Iden ti fi ca tion of Mio cene gas de pos its from seis mic data in the south east ern part of the Carpathian Foredeep
Kaja PIETSCH and Jadwiga JARZYNA
Pietsch K. and Jarzyna J. (2002) — Identification of Miocene gas deposits from seismic data in the southeastern part of the Carpathian Foredeep. Geol. Quart., 46 (4): 449–461. Warszawa.
Seismic interpretations are normally made to help identify and locate structural and stratigraphic traps for oil. We focus on problems in interpreting seismic sections in sandy-shaly Miocene deposits which oc cur in the eastern part of the Polish Carpathian Foredeep. There, the structural picture yielded by the seismic section is not in good agreement with the known structure and a correct interpretation of the seismic wave field, based on seismic modelling, is needed to ensure proper location of exploratory and production wells. We show that the correct choice of petrophysical parameters in these deposits allows interpretation of the seismic image in terms of a multi-horizon gas body. A decrease in velocity, characteristic of gas-saturated beds, was not observed in velocity obtained from sonic measurements.
Therefore, several versions of a seismogeological model were constructed based on the results of integrated log interpretation. A model using seismic wave velocity obtained from acoustic wave velocity and a quality factor Q, as a measure of attenuation of elastic waves, was of particular significance. In addition to the petrophysical parameters, the strata geometry necessary to construct a seismogeological model, was determined. Combining the interpreted geometry and information regarding depths of lithostratigraphic units an anticlinal structure was deduced in the gas-rich zone. A comparison of synthetic seismograms calculated using only sonic velocity and seismic velocity corrected for attenuation, with the recorded seismic traces, shows that the best agreement was obtained for a model which included the attenuation. Differences observed between the synthetic and field sections were a basis for determining local direct hydrocarbon indicators, which were then used to identify hydrocarbon deposits in the recorded seismic section.
Kaja Pietsch, Jadwiga Jarzyna, Department of Geophysics, Faculty of Geology, Geophysics and Environmental Protection, University of Mining and Metallurgy, Mickiewicza 30, PL-30-059 Kraków, Poland, e-mails: pietsch@geolog.geol.agh.edu.pl;
jarzyna@geol.agh.edu.pl (received: July 23, 2001; accepted: May 8, 2002).
Key words: Carpathian Foredeep, seismics, seismostratigraphy, velocity model, acoustic full wavetrain, attenuation of seismic waves.
INTRODUCTION
The role of seismics in oil and gas pros pect ing has long been ac knowl edged. Seis mic sur veys may give di rect in di ca - tions of hy dro car bons via the de pend ence of ve loc ity and at - ten u a tion of P-waves and S-waves on oil and gas sat u ra tion in the pore space. Even tually, as a re sult of seis mic in ter pre ta - tion, struc tural and strati graphic traps may be suc cess fully iden ti fied and lo cated. Some prob lems with in ter pre ta tion of seis mic data in gas fields may oc cur be cause elas tic wave ve - loc ity is sig nif i cantly lower in gas-saturated ho ri zons thus re - sult ing in:
— sag of seis mic ho ri zons be neath a de posit what may give rise to un real pos i tive un du la tions be yond the de posit;
— bright spot events due to an in crease in the ab so lute value of the re flec tion co ef fi cient.
Dif fi culties with the un equiv o cal in ter pre ta tion of seis mic data oc cur, when a seis mic sec tion does not re flect the struc - tural ar range ment of lay ers, for in stance in multi-horizon gas fields. Since the petrophysical pa ram e ters of a hy dro car - bon-saturated rock me dium may be vari able, and the same causes may pro duce dif fer ent ef fects, there is a need to de ter - mine the so-called di rect hy dro car bon in di ca tors (Dilay, 1982; Blackburn, 1986; Sher iff, 1992).
There have been prob lems in the iden ti fi ca tion of small struc tural and strati graphic traps in sandy-shaly Mio cene de - pos its in the east ern part of the Pol ish Carpathian Foredeep. In this area, the struc tural pic ture yielded by the seis mic pro files does not of ten agree with the struc ture as de duced by other means (Borys et al., 1999). Hence, cor rect geo log i cal in ter - pre ta tions of the seis mic wave field, based on 1D and 2D seis - mic mod el ling, are needed for the proper lo ca tion of ex plor - atory and pro duc tion wells (Pietsch et al., 1998).
The com pre hen sive in ter pre ta tion of seis mic data should in clude:
— construction of seismogeological models of reservoir zones,
— modelling of theoretical seismic wave field given in the form of 1D synthetic seismograms and 2D synthetic seismic profiles,
— identification of gas-induced anomalies observed in the theoretical wave field,
— determination of reservoir parameters from seismic data based on criteria for direct hydrocarbon indicators.
GEOLOGICAL SETTING
The Carpathian Foredeep Ba sin in SE Po land orig i nated dur ing the Mio cene as a re sult of flex ural sub si dence in the
T a b l e 1
Re sults of in ter pre ta tion of well logs and in flow mea sure ments in bore hole P-6
No. of gas ho ri zon
Depth of packer in ves ti ga tion
[m]
PHI [%]
SW [%]
RHOB [g/ccm]
V_SONIC [m/s]
GR [API]
Q V_Q
[m/s]
In flow [Nm3/min]
1 1304–1314 7.01 99.35 2.38 3277 73.28 15 2961 280
2 1347–1383 4.27 99.94 2.43 3274 79.18 18 2847 83
3 1430–1437 2.04 99.97 2.49 3174 86.51 19 2795 74
4 1468–1486 6.98 97.07 2.37 3255 68.96 5 2054 323
5 1585–1595 4.3 98.67 2.46 3331 78.91 16 2819 104
6 1667–1692 3.33 98.72 2.54 3253 81.48 19 2861 70
7 2150–2195 1.59 99.52 2.58 3790 92.53 24 3394 25
Fig. 1. Map show ing the lo ca tion of the gas field P and rock units in the Carpathians and the Carpathian Foredeep (af ter Krzywiec, 2001)
front of flysch nappes that were thrust north wards onto the East Eu ro pean Plat form. Near its south ern, ac tive mar gin the ba sin was filled with 2000–3000 m of mainly siliciclastic de - pos its of sub ma rine-fan, slope, and deltaic fa cies. To the south, part of the suc ces sion was re moved due to tec tonic up - lift on the fron tal Carpathian overthrust that forms a ma jor seal for the most hy dro car bon ac cu mu la tions in the Carpathian Foredeep in Po land (Stupnicka, 1989; Krzywiec, 2001) (Fig. 1).
The width of the outer foredeep (out side of the Carpathians) var ies be tween 30–40 km in the west ern seg - ment up to 90 km in the east ern one. The outer foredeep is filled with Badenian and Sarmatian ma rine de pos its, from few hun dred up to about 3500 m in thick ness (Oszczypko, 1996).
Evaporites con sti tute the main cor re la tion level in the Carpathian Foredeep. Tra di tionally, the Badenian de pos its are di vided into lower (sub-evaporatic), mid dle (evaporatic)
and up per (su pra-evaporatic) units. We em ploy this di vi sion, though a more re cent chronostratigraphical sub di vi sion foredeep sed i ments has been made (Steininger et al., 1990 in Oszczypko, 1996).
In the study area, the Mio cene suc ces sion is >2000 m thick and rests un con form ably on Up per Pre cam brian base ment.
The suc ces sion be gins with the lower Badenian Baranów Beds, which are over lain by mid dle Badenian an hyd rites. The up per Badenian con sists of mudstones and shales in ter ca lated with very thin lay ers of fine-grained sand stones. The over ly - ing Sarmatian de pos its are de vel oped as fine- to coarse- grained sand stones and shales, which ex hibit strong fa cies vari abil ity. The sand stones, lo cally up to 150 m in thick ness, dom i nate in the lower part of the Sarmatian, while claystones and mudstones in crease in abun dance to wards its top. There are many small hy dro car bon de pos its in the Sarmatian rocks (Fig. 2) (Karnkowski, 1999).
1- ga s a nd oil de pos its
2- pre s e nt e dge of the Flys h Ca rpa thia ns
6
1 3 2
N
0 10 km
3- P -s tructure
3
Fig. 2. Lo ca tion of the gas field P with seis mic pro files and wells, against a con text of other hy dro car bon de pos its in the SE part of the Carpathian Foredeep (af ter Karnkowski, 1999)
30-1-75 and 28-1-75 — 2D seis mic lines; inl760, inl790, inl820, inl850, inl860, xl 1270, xl 1220, xl 180 — 3D seis mic lines
Fig. 3. In ter pre ta tion re sults of bore hole P-6 (made by Geofizyka Kraków Ltd.)
PHI — po ros ity, SW — wa ter sat u ra tion, GR — in ten sity of nat u ral ra dio ac tiv ity, CAL — real di am e ter of bore hole, RHOB — bulk den sity, V — ve loc ity
SEISMOGEOLOGICAL MODEL
A seismogeological model of the gas field P, needed for con struc tion of the the o ret i cal seis mic sec tion, was made us ing in ter pre ta tion re sults for 2D and 3D seis mic pro files and well logs from the study area (Fig. 2). In ter pre ta tion of 2D and 3D seis mic sec tions, made on a CHA RISMA-GeoFrame 3.8.1 (Schlumberger GeoQuest), gave the ge om e try of seis mic bound aries, which cor re sponded to lay ers with var ied li thol ogy and dif fer ent res er voir pa ram e ters: porosity and gas saturation.
In structure P, located over a small structural elevation of the basement recognised by the seismic survey, several wells were drilled: P-1, P-2, P-3, P-4, P-5 and P-6. These were all logged. Sonic logs were made in the non-productive borehole P-4 and in the productive borehole P-6. The results of comprehensive interpretations of well logs in the boreholes were combined with the seismic results to recognise the mutual relations between the reservoir parameters of layers, the seismic model and the geological model.
WELL LOG DATA
A cor rect se lec tion of petrophysical pa ram e ters of lay ers of a seismogeological model is of great im por tance to that part of the Carpathian Foredeep. There, a de crease in ve loc ity, as is char ac ter is tic of gas beds, was not ob served in ve loc ity curves ob tained from sonic mea sure ments (Pietsch et al., 1998; Bała, 2001). There fore, in this study three seismogeological mod els were con structed based on the re sults of in te grated in ter pre ta tion of logs from wells P-6 and P-4. Quan ti ta tive in ter pre ta tion of well logs was made by Geofizyka Kraków Ltd. In ter pre ta tion re - sults for the 0–2280 m depth in ter val of bore hole P-6 in terms of li thol ogy–po ros ity–sat u ra tion curves are shown in Fig ure 3. The amount of sand stones and shales changes lit tle in the study pro - file. Gas sat u ra tion was ob served in a few lev els of in creased po - ros ity. The vol ume of the in flow was de ter mined from packer mea sure ments. Re sults of log in ter pre ta tions of the gas bear ing ho ri zons in terms of po ros ity — PHI, wa ter sat u ra tion — SW, and qual ity fac tor — Q, as a mea sure of at ten u a tion, to gether with val ues of bulk den sity — RHOB, nat u ral ra dio ac tiv ity —
Fig. 4. Com par i son of GR and sonic curves with mod eled ve loc i ties and cor re spond ing syn thetic seismograms a — VELOCITY_SONIC, b — VELOCITY_RHG, c — VELOCITY_Q; 1–7 — gas ho ri zons
GR, sonic ve loc ity — V_SONIC, seis mic ve loc ity — V_Q, for se lected ho ri zons are shown in Ta ble 1.
VELOCITY FROM LOGS
Ho ri zon ve loc ity (VELOCITY_SONIC) was de ter mined for lay ers dis tin guished based on a VSP-cor rected sonic curve and lithological in ter pre ta tion. Gas-sat u rated lay ers (ho ri zons 1
–7 in Ta ble 1 and in Fig. 4) cor re late with the GR (low am pli - tude) and po ros ity (high am pli tude). How ever, there is no clear re la tion be tween anom a lies in the ve loc ity curve from the sonic log and from the lo ca tion of gas lay ers. The shape of the sonic curve is a re sult of the shal low ra dius of in ves ti ga tion of the sonic log in a flushed zone, where pore space was sat u rated with a mud fil trate, and gas was thus re moved from the vi cin ity of the bore hole walls, caus ing a de crease in tran sit in ter val time. There fore, the ex pected de crease of ve loc ity (in crease of
gas1-t gas1-b gas2-t gas2-b gas3-t gas3-b gas-4t gas4-b gas5-t gas5-b gas6-t gas6-b
gas7-t gas7-t
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P a likówka -6 P a likówka -4
x-scale: 200.00 m/cm de pth s ca le : 100.00 m/cm st a rt x-dist an ce: 0.00 m e nd x-dis ta nce : 1000.00 m s ta rt de pth: -500.00 m e nd de pth: -2655.00 m
1000 m
Fig. 5. Cor re la tion of Mio cene ho ri zons in wells P-6 and P-4, gas field P
TIME[ms]
X DIST. [M] 0 1000 2000 3000 4000 5000 6000 7000 8000 9000
X DIS T. [m]
2000 1500 1000 500
P -2 P -6 P -3
b
0
Fig. 6. The o ret i cal seis mic sec tion, model “without gas”
a — NORMAL INCIDENCE, b — REFLECTION STRENGTH
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tran sit in ter val time) as so ci ated with the gas ho ri zons was not ob served (Pietsch et al., 1998).
In the second case, a velocity (VELOCITY_RHG) was calculated using the Raymer-Hunt-Gardner equation (Raymer et al., 1980) and compared with the VELOCITY_SONIC. In some horizons the decrease in VELOCITY_RHG reaches as much as 20%. A decrease in VELOCITY_RHG and increase in VELOCITY_SONIC was observed for all gas horizons except for horizon 7. Such discrepancy between both velocities (greater for higher porosity) was not observed in non-gas horizons with higher porosity. An exception was the horizon at a 675–825 m depth in which packer measurements were not made. There, both velocities behave as in deposit zones.
A third velocity model (VELOCITY_Q) was obtained from the sonic log with the quality factor, Q, included (Aki and Richards, 1980). The comparison of VELOCITY_Q with two others shows the significant role of this result in the interpretation.
Quality factor was used here as a measure of attenuation of seismic waves. Knowledge of the attenuation and thus the
connection of lithological and seismic parameters is fundamental in lithological and reservoir interpretation of seismic data (Stewart et al., 1984). Various authors have attempted estimation of P-wave attenuation from VSP and acoustic full wavetrains (Hardage, 1985; Toverund and Ursin, 1998). One of the most popular and effective methods for obtaining the Q factor, basing on the logarithmic spectral ratio, was employed in this paper (De et al., 1994).
The interpretation of selected pairs of acoustic full wavetrains recorded with an LSS device (Halliburton) was made for several boreholes to determine the quality factor, Q, for Miocene shaly-sandy rocks (Jarzyna, 1999). The basic difficulty in obtaining correct values of quality factor in this case was the limited number of wavetrains with amplitudes satisfying assumptions for the amplitude spectra ratio method (Bała and Jarzyna, 1992). Thus, values of the quality factor, Q, should be treated as low-confidence estimates of attenuation.
The three above-defined velocities were compared with petrophysical parameter curves in well P-6. The best
TIME[ms]
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de pos it zone 1000
0 2000 3000 4000 5000 6000 7000 8000 9000
500
1000
1500
2000 0
X DIS T. [m] P -2 P -6 P -3
Time [s ]
c
Fig. 7. Theoretical seismic section, model “with gas”
a — NORMAL, b — DIFFRACTION + STOLT MIGRATION, c — NORMAL-REFLECTION STRENGTH
CDP
TIME [ms]
bright spot
shadow zone
sag
P-6 P-6
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0 -1 -2 -3 -5 -6 -7 -8 -10 -11 -12 -13 -15 -16 -17 -18 -20
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700 600
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P-2 P-3 P-3
Fig. 8. Re sults of in ter pre ta tion for 30-I-75K line made on the CHARISMA sys tem for the de posit P
a — seis mic sec tion; marked con firmed gas ho ri zons (z1-z7) in the lower part and prob a ble gas ho ri zons (e1-e4) in the up per part the seis mic sec tion, HS – top of base ment, b — re flec tion strength
e 1 e 1e 2e 2e 3e 3
e 4 e 4
z1 z1 z2 z2 z3 z3 z4 z4 z5 z5
z7 z7 z6 z6 HS
P-6 P-6 P-2
P-2 P-3P-3
CDP
TIME [ms]
a
correspondence was observed between VELOCITY_Q and GR and porosity and inflow values.
SEISMIC MODEL — GEOMETRICAL ARRANGEMENT
The geometric arrangement of boundaries in the seismogeological model was assumed on the basis of the interpretation of the seismic profile 30-1-75K. The time-depth conversion and geological correspondence of seismic layers was determined using velocity curves and synthetic seismograms calculated for wells P-6 and P-4. The results of this interpretation were used to determine the geometry of seismic boundaries outside the structural elevation in the basement, the P-structure, in which the deposit is located. In the P-structure itself, an anticlinal geometry of strata was assumed, corresponding to the structural plan of the basement, even though it was not visible on the seismic image. The width of the deposit zone was adopted from interpretation of well logs in well P-2 (productive) and in well P-3 (non-productive) as well as from preliminary interpretation of seismic profiles (location of anticlinal arrangement of basement, presence of sag times and bright spots in Miocene beds).
The as sumed ge om e try is con sis tent with the cor re la tion of dis tin guished lay ers in the cor re la tion pro file P-6–P-4 (the part of this pro file in the de posit zone is shown in Fig ure 5). The cor re la tion pro file was pre pared us ing GeoGraphix Model Builder (Land mark Graphics Co.). Cor re lated seismo-lithological lay ers cor re spond to bound aries, which are dis tinctly vis i ble on all in ter preted seis mic pro files in the field P (Fig. 2). The block ve loc ity of the lay ers was ob tained from cal - i brated sonic logs, VELOCITY_SONIC. In the gas ho ri zons, apart from VELOCITY_SONIC (black blocks) VELOCITY_Q (check ered blocks) were also in cluded.
SEISMIC MODEL — VELOCITY
The ve loc ity curve from the acous tic log (VELOCITY_SONIC) was as sumed as cor rect in the vi cin ity of the gas de posit. It was the re sult of de tailed anal y sis of the acous tic log in the well P-6, in which the ex pected in crease of tran sit in ter val time in gas ho ri zons was not vis i ble. The VELOCITY_Q curve, as a re sult of in clud ing of at ten u a tion in pro cess of wave prop a ga tion, was adopted in the de posit zone. This ve loc ity cor re sponds to other pa ram e ters in gas ho - ri zons (Ta ble 1).
SEISMIC MODELLING
Seismic modelling enables the correlation of seismic anomalies with their sources. In this discussed case study, 1D seismic modelling (synthetic seismograms) was applied to determine the effect of gas saturation on seismic data, whereas 2D seismic modelling (synthetic sections) was used to evaluate the seismic pattern deformation, which may be observed in zones of multi-horizon gas deposits. First, calculations were
carried out for a seismogelogical model from which gas was removed (model “without gas”), then for a model representing the actual deposit (model “with gas”). Comparing the calculated theoretical wave fields one can see the scale of gas-induced seismic anomalies.
The theoretical wave field was calculated in the GeoGraphix system (Landmark Graphics Co.). 1D modelling was made using the LogM software, while 2D modelling was made using the STRUCT 2D:
— NORMAL INCIDENCE — synthetic section was computed based on the ray propagation theory with an assumption of ray incidence normal to seismic boundaries; it corresponds to an unmigrated seismic section after stacking but without diffraction waves,
— DIFFRACTION — synthetic section was computed based on the solution of the Hilterman wave equation, in which it is assumed that energy is dispersed in each point on the reflecting surface. The result of computing represents an unmigrated seismic section after stacking but with all diffraction waves. Generated seismic traces must be additionally migrated. The GeoGraphics system has an option with f-k migration using the Stolt method.
The signal estimated from seismic line 30-I-75K with LogM software was used in 1D and 2D modelling.
SYNTHETIC SEISMOGRAMS
Syn thetic seismograms were cal cu lated for the three ve loc - ity mod els given above. The seismograms are shown in Fig - ure 6 to gether with the ve loc ity curves (VELOCITY_SONIC, VELOCITY_RHG and VELOCITY_Q), GR, and the seg - ments of seis mic line 30-I-75K. Bound aries of gas ho ri zons are marked in the fig ure. Com par ing syn thetic seismograms for mod els “with out gas” (as sum ing that in the VELOCITY_SONIC there is no in flu ence of gas) and “with gas” (as sum ing that in the VELOCITY_Q the in flu ence of gas is in cluded) one can see a dis tinct in crease in am pli tude of sig - nals from res er voir ho ri zons. Also, en hanced cor re la tion be - tween seismograms for a model “with gas” and seis mic re cords was ob served. The best cor re la tion was ob tained for a model in which VELOCITY_Q was used in mod el ling the the o ret i cal wavefield (Fig. 4).
THEORETICAL WAVEFIELD — MODEL “WITHOUT GAS”
A seismogeological model named “with out gas” was adopted in zones out side gas ho ri zons. The syn thetic sec tion in these zones was con structed us ing ve loc ity dis tri bu tion in lay - ers de scribed by a VELOCITY_SONIC curve. Cal cu lated the - o ret i cal seis mic sec tions are shown in Fig ure 6a (NORMAL INCIDENCE) and in Fig ure 6b (REFLECTION STRENGTH). In stan ta neous char ac ter is tics il lus trate the bright spots in the best way (Taner and Sher iff, 1977). Com par - ing syn thetic sec tions with a seismogeological model one can see that al most all bound aries are dis played in the seis mic sec - tion and it is pos si ble to cor re late them along the en tire line. The
sig nal am pli tudes are strictly as so ci ated with acous tic im ped - ance con trasts at layer in ter faces.
THEORETICAL WAVEFIELD — MODEL “WITH GAS”
A seismogeological model named “with gas” was adopted in gas ho ri zons. In con struct ing the o ret i cal seis mic sec tions for this model, ve loc ity dis tri bu tion in lay ers was de scribed by the VELOCITY_Q in the de posit, and by VELOCITY_SONIC curve in the other zones. The the o ret i cal sec tions are shown in Fig ure 7a (nor mal) and in Fig ure 7 b (dif frac tion+Stolt mi gra - tion) and in Fig ure 7c (nor mal-re flec tion strength).
CRITERIA FOR DETERMINATION OF RESERVOIR ZONES FROM SEISMIC SECTION INTERPRETATION
Based on both syn thetic sec tions: “with gas” (Fig. 7) and
“with out gas” (Fig. 6), cri te ria for de ter min ing res er voir zones from seis mic data in ter pre ta tion may be es tab lished for gas field P. The fol low ing char ac ter is tic anom a lies were ob served in the de posit zone:
— A distinct increase in amplitudes of signals for reservoir layers (bright spot) in a time interval of 1000 to 1300 ms. It corresponds to a zone of major gas-bearing horizons. Beyond the deposit amplitudes are lower and their maxima are shifted in relation to the position of layers for a model “with gas”. A signal amplitude change is best seen in reflection strength sections. This change may be due to lower seismic wave velocity in gas horizons.
— Small decrease in amplitude of reflections beneath the deposit series (dim spot).
— Lo cal sag as so ci ated with lon ger time of seis mic wave prop a ga tion in gas-bearing ho ri zons, e.g. dou ble re flec tion at a 1050 ms time.
— Bright spot events in zones of horizontal and synclinal arrangement of seismic boundaries.
SEISMIC DATA INTERPRETATION
Re sults ob tained for the gas field P con firmed that cri te ria for de ter min ing res er voir zones from seis mic data in ter pre ta - tion were cor rect. In ter pre ta tion of the 30-I-75K line made in the CHARISMA sys tem shows how de posit P is rep re sented in the seis mic sec tion. A part of a pro cessed sec tion is shown in Fig. 8a (seis mic sec tion) and in Fig. 8b (re flec tion strength).
The fol low ing char ac ter is tics are ob served in the time in ter val 600–1400 ms:
— bright spot for horizontally arranged seismic boundaries at a shallow depth, and for synclinally arranged seismic boundaries at a greater depth,
— lo cal sag, e.g. a sig nal re corded at a 1050 ms.
Marked gas horizons in the lower part of seismic section (z1-z7) were confirmed by results of well log interpretations and by inflow observed in borehole P-6. Probable gas horizons (e1-e4) in the upper part of seismic section are the result of interpretation with adopted criteria. In this interval the packer measurements were not done.
A distinct decrease in signal amplitudes was observed for times greater than 1400 ms. This may be caused by enhanced attenuation of seismic waves propagating through the gas horizons. Moreover, strong, inclined reflections from the basement suddenly disappear beneath the deposit zone. These effects were not observed in theoretical sections, because no seismic wave attenuation was included in modelling. The attenuation was indirectly introduced only by decreasing the VELOCITY_Q value.
The sim i lar ity be tween the layer ar range ment in a re corded seis mic sec tion (Fig. 8) and the o ret i cal sec tions (Figs. 6 and 7) proves that the ve loc ity model and cri te ria for seis mic data in - ter pre ta tion were cor rectly de fined. A small dis agree ment be - tween re corded and the o ret i cal wave fields may be caused by in ac cu rate ap prox i ma tion of layer ar range ment by a geo met ric model and by in ad e quate iden ti fi ca tion of petrophysical pa ram - e ters of the rock mass.
To correctly approximate the velocity field for a deposit with dimensions similar to those of gas field P, acoustic log data should be available from at least three boreholes in the deposit zone and beyond it. Since our studies were based on acoustic log from borehole P-6 alone and no acoustic logs were made in wells P-2 and P-4, the credibility of the model adopted is lower.
CONCLUSIONS
We show how the correct selection of elastic and reservoir parameters for Miocene sandy-shaly deposits affects the interpretation of a seismic image of a multi-horizon gas deposit.
Including information about attenuation of elastic waves improves the velocity model and makes it more credible. The interpretation criteria determined in this study may be used for similar cases of gas fields in the Carpathian Foredeep.
Acknowledgements. The authors thank the Geophysical MicroComputer Application (International) Ltd. and the Schlumberger Company for use of their software in preparing this paper. The authors are also grateful to Geofizyka Kraków, Ltd. and POGC, Warsaw, for access to seismic data, logs, and geological data. Results were obtained in scientific projects No 9T12B01011 and 9T12B01517 financed by the Polish State Committee for Scientific Research.
Results were partially presented in the following conferences: 70th Annual Meeting of SEG, Calgary, 2000, Petrophysics Meets Geophysics, Paris, 2000 and Małopolska Prowincja Naftowa: Geologia i Złoża Węglowodorów, Warszawa, 2001.
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