analysis of p-p and p-sv seismic data from lousana, alberta · 2014-04-09 ·...

P-PandP-SVseismicdatafromLousana

Analysis of P-P and P-SV seismic data from Lousana,Alberta

Susan L.M. Miller, Mark P. Harrison*, Don C. Lawton, Robert R.Stewart, and Ken J. Szata**

ABSTRACT

Two orthogonal three-component seismic lines were shot by Unocal CanadaLtd. in January, 1987, over the Nisku Lousana Field in central Alberta. The purpose ofthe survey was to investigate a Nisku patch reef thought to be separated from the Niskushelf to the east by an anhydrite basin. These data have been reprocessed and arecurrently being analyzed and used to develop methods for multicomponent seismic dataanalysis. The data are of overall good quality and major events were confidentlycorrelated between the P-P and P-SV sections using offset synthetic seismograms. P-SV offset synthetic models were also used to extract interval Vp/Vs values from the P-SV data.

Forward log-based offset P-P and P-SV modelling shows character, isochron,and Vp/Vs variations associated with lithology and porosity changes between reservoirand non-reservoir rock in the Nisku. The modelling results indicate thatmulticomponent seismic analysis is applicable in carbonate settings, even when thereservoir intervals are relatively thin (23 m). To date, we have had difficulty applyingthe modelling results to this data set because of multiple contamination in the zone ofinterest and work is ongoing to resolve this issue. Full-waveform modelling is plannedto better understand the multiple activity on both components. However, there areobservable anomalies on the data which are currently under investigation.

Multicomponent recording provides additional seismic measurements of thesubsurface to assist in developing an accurate geological model. Rock properties whichcan be extracted from elastic-wave data, such as Vp/Vs, reduce the uncertainty inpredictions about mineralogy, porosity, and reservoir fluid type. Joint interpretation ofP-P and P-SV data was helpful in horizon picking and in providing feedback on thevalidity of the interpretation. The interpretation techniques described in this paper canbe usefully applied to other areas, including other carbonate plays.

INTRODUCTION

In 1987, Unocal acquired two three-component lines across the Lousana Field.Unocal donated these data to the CREWES Project, and in 1993 they were reprocessedusing IT&A's software and converted-wave code developed in CREWES. Morerecently, the data were reprocessed again using the ProMAX processing package andthe same converted-wave algorithms. The reprocessed sections exhibit significantlybroader bandwidth on both the vertical and radial components. Modelling andinterpretive studies are ongoing, with the following objectives:

* SensorGeophysicalLtd.,Calgary,Alberta**ChevronCanadaResources,Calgary,Alberta

CREWESResearchReport Volume6 (1994) 7-1

Miller,Harrison,Lawton,Stewartand Szata

• identify character, amplitude, or time interval variations which may beassociated with productive zones, and attempt to calibrate these variations tochanges in geology through forward modeling,

• use full-waveform modelling to study the signal interference caused byinterbed multi-converted multiples,

• develop and test techniques for analyzing multicomponent data,• evaluate the benefit of multicomponent versus conventional recording.

Benefits of multicomponent recording

This discussion concentrates on the benefits of multicomponent recording as itapplies to data interpretation. Recording multicomponent data provides complementaryimages of the subsurface for a relatively small additional cost, as conventional sourcesand receiver geometries are employed. Due to the difference in travel path, wavelength,and reflectivity, P-SV seismic sections may exhibit geologically significant changes inamplitude or character which are not apparent on conventional P-wave data. Horizonsmay be better imaged on one of the components because of different multiple paths andwavelet interference effects.

This additional seismic measurement of subsurface rock properties can reducethe uncertainty in predictions about mineralogy, porosity, and reservoir fluid type. TheP-SV seismic data can be used in conjunction with the P-P data to determine other rockproperties, such as Vp/Vs (or similarly, Poisson's ratio). Vp/Vs is sensitive to changesin a number of rock characteristics, including lithology, porosity, pore shape, and porefluid (e.g. Pickett, 1963; Nations, 1974; Tatham, 1982; Eastwood and Castagna, 1983;Miller and Stewart, 1990). A number of studies, some of which are described below,have indicated that multicomponent seismic technology has practical applications incarbonate settings (Rafavich et al., 1984; Wilkens et al., 1984; Goldberg and Gant,1988; Georgi et al., 1989; Wang et al., 1991).

Robertson (1987) interpreted porosity from seismic data by correlating anincrease in porosity with an increase in Vp/Vs. He based his interpretation on the modelof Kuster and Toks6z (1974), which incorporates pore aspect ratio as a factor invelocity response. According to this model, Vp/Vs will rise as porosity increases in abrine-saturated carbonate with flat pores, but will decrease slightly if the pores tend tohave a higher aspect ratio (are rounder). If the carbonate is gas saturated, Vp/Vs willdrop sharply as porosity increases if pores are flat, and drop slightly if pores arerounder.

Multicomponent data have been used successfully to differentiate tight limestonefrom reservoir dolomite in the Scipio Trend in Michigan. Pardus and others (1991)mapped the variation in the Vp/Vs ratio across the interval of interest on P- and S-waveseismic data and related it to the ratio of limestone to dolomite. Limestone tends to aVp/Vs value of 1.9, dolomite of 1.8. Petrophysical studies suggest that anhydrite alsohas a higher Vp/Vs ratio than dolomite and thus might be used in a similar manner totrack dolomite/anhydrite variations (Miller, 1992).

Finally, having two or more seismic measurements of the same geology givesthe interpreter additional hard data to incorporate into the geological model. There isanother data set to work with in areas where the data quality is poor or the interpretationis unclear. The P-SV section can be used to guide and constrain conventional datainterpretation. Joint horizon interpretation offers the interpreter feedback in the form ofthe resultant interval Vp/Vs values. Vp/Vs values which lie outside the range expected

7-2 CREWESResearchReport Volume6 (1994)


due to geological variations indicate mispicks, whereas reasonable values increaseconfidence in the interpretation.

Methods for multicomponent analysis

This paper discusses several techniques which are useful in the analysis ofmulticomponent seismic data. These can be summarized as:

• correlation of vertical and radial components through P-P and P-SV log-based offset synthetic modelling,

• matching P-SV offset synthetic seismograms to P-SV stacked seismic dataand iteratively adjusting interval Vp/Vs values on the model to tie the data,

• workstation interpretation of horizons on P-P and P-SV components andcalculation of interval Vp/Vs values using P-P and P-SV isochrons,

• log-based forward modelling to determine amplitude, character, and travel-time response of each component to a variety of geological settings,

• use of this model to determine the expected variation in Vp/Vs acrossdifferent time intervals for a range of rock properties.

GEOLOGY

The Lousana field is located west of the Fenn-Big Valley and Stettler Oil Fieldsin Township 36, Range 21, West of the 4th Meridian, in central Alberta (Figure 1). Thetarget is a Nisku dolomite buildup which is separated from the Nisku carbonate shelf tothe east by an anhydrite basin, which forms the seal for the reef.

The Nisku is an Upper Devonian Formation in the Winterburn Group, whichconformably overlies the shales and carbonates of the Woodbend Group (Figure 2).Immediately underlying the Nisku is the Ireton Formation, a calcareous shale. Theinitial deposition of the Nisku occurred during a relative rise in sea level. Theantecedent topography of the Leduc reef complexes served as sites for deposition of theNisku carbonates, which was concentrated in shallow waters. Deposition was restrictedin deep-water areas to pinnacle reefs and small carbonate outliers (Stoakes, 1992).

During a later, regressive phase of deposition, sea water circulation wasrestricted by the growth of the shelf margin in the northwest, the current location of theWest Pembina Field. Progressive evaporation resulted in the hypersaline deposits of anupper evaporitic unit in the Nisku. Brining upward cycles of dolomudstones andanhydrites infilled the lows between isolated reefs and platforms and capped thesuccession. Basin infilling was completed by the deposition of the silts and shales ofthe overlying Calmar Formation (Andrichuk and Wonfor, 1954).

The nearby Fenn West, Fenn-Big Valley, and Stettler Fields are structural trapsformed by the porous dolomitized Nisku shelf carbonates draping over Leduc reefs(Rennie et al., 1989). There is no underlying Leduc at Lousana, which is west of theLeduc edge, and the stratigraphic trap is a biohermal buildup of dolomite, perhaps oneof the carbonate outliers described by Stoakes (1992). The hypersaline deposits of theupper phase of the Nisku filled the basin between the shelf and the buildup at Lousana,and sealed the trap. The Nisku is 50 - 60 m thick in this area and the overlying CalmarFormation is generally only about 3 m thick.

The Nisku at Lousana is a dolomite oil reservoir with up to about 25 m ofporosity in producing wells. The two oil wells in the field, 16-19-36-21W4 and 2-30-


Miller, Harrison, Lawton, Stewartand Szata

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Nisku _ __ ? /IBasin ,_Jl,ooe ?@'X: '°

u,,du\ +

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Carbonate edge R21W4 Scale: 1:50,000

10 m porosity contour

FIG. 1. Shotpoint map of the Lousana survey showing seismic lines EKW-00I andEKW-002, cross-section A-A', and well control. Carbonate edge and 10 m porositycontours arc approximate only.

DEVONIAN STRATIGRAPHY

WABAMUN WABAMUNGROUP

GRAMINIA2_CALMAR

WINTERBURNGROUP NISKU

Z

°"' WOODBEND IRETONa: GROUP LEDI,iJa.a.

DUVERNAY

COOKING LAKE

-BEAVERHILL WATERWAYSLAKE GROUP

SWANHILLS _

FIG.2. Stratigraphic chart of the Upper Devonian succession in central Alberta.(after Stoakes, 1979)

7-4 CREWES ResearchReport Volume6 (1994)


36-21W4, have about 25 m of porosity and 10 m of pay. The 16-19 well went onproduction in 1960 and has produced about 78,000 m3 of oil. The 2-30 well hasproduced over 58,000 m3 of oil since it went on production in 1962. The porosity isprimarily vuggy, ranging to intergranular, and averages about 10% in the twoproducing wells. Dry holes in the field are either tight or wet carbonate, or massiveanhydrite at the Nisku level. There is also gas production from the Viking and otherLower Cretaceous formations in this area.

Geological cross-section A-A', shown in Figure 3, roughly parallels line EKW-002, and is built from well logs from each of the shelf, basin, and reef environments.

DATA ACQUISITION AND PROCESSING

Data acquisition and processing are described in greater detail by Miller et al.(1993). The field acquisition parameters are summarized in Table 1. For both lines thegeophones were oriented at 45 degrees to the line direction, giving comparable amountsof shear energy on H1 and H2. The H1 and H2 components were rotated into radialand transverse directions. Vertical-, radial-, and transverse-component gathers areshown in Figures 4, 5, and 6 respectively for one sourcepoint from each of the twolines. A time-variant gain function followed by individual trace-balance scaling hasbeen applied to these field records. The radial-component records are seen to havegood signal strength, with events that roughly correspond to those on the verticalcomponent. There is little signal evident on the transverse component.

Table 1. Field acquisition and recording parameters for the Lousana survey.

Energysource dynamiteSource pattern, EKW-001 single hole, 2 kg at 18 mSource pattern, EKW-002 4 holes, 0.5 kg charges at 5 mAmplifiertype 2 - SercelSN348Numberof channels 2 x 240Samplerate 2msRecordingfilter Out-125HzNotchfilter OutGeophones per group 6 spread over 33 mType of geophones used OYO 3-C, 10 HzNumber of groups recorded 110Groupinterval 33mNormal source interval 66 mNominalCMPfold 27Spread 1799-16.5-SP-16.5-1799m

The data were processed in 1993 on the IT&A system, as reported in Miller etal. (1993). Subsequent to that report, tests for the removal of multiples were performedusing the parabolic radon transform method (Hampson, 1986) and predictivedeconvolution. The results for both methods were inconsistent, with some portions ofthe data showing slight improvement while others were degraded.

The data have since been fully reprocessed on the ProMAX system usingsimilar flows to the IT&A processing for both the vertical and radial components.Vertical and radial component flows are outlined in Figures 7 and 8 respectively. Thefinal migrated stacked sections are shown in Figures 9-12.



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P-Pand P-SV seismic data from Lousana

I 21 41 GI 81 I01 121 141 181 181 2010,00

0.215

0.50

0.75

LO0

1.25

N1.75

2.76

3,00

FIG. 4. Vertical-component records from line EKW-00I, shotpoint 181.5 (left) andline EKW-002, shotpoint 185.5 (right).

1 il 41 61 81 101 _ I_.1 Im 181 _10.01

o.'1!

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3,2,!

3.11

!11

FIG. 5. Radial-component records from line EKW-001, shotpoint 181.5 (left) and lineEKW-002, shotpoint 185.5 (right).

CREWES ResearchReport Volume6 (1994) 7-7


I 2[ 41 61 _ 1Ol I_ 141 161 NI 2010.I0

0J5

0A0

0.15

1,00

1.2S

1.80

A

UJ 2.,om

_" 2.15

2.10

2.15

3.10

8.10

4,10

FIG. 6. Transverse-component records from line EKW-001, shotpoint 181.5 (left) andline EKW-002, shotpoint 185.5 (right).



Demultiplex It2 spreading gain

Surface-consistent deconvolution

Source, receiver & CDP components80 ms operator, 0.1% prewhitening

Elevation & refraction statics I

IInitial velocity analysis

Automatic surface-consistent statics I

F700-1800 ms window, maximum 20 ms shift

Velocity analysis INormal moveout removal

Zero-phase spectral whitening4/8 - 100/110Hz500 ms AGC

I Apply mute function IStack by CMP, offsets 0-2640 m

Zero-phase spectral whitening4/8 - 100/110Hz500 ms AGC

I f-x prediction filterPhase-shift migration

Trace equilization

10/14-70/80 Hz bandpass filter

FIG. 7. Processing flow for vertical-component data.



I Demultiplex ]45 deg geophone rotation I Velocity analysis I

Normal moveout removalI IReversepolarityof leadingspread

Zero-phasespectralwhitening

I 6/10-45/55 Hz

Apply final P-P static solution 700 ms AGC

Applyreceiver-stackstatics

[ Apply mute function

Initial velocity analysis I I Compute ACP trim statics

t z.5spreading gain ] Depth-variant CCP stack [Trace equilization Offsets33-2640m

Normal moveout removal Zero-phase spectral whitening

Shot-mode f-k filter 2/6-50/60 HzRestore normalmoveout 700 ms AGC

Surface-consistent deconvolution ] f-x prediction filter ]

Source,receiver&CDPcomponents

120ms operator, 0.1%prewhitening Phase-shift migration

Modified velocity function

Asymptotic-conversion-point binning ]

Vp/Vs value of 2.29 I ] 4/8-40/50 Hz bandpass filter ]

TraceequilizationI II Automatic surface-consistent statics I

1100-2300 ms window, maximum 20 ms shift

I

FIG. 8. Processing flow for radial-component data.

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sw NE

l--

FIG. 9. Migrated P-P stacked section for line EKW-001 after reprocessing.

NW 16"19 12"20 seID|

EI=

FIG. 10. Migrated P-P stacked section for line EKW-002 after reprocessing. The wellsshown are used in correlation and modelling.


Miller, Harrison, Lawton, Stewart, and Szata

$W N:E

i=

FIG. l 1. Migrated P-SV stacked section for line EKW-O01 after reprocessing.

,M_ _ 16-19 12-20 seIQZ

20a

xal

I-- a,**l

_na

2_m

2z_u

2,L01

FIG. 12. Migrated P-SV stacked section for line EKW-002 after reprocessing. Thewells shown are used in correlation and modelling.

7-12 CREWES Research Rel_ort Volume6 (1994)


For both components, the most recently processed data has significantlybroader bandwidth. There is also improved imaging of some of the reflectors, mostnotably the Wabamun at about 1775 ms. Figure 13 shows a comparison of 1993 and1994 processing on the radial component of line EKW-002.

EKW-_2 Radial18B3 EKW-_P_2Ri_zl I_B419B i_ _BG i|_ LT_ 1_ _P IgS LgO iJ5 iBO iTS L_

2 ZTSZ_ ZTZZ?O r _4 Z_tZSSZSSZ] Z9 CP Z_rZ_ Zqtzqe Z_ zn 3_

FIG. 13. Comparison of the radial component stacked section from line EKW-002processed in 1993 (left) to the same data reprocessed in 1994 (right). In addition to thebroader bandwidth, note the improved imaging of the Wabamun horizon, the peak atabout 1775 ms. The peak at 1900 ms is the Nisku event.

MODELLING AND INTERPRETATION

Correlation of P-P and P-SV sections

Correlation between P-P and P-SV events was initially done on the IT&A-processed data, and did not need to be repeated for the data reprocessed on ProMAX.Thus, the figures in this section show data from the 1993 processing, but the techniqueapplies to any data set.

Horizons were interpreted on the vertical (P-P) component in the conventionalmanner of matching a synthetic seismogram created from a sonic log to the seismicdata. The data shown use the polarity convention that a peak represents a positiveincrease in acoustic impedance. The P-wave offset synthetic seismogram, from 16-19-36-21W4, matches the seismic very well through most of the section (Figure 14).However, the Wabamun, a high amplitude peak on the synthetic seismogram, is poorlyimaged on the actual data. This is mostly likely due to interference from short pathinterbed multiples, probably originating in the coal beds of the Mannville Formation.This multiple energy also appears to interfere with the primary reflections from deeperin the section. A mistie occurs at the Nisku, causing uncertainty in the seismic pick forthis horizon. Problems with the data quality below the Mississippian.motivated thereprocessing of the data. The Wabamun event was improved on both components,particularly the P-SV data, but the Nisku mistie is still unresolved.


Miller,Harrison,Lawton,StewartandSzata

There are no full-waveform sonic logs or 3-C VSPs available from this area toserve as control for the P-SV, or radial component, interpretation. Therefore, offset P-P and P-SV synthetic seismograms were created from the 16-19 well, using the offsetsynthetic modelling program described by Lawton et al. (1992). Ricker wavelets wereused, with a central frequency of 30 Hz for the P-P seismogram and 15 Hz for the P-SV seismogram. The offsets are from 0 to 1650 m, with a group interval of 66 m.Normal moveout corrections and mutes were applied prior to stacking the offset traces.For the initial correlation, the P-SV seismogram was calculated using the 16-19 soniccurve for the P-wave velocities and assuming a constant Vp/Vs of 2.00.

Our polarity convention is that a peak on the P-SV section represents anincrease in acoustic impedance, as do peaks on the P-P data. Because both syntheticseismograms were created from the same depth model, the correlation between theoffset synthetic stacks was straightforward. The P-SV synthetic seismogram was thenused to identify the events on the P-SV seismic section (Figure 15).

Vp/Vs through offset synthetic modelling

Figure 15 indicates a time-variant shift between the events on the P-SVsynthetic stack and the P-SV seismic data. This occurs because the synthetic model wascreated using a constant Vp/Vs value, whereas Vp/Vs varies with depth in the earth.The interval Vp/Vs can be adjusted to stretch or squeeze the synthetic stack in a time-variant manner such that the events line up with those on the data. To determine thecorrect interval Vp/Vs, a suite of P-SV offset synthetic stacks was generated usingconstant Vp/Vs values ranging from 1.60 to 2.20 in steps of 0.05 (Figure 16). Vp/Vswas taken from the synthetic stack which best fit the stacked section across a giveninterval. From this analysis, an S-wave transit time curve was generated from the P-wave sonic curve and the depth-variant Vp/Vs values. The final P-SV offset syntheticseismogram was generated using both P- and S-wave sonic curves and properly ties theP-SV data (Figure 17). The resultant interval Vp/Vs is interesting in that it is quite lowin the Nisku to Cooking Lake interval, although the mistie at the Nisku horizon callsinto question the validity of this result. Future plans include repeating this analysis onthe newly reprocessed data.

Forward P-P and P-SV modelling

Well log curves were modified to simulate a variety of geologic conditions inthe Nisku formation. The base P-wave sonic curve was from the 12-20-36-24W4 wellwhich is on line EKW-002 (S.P. 175) and matched the P-wave seismic data reasonablywell. The 12-20 well is in the basin with anhydrite at the Nisku level. This well onlypenetrated to the Ireton Fm, and so a composite curve was made using the 16-19-36-24W5 sonic log from the Ireton to the base at the Beaverhill Lake Fm. Since there wasno density log available, the density log from 8-20-36-24W5 was modified to match the12-20 depths. The values in Table 2 were used in the Nisku Formation to simulateanhydrite, tight limestone, tight dolomite, and dolomite with 10% porosity, both oil-filled and gas-filled. The values were obtained from well logs from the Lousana field,the Mobil Davey well at 3-13-34-29W4 (see Miller, 1992), Schlumberger (1989), andpetrophysical relationships such as the time average equation (Wyllie et al., 1956). TheNisku porosity is primarily vuggy, so Roberston's Kuster-Toks6z models (1987) forrounded pores in dolomite were used to estimate the variation in Vp/Vs with porosityand pore fluid.


P-P and P-SV seismic data from Lousana

P.P OffSet seismogram Shot point s

L91 tgB 186 183 tO1 175 |78 173 L/! 168 186 863 ;81 15B 156 153 ;51 148 146 143 141"

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....................... . ....... ,..._, ,,.,_..... ...... •.... _ .......... _,._._;_;,, ,,:.... ,... ,.:,

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................ ;i_'_ ''lll;_l't:t '"lil"_"lililllmltllllll''_'tiltiilllltllliil''''';i_'illlllililllliiil 13801380

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FIG. 14. Correlation of P-P offset synthetic seismogram and synthetic stack with P-Pseismic data. The synthetic is generated from the 16-19 well, and spliced into lineEKW-002 at S.P. 191. The tie is good except for the Wabamun and the mistie at theNisku. The seismic data were processed using IT&A software.

Shotpoint_

Constant Vii/Vii2.0 191 lOS 186 183 181 178 176 173 171 ;68 108 16_ tO1

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................. _ _I,,_,,_,_,,*,_ ' 708

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130R .............. " _i '_' _t_ ii, . II . " I,.,,,,.)**,,I,l_l_t)_)l)l ii ;ttP,I, wt i................... ll'll'tlr ........... ;;_,_'lllilliltlllLli_tI IIit 2_08

EKW-002 Radial

FIG. 15. The /'-/7 seismogram is first tied to the P-SV seismogram, which wasgenerated using the P-wave sonic log from 16-19 and a constant Vp/Vs of 2.0. It isplotted at 2/3 the scale of the P-P seismogram. The P-SV seismogram is then tied to theP-SV data at shotpoint 191. The correlation is straight forward, but there are timemisties due to the assumption of constant Vp/Vs. Processing was on IT&A's software.

CREWESResearch Re_ort Volume6 (1994) 7-15

Miller, Harrison, Lawton, Stewart and Szata

vp/vgOffset (m)

0 528 1056 1584 ,0 1.6 1.7 1.8 1.9 2.0 2.1 2,2

m ')DPPJp_+_'_pl'_ill )+i! 'HDt _ --

+...,.,.',.,,,i,,,,,i,',i I i- ll+l+i "

8_e )l/lltll'lll'l'lll'l'llll ItilL '1'1'11

--- _'"""""'"''""' i'i' '- ,- ;;_5- , +_.-.,,nnv,,,. _..... '--'"'/L.......... ',k,.. ii P _ p _p r__

++....... ,+++.MIsslsslpptan -- ---"-'--" ....

"'PPPPPPPPPPPPPHq' H'H';'_, PI P tttt liD' Inn, )))l I,I,_!

Wabamun '" : liB, I_-

l_OgOI |, I'_;tt'(t';l_ll;l('_ I ) ) I

coo+,+ fl????:li/':, r I'1//

P-P offset seismogram P-SV offset seismograms

FIG. 16. The P-P offset synthetic seismogram and stack is compared to a series of P-SV offset synthetic stacks. P-SV stacks were generated using the P-wave sonic logfrom 16-19 and constant Vp/Vs values ranging from 1.60 to 2.20 in steps of 0.05;every other one is shown here. The seismogram stretches in time as Vp/Vs is increased.

P-SV offBe! lellmogrllm ShotpoIntl191 _..=_J1_6 t_3 l_l 178 _76 173 171 168 166 t63 t6t 158 $5e t53 LSL:4_ l_ 343 141

v.,t._, v_v. ,,:,-'._...,..._,..,.+-,::, .;-_+...;::_,_:-:'-+_",,:;_ :','_.;. ': :,, "",;+:'_'_.•__-.-.... ;_:_:.._,.,,.,___.._.., .., ,' ,'d..'_;.,.._._._...+__-.:,_-,u-._= so.

.'"_)_' "" _l.l_q__Ik "'._'"-..... •..... '-- " ;! t,mt"_-._ _+ _2J'n'_E_l_el_- 7_

!.,li Jill II ill) .'.-,,,,_ ,._;..+.,_' _,,...._,,._,.,._'._::__..,;kn_i_'_._._q_,,_-=ee; • .. , IlK ,L ., , ..... r_r,"7 .... '11,'ff.__...... , " " . '-' -"kE -_ ..... _--q-I_._":: , d', " .,._"+_."_ .... ,._, 4,1-- , I,_ ,..+,, ,t;_,_,,

))L_t*l)) k+It'.'rl_li.111,111111 + i_l__ ++'+" ' i*, I ..k_'-."_- _

• - '"' +_" ""kHk_--'"'+''; "- _+_.' _lt'" " .,.L;Ptli+m_ip_l.lk._T;T,,._],vr_p_iTl _ t............... "'"'" _,_p)+ffmv , m,l_+|_,. +iL_m,..., tE,,+,;iilt+.-.. - • ., ;_ " , .-:_._II_+I III llllllllll)lI)lll IIII ,., " ',...-", .... h,_,,,, ' .... '

V_ldh(I ili)[_" ........... ' I "_ i+_Hlllll)llN_l_l_i;)ll li_l_ll_+iit+l+li)lql_N_)bil''*b'l+'l'''i_tpllll_ll_IPlli''+.+_

............ mllp---.--ww,m-_l, peel,l,.,,,- i.) +L .- .... :- +- FIT.. ""+ ,,,_r Ih(: "'."_..+L+-::3-'C_. ....... ,r , _ ' _ _'_"_ 21_eCo_nng _ko ,_it, t,ll [.:_L ............ . ........ _l_'_P lk:''; -+._;,,,.,,'h,,,l,_h,,,,,l''_ _m ...... ,' ....... *_i,l@L" -'-_)l

+_.ol+.l, ...... """;::_l_41111_$_Ll_rlPI)l tYp_lq Ilulllls_ ILlllllllllll_ :?_.aEKW-Q02 Radllll

FIG. 17. P-SV synthetic stacks from Figure 16 were compared to the P-SV data todetermine optimum intervalVp/Vs values. The interval Vp/Vs values were used togenerate a new S-wave sonic curve for input to the P-SV synthetic seismogram. Theresultant P-SV seismogram shown here ties the data correctly. In this case, the mistie atthe Nisku (Figure 14) still causes some uncertainty in the results.

7-16 CREWES Research Report Volume6 (19941


Table 2. Rock property values used in P-P and P-SV offset synthetic models.

A tp (_ts/ft) A ts (_ts/ft) Vp/Vs density (g/cm3)[Its/m] [_s/m] [kg/m3]

Anhydrite 50 [164] 1001328] 2.00 2.96 [2960]Tightlimestone 47 [154] 90 [295] 1.90 2.70 [2700]Tightdolomite 43 [141] 77 [253] 1.80 2.85 [2850]Dolomite:10% 57 [187] 1001328] 1.75 2.65 [2650]porosity - oil filledDolomite:10% 65 [213] 100 [328] 1.55 2.56 [2560]porosity - gas filled

The Nisku interval is 162 ft (50 m) thick, from 5828 to 5990 ft (1776-1826 m).Two porosity thicknesses were simulated: 75 ft (23 m) from 5900 to 5975 ft (1798-1821 m) as is found in the 16-19 well, and 125 ft (38 m) from 5850-5975 ft (1783-1821 m) where depths are relative to kelly bushing. The porosity was overlain byanhydrite and underlain by tight dolomite within the Nisku. All the logs are identicaloutside of the Nisku formation.

The P-P model consisted of 25 receivers with 66 m spacing, with a far offset of1650 m. Because of phase changes at far offsets, the spread was reduced for the P-SVmodel to 22 receivers at 66 m spacing for a far offset of 1452 m. A 40 Hz zero-phaseRicker wavelet was used for the P-P model and a 25 Hz zero-phase Ricker for the P-SV model to best match the reprocessed data.

The offset synthetic stacks are shown for the P-P case in Figure 18 and the P-SV case in Figure 19. Both components exhibit changes at the Nisku level withchanges in lithology, porosity, and pore fluid. When porosity is introduced, a troughfollowed by a peak develops below the peak at the top of the Nisku, indicating porosityand the base of porosity respectively. The amplitude of this lower peak brightens eitherby thickening the porosity from 23 m to 38 m or by the replacement of oil with gas.The modelled P-P response is similar for anhydrite and tight limestone: a tight doublet.A slight difference in the character of the doublet is evident on the P-SV model. In tightdolomite a single broad peak/trough pair is evident on both models.

Horizons were picked on both models and P-P and P-SV isochrons used tocalculate Vp/Vs values across several time intervals for each stack. For converted-wavedata, Vp/Vs is equivalent to (2Is/Ip) -1, where Is and Ip are the P-SV and P-P intervaltransit times respectively (Garotta, 1987). Horizons were chosen which bracketed theNisku and can be readily identified on the real data. Figures 20 and 21 show Vp/Vsvariations across six intervals with the following thicknesses:

Mississippian- CookingLake: 1906ft 581 mWabamun- CookingLake: 1484ft 452mWabamun salt marker - Cooking Lake: 1076 ft 328 mMississippian- Ireton trough: 1200ft 366 mWabamun- Iretontrough: 778ft 237mWabamun salt marker - Ireton trough: 370 ft 113m

There are several observations to note from these plots. Vp/Vs varies withlithology, porosity, and pore fluid, with the degree of variation diminishing as thethickness of the interval increases. The general trend is for Vp/Vs to decrease to theright as the model changes from nonreservoir rocks to reservoir rocks. Thickening theporous zone from 23 m to 38 m decreases Vp/Vs more than replacing the oil with gas.



(h)

Ck¢

FIG. 18. P-P model results showing (a) offset synthetic seismogram for anhydrite, andoffset synthetic stacks for (b) anhydrite; (c) tight limestone; (d) tight dolomite; (e) 23 moil-filled porous dolomite; (f) 38 m oil-filled porous dolomite; (g) 23 m gas-filledporous dolomite; (h) 38 m gas-filled porous dolomite.

Miss'

Wab,

Salt.Nisku

Ireton

FIG. 19. P-SV model results for the same geological settings as in Figure 18.

7-18 CREWESResearch Report Volume6 (1994)


1.84

1.82

1.76. -- Miss - Ckg Lk m,& Wab - CkgLk

o Salt - Ckg Lk1.74, I . I • I •

(b) (c) (d) (e) (f) (g) (h)

FIG. 20. Plot of Vp/Vs measured across intervals on the model data in Figures 18 &19. The lower horizon is the Cooking Lake (Ckg Lk) and the upper horizons are theMississippian (Miss), Wabamun (Wab), and Wabamun salt marker (Salt). The Niskusimulates (b) anhydrite; (c) tight limestone; (d) tight dolomite; (e) 23 m oil-filled porousdolomite; (f) 38 m oil-filled porous dolomite; (g) 23 m gas-filled porous dolomite; (h)38 m gas-filled porous dolomite.

2.00., -- Miss- Ireton

1.95. ,t Wab - Ireton

. \ o Salt- Ireton[

1.90.. _

1.70'

1.65'

1.60'

(b) (c) (d) (e) (f) (g) (h)

FIG. 21. Plot of Vp/Vs measured across three more intervals on the model data withthe Ireton as the lower horizon. The upper horizons and the geological settings are thesame as in Figure 20.



If one of the bounding horizons is too close to the zone of interest, waveletinterference from that zone may affect the picks and cause unexpected results. This isprobably the case for the sharp decrease in Vp/Vs which occurs for tight dolomite whenthe bottom horizon is the Ireton, which underlies the Nisku. This may also be the causeof the increase in Vp/Vs across the Wabamun salt to Cooking Lake interval when gasreplaces oil with 23 m of porosity. Using this salt as the top marker and the Ireton asthe base marker produces the thinnest interval and thus the largest variations in Vp/Vs,however the values may be in error due to wavelet interference.

Our experience is that changes of 0.05 in Vp/Vs are observable on seismicsections. The intervals which use the Ireton as the lower bounding horizon show atleast this much change between the anhydrite and 23 m of porous dolomite (as occursin the oil wells). This suggests that the difference may be detectable on seismic data,although possible wavelet interference from the Nisku should be considered. Thegreatest contrast for all intervals occurs between tight anhydrite or limestone and the 38m thick gas-filled porous dolomite. This effect might be observable even in oil plays ifthe gas to oil ratio is high enough. Gas saturation as low as 5-10% will result in a sharpdecrease in Vp/Vs (Gregory, 1977). The modelling results indicate that Vp/Vs analysisis applicable in carbonate settings, even when the reservoir intervals are relatively thin(23 m) and comprise only 15-20% of the total measured interval.

Workstation interpretation

The reprocessed migrated sections of both components were loaded onto theLandmark workstation and the correlated events interpreted on both lines. Interpretationof the new data is in progress, but P-P and P-SV interpretations to date for line EKW-002 are shown in Figure 22. The P-SV data are plotted at 2/3 the scale of the P-P data,and the excellent tie between the two components is evident.

Isochrons and Vp/Vs ratios were calculated for several intervals between theinterpreted horizons. If the event correlations between the components are accurate, thedimensionless ratio will be free of effects due to depth or thickness variations, as thesewill affect both components equally. Variations may be caused by factors such aschanges in lithology, porosity, pore fluid, and other formation characteristics (Tathamand McCormack, 1991 ).

Figure 23 shows Vp/Vs along line EKW-002 for two intervals. The top curve isthe interval Vp/Vs for the portion of the Cretaceous section from the Second WhiteSpecks to the Mannville. The high Vp/Vs values from about 2.2 to 2.4 are consistentwith a shaley section. By contrast, the Vp/Vs values from an interval within thePaleozoic, the Mississippian to the Nisku, average about 1.8, a reasonable value forcarbonate rocks. In the Paleozoic interval, Vp/Vs decreases to about 1.6 betweenshotpoints 182 and 202. Examination of the isochrons shows a thickening of theisochron on the P-P data and a thinning on the P-SV data, both contributing to theoverall drop in Vp/Vs. The 16-19 well at shotpoint 191 is located on a structural high,suggesting that the P-SV time structure is consistent with the geological structure.Pushdown of the Nisku horizon on the P-P section may be a velocity effect caused byfeatures in the shallower part of the section. This Vp/Vs anomaly occurs in the sectionimmediately above, but not including, the Nisku reservoir. With further analysis wehope to determine if it is related to the dolomitic buildup in the underlying section.

To date, we have been unable to measure Vp/Vs across an interval whichincludes the Nisku reservoir. Several of the horizons which bound the Nisku have beendamaged by multiple interference and cannot be picked with the level of certaintyrequired for meaningful results. We are still working with the data


P-P and P-SVseismic data from Lousana

FIG. 22. Interpretation of the P-P data (left) and P-SV data (right) is shown for part ofline EKW-002. To facilitate interpretation, the sections are displayed on the workstationsimultaneously and the P-SV data plotted at 2/3 the scale of the P-P data.

(Oi>

3.0

2.8-

2.6-

2.4-

2.2-

2.0-

1.8-

1.6-

1.4-

1.2-

1.0-

Cretaceous

Paleozoic

O J O J < N C \ J O J C M C \ J C \ J C S J C \ J C S J C \ J ( N C J O J

Shotpoint

FIG. 23. Vp/Vs values on line EKW-002 for two intervals: Second White Specks toMannville in the Cretaceous section and Mississippian to Nisku in the Paleozoic. ThisCretaceous interval is primarily shales, whereas the Paleozoic consists mainly ofcarbonate rocks. The difference in lithology is reflected in Vp/Vs.

CREWES Research Report Volume 6 (1994) 7-21


in an attempt to use horizons which bound the Nisku and can be picked with areasonable level of confidence.

The advantages of having two seismic data sets to work with became evidentduring the course of this study. It was useful to display both components on the screensimultaneously while picking horizons, especially in zones where the horizons weredifficult to track. In the case of the Wabamun and Nisku horizons, the pick was cleareron the P-SV data than on the P-P data. Also, the calculated Vp/Vs values were used asa quality control check on the horizon interpretations. Vp/Vs values which were outsidethe range expected due to geological variations indicated mispicks and the need torevisit the data. For example, on both components there were two possible picks for theWabamun event. Choosing the wrong cycle on either section resulted in unreasonableVp/Vs values for the intervals above and below the pick. Vp/Vs provided a usefulconstraint on the interpretation of this and other horizons. By providing the interpreterwith feedback on the validity of horizon picks, Vp/Vs can increase confidence in thefinal interpretation.

SUMMARY

Two orthogonal three-component seismic lines were shot by Unocal in January,1987, over the Nisku Lousana Field in central Alberta. The purpose of the survey wasto investigate a Nisku patch reef thought to be separated from the Nisku shelf to the eastby an anhydrite basin. These data have been reprocessed and are currently beinganalyzed and used to develop methods of multicomponent seismic data analysis.Several techniques which are useful in the analysis of multicomponent data aredescribed and applied in this study. The data are of overall good quality and majorevents were confidently correlated between the P-P and P-SV sections using offsetsynthetic seismograms. P-SV offset synthetic modelling was also used to extractinterval Vp/Vs values from the P-SV data.

Forward offset synthetic modelling showed character, isochron, and Vp/Vsvariations associated with changes in lithology, porosity, porosity thickness, and porefluid. The modelling results indicate that multicomponent seismic analysis is applicablein carbonate settings, even when the reservoir intervals are relatively thin (23 m) andcomprise only 15-20% of the total measured interval. To date, we have had difficultyapplying these results to this data set because of multiple contamination in the zone ofinterest, however work is ongoing to resolve this issue.

Multicomponent recording provides additional seismic measurements of thesubsurface to assist in developing an accurate geological model. Rock properties whichcan be extracted from elastic-wave data, such as Vp/Vs, reduce the uncertainty inpredictions about mineralogy, porosity, and reservoir fluid type. Joint interpretation ofP-P and P-SV data was helpful in horizon picking and in providing feedback on thevalidity of the interpretation. The interpretation techniques described in this paper canbe usefully applied to other areas, including other carbonate plays.

FUTURE WORK

The results of forward modelling encourage us to continue working with thisdata set. We hope to obtain further results from interpretation methods described in thispaper, such as interval Vp/Vs analysis. Multiple contamination has complicated theapplication of these methods, so we plan to pursue full-waveform modelling to try to


P-P and P-SV seismic data from Lousana

understand the effects of interbed multiple energy on both components. ContinuedAVO modelling and AVO inversion may help us further understand the effects ofmineralogy, porosity, and pore fluid in carbonate settings. In keeping with ourobjective to develop multicomponent interpretation techniques, we plan to enhance theoffset synthetic modelling program as a tool for extracting interval Vp/Vs values.

ACKNOWLEDGMENTS

We would like to acknowledge Unocal Canada Ltd. for donating the seismicdata to the CREWES Project. We are grateful to Chevron Canada Resources andSensor Geophysical Ltd. for participating in this study, particularly for the time andeffort spent by Mark Harrison on reprocessing the data at the University of Calgary.Our thanks to the companies which donated software used in this work: Landmark,GMA, and Hampson-Russell. Thank you to Andrea Bell of Norcen Energy ResourcesLtd. for useful discussions on the geology of the Lousana Field. We thank theCREWES Sponsors for their financial support.

REFERENCES

Andrichuk, J.M. and Wonfor, J.S., 1954, Late Devonian geologic history in Stettler area, Alberta,Canada: AAPG Bull., 38, 2500-2536.

Domenico, S.N., 1984, Rock lithology and porosity determination from shear and compressional wavevelocity: Geophysics, 49, 1188-1195.

Eastwood, R.L. and Castagna, J.P., 1983, Basis for interpretation of Vp/Vs ratios in complexlithologies: Soc. Prof. Well Log Analysts 24th Annual Logging Symp.

Garotta, R., 1987, Two-component acquisition as a routine procedure, in Danbom, S.H., andDomenico, S.N., Eds., Shear-wave exploration: Soc. Expl. Geophys., Geophysicaldevelopment series 1,122-136.

Georgi, D.T., Heavysege, R.G., Chen, S.T., and Eriksen, E.A., 1989, Application of shear andcompressional transit-time data to cased hole carbonate reservoir evaluation: Can. WellLogging Soc. 12th Formation Evaluation Symp.

Goldberg, D. and Gant, W.T., 1988, Shear-wave processing of sonic log waveforms in a limestonereservoir: Geophysics, 53,668-676.

Gregory, A.R., 1977, Aspects of rock physics from laboratory and log data that are important toseismic interpretation, in Seismic Stratigraphy - Applications to Hydrocarbon Exploration:AAPG Memoir 26, 15-45.

Hampson, D., 1986, Inverse velocity stacking for multiple elimination: Can. J. Expl. Geophys., 22,44-45.

Kuster, G.T. and Toks6z, M.N., 1974, Velocity and attenuation of seismic waves in two-phase media:Part 1. Theoretical formulations: Geophysics, 39, 587-606.

Lawton, D.C. and Howell, C.E., 1992, P-SV and P-P synthetic stacks: Presented at the 62th AnnualSEG Meeting.

Miller, S.L.M. and Stewart, R.R., 1990, Effects of lithology, porosity and shaliness on P- and S-wavevelocities from sonic logs: Can. J. Expl. Geophys., 26, 94-103.

Miller, S.L.M., 1992, Well log analysis of Vp and Vs in carbonates: CREWES Research Report 4.Miller, S.L.M., Harrison, M.P., Szata, K.J., and Stewart, R.R., 1993, Processing and preliminary

interpretation of multicomponent seismic data from Lousana, Alberta: CREWES ResearchReport 5.

Nations, J.F., 1974, Lithology and porosity from acoustic shear and compressional wave transit timerelationships: Soc. Prof. Well Log Analysts 15th Annual Symp.

Pardus, Y.C., Conner, J., Schuler, N.R., and Tatham, R.H., 1990, Vp/Vs and lithology in carbonaterocks: A case study in the Scipio trend in southern Michigan: Presented at the 60th AnnualSEG Meeting.

Pickett, G.R., 1963, Acoustic character logs and their applications in formation evaluation: J.Petr.Tech., June, 659-667.

CREWES Research Report Volume 6 (1994) 7-23


Rafavich, F., Kendall, C.H.St.C., and Todd, T.P., 1984, The relationship between acoustic propertiesand the petrographic character of carbonate rocks: Geophysics, 49, 1622-1636.

Rennie, W., Leyland, W., and Skuce, A., 1989, Winterburn (Nisku) Reservoirs in Anderson, N.L.,Hills, L.V., and Cederwall, D.A., Eds. The CSEG/CSPG Geophysical Atlas of WesternCanadian Hydrocarbon Pools, 133-154.

Robertson, J.D., 1987, Carbonate porosity from SIP traveltime ratios: Geophysics, 52, 1346-1354.Schlumberger, 1989, Log Interpretation Principles/Applications: Schlumberger Educational Services.Stoakes, F.A., 1979, Sea level control of carbonate-shale deposition during progradational basin-

filling: the Upper Devonian Duvernay and Ireton Formations of Alberta, Canada: Ph.D.dissertation, Univ. of Calgary.

Stoakes, F.A., 1992, Winterburn megasequence in Wendte, J., Stoakes, F.A., Campbell, C.V.,Devonian-Early Mississippian carbonates of the Western Canada sedimentary basin; a sequencestratigraphic framework: SEPM short course No. 28,207-224.

Tatham, R.H., 1982, Vp/Vs and lithology: Geophysics, 47, 336-344.Tatham,R.H. and McCormack, M.D., 1991, Multicomponent Seismology in Petroleum Exploration:

Soc. Expl. Geophys., Investigations in geophysics no. 6.Wang, Z., Hirsche, W.K., and Sedgwick, G., 1991, Seismic velocities in carbonate rocks: J. Can.

Petr. Tech., 30, 112-122.Wilkens, R., Simmons, G., and Caruso, L., 1984, The ratio Vp/Vs as a discriminant of composition

for siliceous limestones: Geophysics, 49, 1850-1860.Wyllie, M.R.J., Gregory, A.R., and Gardner, L.W., 1956, Elastic wave velocities in heterogeneous and

porous media: Geophysics, 21, 41-70.

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