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Seismic modeling of TS-4 sands embedded within Lower Clay Marker (LCM) unit and its

delineation using relative acoustic impedance in Lakwa area, A & AA Basin

– A case study.

Anoop Singh*, Pragati Mitra, A K Srivastava & Chakradhar Mahapatra Oil and Natural Gas Corporation Limited, A & AA Basin, Jorhat

*singh_anoop@ongc.co.in

Keywords

LCM, seismic modeling, relative acoustic

impedance

Summary

The TS-4 sands of Tipam Formation in North

Assam Shelf area of A&AA Basin occurs as

lenticular sand bodies embedded within Miocene

clay unit known as Lower Clay Marker. The

LCM unit is deposited in low energy flood plain

environment that is sandwiched between two

thick high-energy braided sandstone reservoirs of

TS-3 and TS-5. The mapping of these lenticular

TS-4 sands occurring within LCM unit in Lakwa

area poses a challenge in terms of understanding

the seismic response of these sand bodies

encased within a clay medium. 1-D seismic

modeling has given a quick insight to the seismic

response of these reservoir sands. Based on this

understanding amplitude anomaly and relative

acoustic impedance attribute was used to

delineate these reservoir sands.

Introduction

The study area falls in the Lakwa acreage of A &

AA Basin (Fig.1).

The Lower Clay Marker (LCM) unit is deposited

in a low energy flood plain environment overlain

by TS-3 and underlain by TS-5, both thick high

energy braided sandstone reservoirs (Fig. 2).

Within this LCM pack, TS-4 sands are deposited

as channel fill in meandering fluvial systems.

These TS-4 channel sands are thin to moderately

thick bedded encased within the LCM

clay/claystone sediments. Out of three laterally

restricted TS-4 sands units, viz. TS-4A, TS-4B &

TS-4C identified within the study area, TS-4B is

the most extensive (Fig. 3).

The thickness of LCM unit varies from 20 to 100

m in Lakwa area. The net thickness of TS-4

sands varies between 4- 40m.This typical

discrete and discontinuous deposition of TS-4

sands poses a challenge in understanding the

seismic response of these sands.

To overcome this challenge 1_D seismic

modeling has been carried out which helped in

understanding the seismic response of the TS-4

sand bodies. Based on this understanding

amplitude anomaly and relative acoustic

impedance was used to delineate these sand

bodies.

Methodology

The clay of LCM unit is of low impedance

compare to TS-4 sands and it is sandwiched

between high impedance TS-3 and TS-5 sands

(Table 1).

Figure 1. Study area

Figure 2. Envisaged deposition setup of LCM/TS-4

(Courtesy M M Rajkhowa)

As per polarity convention used in ONGC an

increase in impedance is recorded as negative

amplitude and plotted as trough on seismic

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section whereas decrease in impedance is plotted

as trough on seismic section whereas decrease in

impedance is recorded as positive number and

plotted as peak in seismic section. Based on this

convention LCM top corresponds to peak and

bottom corresponds to the trough on the seismic

section. Synthetic seismograms were prepared

for key wells (Fig. 4) and the top and bottom of

the LCM unit is calibrated and mapped with

good confidence (Fig. 5).

Figure 3. LCM litholog in Lakwa area and stratigraphic

correlation showing deposition pattern of TS-4 sands within

LCM.

Table 1. Impedance of TS-3, TS-4, TS-5 sands & LCM

Figure 4. Synthetic seismogram prepared to calibrate

reservoir Top & Bottom.

The interval velocity of the TS-4 sands in LCM

unit is approximately 3500 m/s and the dominant

frequency is 25 Hz. The vertical seismic

resolution limits as per the λ/4 criteria is around

35 m.

The thickness of individual TS-4 sand units is

below this resolution limit; however the seismic

response of TS-4 sands is embedded in the peak

& trough corresponding to the top and base of

LCM (Alister Brown, 2011, Chapter 6 pp 203-

216).

Figure 5. TWT time map prepared at the top of LCM

The occurrence of TS-4 sands near the top of

LCM unit, will create lowering of the amplitude

of peak generated corresponding to LCM Top

due to destructive interference (Fig. 6a and Fig.

7).

Figure 6: Amplitude anomaly due to (a) destructive interference – decrease in amplitude (b) constructive

interference – increase in amplitude.

Figure 7: Lower in amplitude due to destructive interference (a) without considering the effect of TS-4 sands (b) after

considering the effect of TS-4 sands.

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To delineate these TS-4 sands, window based

seismic attribute analysis corresponding to LCM

Top has been carried out by capturing low

amplitude anomalies. The maximum positive

amplitude has been computed in a+/- 8ms

window from LCM Top. The low amplitude

ranges (plotted in red and yellow) bring out the

channel features, indicating presence of TS-4

sands (Fig. 8).

When the TS-4 sand bodies are occurring in the

lower part of LCM unit, it will create boosting of

the amplitude of trough generated corresponding

to TS-5 Top due to constructive interference (Fig

6b). To completely delineate the sand geometry

of TS-4 sands relative acoustic impedance

seismic attribute was used.

Figure 8: Maximum amplitude attribute- window +/- 8ms

from LCM Top. (LCM time map superimposed)

Relative Acoustic Impedance (RAI) for

Delineation of TS-4 Sands

RAI can be obtained by integrating the real part

of the seismic trace (Berteuseen, K. A. & Ursin

B., 1983, Becquey M., Lavergne M. & Willm C.,

1879, Sanceveo S. S., 2003). It is mathematically

expressed as

In(ρν) = 2 ∫ ( )

The integration of the trace gives estimate of the

natural log of the acoustic impedance log. Since

Seismic data is band limited the result is only

estimate of zero offset reflectivity and not the

absolute impedance. The attribute shows

apparent acoustic contrast. It relates to porosity

(Bag S. et. al., 2008), high contrast indicates

possible sequence boundaries, unconformity

surface and discontinuities.

High impedance TS-4 sands are embedded

within low impedance LCM clay that is

sandwiched between high impedance TS-3 &

TS-5 sands.

Due to deposition of TS-4 sands the average

absolute value of relative acoustic impedance

decrease between top and base of LCM unit.

When the brine is replaced by HC average

absolute value RAI further decrease (Fig. 9).

Figure 9: Change in average absolute value of RAI due to

TS-4 sands.

After converting the amplitude data into relative

acoustic impedance data average absolute value

of RAI is extracted between LCM top and TS-5

Top. The lower average absolute value of RAI

plotted in orange and yellow color brings out the

sand geometry of TS-4 sands embedded within

LCM as shown in figure 10 which corroborate

with the sand isolith map of TS-4 sand in study

area. Other workers also shows the application of

relative acoustic impedance in thin reservoir

characterization (Satider Chopra et al, 2009,

Cooke et al, 1999, Rahmani et al, 2006, Brown

et al, 2008).

Figure 10: Average absolute value of RAI extracted between LCM Top & TS-5 Top

Conclusion

In the present study 1-D seismic modeling

significantly helped to give insight into how the

response of reservoir sand is getting registered in

seismic data. The seismic modeling facilitated

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judicious selection of RAI seismic attributes for

bringing out the TS-4 sands geometry in the

study area.

References

1. Bag S., Murthy B. V. R. R. K and Anand S.

K., 2008, Seismic Attribute Analysis- A

valuable tool for accurate Placement of

Horizontal Drainhole: An Elucidation

through a case study, 7th

International

Conference & Exposition on Petroleum

Geophysics.

2. Becquey M., Lavergne M. & Willm C., 1879,

Acoustic impedance logs computed from

seismic traces, Geophysics, VOL. 44, NO.

09, September 1979; P. 1485-1501.

3. Berteussen K. A. & Ursin B., 1983,

Approximate computation of the acoustic

impedance from seismic data, Geophysics,

VOL. 48, NO. 10, October 1983; P. 1351-

1358.

4. Brown A. R., 2011, Interpretation of Three-

Dimensional Seismic Data, 7th Edition, SEG

Investigations in Geophysics 9 and AAPG

Memoir 42.

5. Brown, L.T., J. Schlaf, and J. Scorer, 2008,

Thin-bed reservoir characterization using

relative acoustic impedance data, Joanne

Field, U. K., 70th EAGE Conference &

Exhibition, Rome.

6. Copra S. Castagna J. P., Xu Y., 2009, When

Thin Is In – Relative Acoustic Impedance

Helps, CPSG CSEG CWLS Convention,

Calgary, Alberta, Canada.

7. Cooke D., A. Sena, G. O’Donnell, T.

Muryanto and V. Ball, 1999, What is the best

seismic attribute for quantitative seismic

reservoir characterization, 18, no. 1, P1588-

1591.

8. Rahmani, A., A. Belmokhtar, A. Murineddu,

J, Khazanehdari, J. English, H. Roumane, B.

Godfrey, 2006, The art of seismic inversion –

an example from Erg Chouiref, Algeria, 25,

no. 1, p264-268.

9. Sanceveo S. S., 2003, Comparison between

recursive and sparse-spike inversion

performed in a synthetic reference model,

Presented in 8th

International Congress of The

Brazilian Geophysical Society, 14-18

September 2003.

Acknowledgments

Authors are grateful to Oil and Natural Gas

Corporation Limited, India for providing the

necessary facilities to carry out this work and

giving permission to publish this paper. The

authors are thankful to Shri M M Rajkhowa for

giving permission to use the figure of deposition

model of TS-4 sand Shri H K Mehanta for

helping in preparing the LCM lithologs.

The views expressed in this paper are those of

authors only.

11th Biennial International Conference & Exposition