the limestone velocity-vsporosity plots accurately predict calcite matrix velocities.

P-wave and S-wave

velocities

decrease

as porosity

increases in the carbonate/elastic lithologies of the Nordegg, Shunda and Detrital Formations. VP/K decreases as porosity increases in the Detrital but does not demonstrate a porosity trend in either the Nordegg or the Shunda. A number gesting that of trends were visible in these field data, sugP- and S-wave data can contribute valuable infor-

mation about tbe subsurface. In particular, velocities contain information on lithology, porosity and the degree of shaliness.

REFERENCES Anno, PD., 1985.Exploration of the HuntonGroup.AnadarkoBasin, usingshear waves: Presented the53rdAnn. Internat. at Mg., sot. Expl. Geophys., Vegas. Las Castagna, Batrte, M.L. and Eastwood, J.P., ILL., ,985. Relationships between compressional-wave shear-wave and vetocides in clasficsificaterocks:Geophysics 50.571-581,

these data the effect of the shale is substantially smaller. Data from the Pekisko limestone indicate that both V, and Vs decrease as porosity increases but the VW, ratio exhibits mm Y I5264 ,331 R = 0.79 I little trend with rising porosity. This is the

o y = 5651 49x R = 0:4 z E

.C
i g

moo 4000

0
o VP
I

y = 3339 36X
3m . . .

R = 0.75

rami 0

4
% Porosity

I 8
Fig. The one and

j--::-nln

2ooo x Poro*lty

30

Fig, 11. V, and !J, YS percent porosity for the Pekisko limestone. v,, and V. both appear to be dependent on porosity variations: y-inters cepts represent limestone matrix velocities.

13. V,, and VI YS percent Porosity tar the Nordegg Formation. Nordegg is a mixture of sandstone, limestone and cheti and is of the oil-producing zones in the Medicine River field. Both v, VI show a similar degree of dependence on porosity.

A 1.9 ;: ii z h % 1.8 l-----d A y = 1.91 5.868-311 R i 0.24 A A A

z WI0 z 4wo .c
3wo

126X y=5755 R=0.82 A A

y = 3182 57x R r 0.88 A A VP I

2ma4 2

, 4 x

, 5 POrOsfty

I 8

I 10
Shunda Formation. shale stringers. The but both vu and VI with V, responding

Porority

Fig. 12. VpWv YS percent porosity for the Pekisko limestOne. Both V, and Vs show a similar response to Pcrosity variation so there is fidle overall trend for the vpIVr ratio.

Fig. 14. V, and Vr vs percent porosity for the The Shunda is a Mississippian carbonate with range of variation of porosity is very limited, show a linear decrease as porrxity increases slightly more than Vs.

102

s. L.M. MLLLEK and K. K. SlmvAKT

6000 0 II 5000 48X 4000 y=3550 R=0.83 3000 . ~---LA

2000 1 0 10 % Porosity

yi 6739 R=0.98 124x

q n VP I

20

Fig. 15. V,>and VS vs percent porosity for the Detrital. The Detrital refers to rock detritus on top of the Mississippian unconformity. It is composed largely of dolomite in the well from which the data points are taken. The velocity data exhibit a dependence on porosity; the more rapid decline of ,vja results in a decrease in the \vi,:i, ratlo.

Ehcrharr-Phillips. Us. Han. ,l~H lln* XhilCk. M~II.. IW,, Fmplrlcal r&ltimihips among srismic YcICi,y, efkcrive pressure. ,mnrsiry. an* clay CntCllt in iilndirnc: Gcuphysics 54. 82~8). E\lry. K.A., ,984, Cmpari>n ,,I P- n,,d s-wiwc SCiSlniC(lilti,: a new mcthud fur detscting fh iCbCrOirS: Gcuphyaica 49. 1420~143I. pi 1985, Evoluarion uf dirccr lhydlocarbon hdicawrs through cump&on of comprcrsimdl- ad \hcwwwc seismic data: il CilSe s,dy Ol the Myrnam gas field. Aiherta: Genphyrics 50, 37~4X. Garot,a, R., Miwxhid. P. ;m<,Megesan, M., 19x5. Tw~cmpnent acquisi. tim as 2 routine procedure hr recording P-wi(v(r und convcrk~I wiwcs: I. Cm sue. Expl. Ccophys. 2,. 403. Grorgi. D.T., Heilvysegc. R.G.. Chrn. S.T. and Erikien. E.A.. 19X). Application of shcir and coiiiprcssioniil fransit-time data 10 cased hole cartmnarr rrSrrOir evdil,i<,n: ,?,h Frma,in Ea,a,i Symp.. Can. well Logging sot.. Calgary. Gregory. A.R.. 1977, Aspects frock physics from iahoratory an* log data that are Impwranf to irismic inrerprerarion, in seismic srmigraplly appliciltions to hydmcdrh~ cnpkmtim Am. Aasn. Pctr. Gcd mm. 26, 15.41. Ha. D.H.. Nur, A. arKI Murgan. D., 1986. Efkch i~f pomsity and clay ColCllt WUYC &CitiC\ in a;mdsa,ncs: Geophysics 51. ?)32,,~7. Hcslup. A., 1974, Gmlmiiwy Ig ropunse f rhaly sands,orE, ,Slll Arm. Symp.. sot. PRlf. Well Log Analyals. MCAk. Teas. Ikwuakor. KC.. 19X8. C>ilK revisited: pirhlls and new inwprcmion lechniqoes: World Oil 207. 3. 41.46. King, M.S.. Stauffer. M.R., Yam+ H.J.P. fm3 Hajnal. L., IWX. E,lustic~wdve and related prupertiea 01 clamc rocks fmm the Arhahaica hasm. wit~rn chada: can. I. Expl. Cwphys., 24. / I., Ih.

Kithas. B.A., 1976. Lithdogy. gas detection, and rock properties from amustic logging systcmn: 17th Ann. Symp., Sot. Prof. Well Log Analysts. L%KT Kukal. GC. a,,* Hill. Kit. 1986, Lug analysis of&y lume: a c;lluatin of iechriiqucs and assumpiions USC* in an Upper Cretaceoui sand-shale sequence: Z7,h Arm syrnp., s,,c. Pruf. WC,, Lug Andysts, HS,,,. Kuster, G.T Bd TkSOZ. M.N.. 1974. wucirv and atfcnuati of SCiSrniC waves in two-phme media: Part I, Theorefiid formukirioni: Gsophyrics 39, SX,.huh. McCormack. iaD.. Dunhar. ,~A. ;ald Sharp, W.W.. 19x1. A case wily f Waiipraphic inwpretarion using shear and comprciiional ~eiimic data: Gsophysics 49, SW-520. and ~, I)HS. A srratigraphic irlrelprr,a,io I Shear and curn~rcssiur~~l wave beismic data for rhc Pcnnwlv;mi;m Mnrrnu formnfion <ii southexrern New Mexico. in Seismic Srratigrayhy II: Am. AWL Pew Ged, bknl 19. 225.219. Minear, I.W 19x2, ciay mtMs and Bcostic ck>ciliw 57th Ann. Tech. Conf. an* EXhib., sot. PC,,, Fng. fA,ME. NW O,,eans. Narions. I.F.. 1974, Lithdogy and porosiry liom i~~wfi~ rhrar and corm pre~~iimai wave traixit time relationships: lSth Ann. Symp.. SW Prof. Well Lug Ardysts. McAkn. 1SnaS. Pickcir. G.R.. IOh3, Acoustic charxter logs arid their applicirrions in formirfiun evduafion: J. Cm Petr. Rcb. 15,659 667. KldiCh. F.. Kcnddl. C.H.S.C. and Tudd, T.P.. ,984. TX lelatiunahip herwen KOSdC prpEitio an.3 ,hC prrrogrqhic characrer ofcarhriare rocks: Geophysirr 49. ,622 ,636. Kaymrr. L.L... Hun,, E.R. and Gdner. 1,s.. 19X. An improved sonic mnsi, ,ilTw-pi~rsi,y ,mnGx1ll ?Is, A. Symp., sot. Prof. we,, Log Anlysf\. Latiyette. Rohrlmun, J.L>., 14x7. rarhooate porosi,y f,nl S/P ,rae,,imc ratior: Gcophybicr 52. 12X-1254. ~nrham, R.T.. ,982. WV, and litholgy: Geophysics 47. 336.344. Trr Berg, L.I.A.. 1966, Mcdicinc River field, in Century, J.K., Ed., Oil fields of Alberra rupplemeni: Alhcrm Society of Pctmleum Geologists. 7? I. Tokr&. MN.. Cheng. CH. and limur. A.. 1Wh. Vdocitics of seismic waves m poroui rocks: Gmphyrics 41.621-645. Tusaya, C. and Nur. A., 14X*. Effecrs of diapesib and clays on compresrional velocities in rocks: Geophy5. Kcs. Len 9. 5-X. Trou,m;m B.M. 2nd Wiihms, CP,, 19x7. Fi,,i,g straight linei in the earth screncc~. in Six, W.R., Ed.. llse and ahuse of stnfixticni methods in Ihs rarrh scimccb: Sruilia in ~nahcmtical grulogy No. I. Oxford vniv. Pies,. i,lC, 1,1,-128. Witkhs. P.R.. IV66 Medicine River oil kid. irr Century, J.R.. Ed., Oil fields f Albert* WpplcrnrrN Aber,a S&q f R,,<,,c,,, Geologir,s. h?.hi~ Wilkms. R.. Simmos. G. and Caruso. L., 19x4. The lilti )./V. as a discrmunani of cnmposilion fbr ~ikrws limesrunes: Geophysics 49. IXSOI X6,,. Wyllie. M.K.J., Gregory, A.K. and Cardncr, LW., 1%h. Elasric wave veloci~ IiCI in hctcrgcncus and pomus media: Geophysics 2,. 41.7

APPENDIX -.
Lithology Sandstone Sandstone S.WldEfOe

Formation

well bzition

Depthjm,

limestooe Limestone Limestone Limestone

%% ShWdil Sh, Oeha, Lmrita, Detrifal Detrital