de-risking seismic amplitude interpretation by 3d … may 2016 © earthworks environment &...

24 May 2016 © Earthworks Environment & Resources Ltd. All rights reserved 1

De-risking Seismic Amplitude Interpretation by 3D Seismic

Volume Detuning

DEVEX, 18 May 2016

Ashley Francis


Introduction

• Mapping and interpretation of amplitudes from

seismic data is a valuable tool for understanding

subsurface geology

• Amplitudes are sensitive to many geological factors,

and their interpretation is not unique

• Thickness changes can result in very large

amplitude variation – this is called the Tuning Effect

• Amplitudes are always contaminated by tuning

effects and a proper geological interpretation can

only be made when tuning effects are removed from

seismic amplitude data


Uses of Seismic Amplitude

• Geological Interpretation of Seismic Amplitudes

• Lateral prediction of rock properties

– Lithology

– Porosity

• Direct hydrocarbon indicators (DHI)

– Fluids – Gas

– Fluids – Oil

– Conformance to Structure


What is Tuning?

• The largest physical effect controlling seismic

amplitude variation is thickness change

• Thickness related tuning is where, if all other

properties stay the same, the amplitude response of

a seismic event varies as a function of thickness of

the interval

• Thickness related tuning can double seismic

amplitudes

• The tuning response is a characteristic curve which

is a function of bandwidth


Wedge Model Kadanwari Field, Pakistan

• Following example is from modelling a reservoir

sand in the Kadanwari field, Onshore Pakistan

– Stratigraphically trapped lower Cretaceous reservoir

– Softer (-ish) sand between harder encasing shales

• Gas sands are expected to have a bright er

(negative) acoustic impedance response than a

brine sand

– Seismic amplitude response of about +20 – 25%

• So we expect bright negative amplitudes are gas

• Except…thickness changes in the same interval can

change amplitudes by 100%!


Kadanwari Field Sand AVO / Fluid Model

-0.3000

-0.2500

-0.2000

-0.1500

-0.1000

-0.0500

0.0000

0.0500

0.1000

0.1500

0.0000 0.0500 0.1000 0.1500 0.2000 0.2500 0.3000

Sin2q

Re

fle

ctio

n C

oe

ffic

ien

t

GW

C

Hard Shale / Gas Sand

Hard Shale / Brine Sand Soft shale / Gas Sand

Soft Shale/ Brine Sand

Hard shale/soft sand

Gas is about 25%

brighter than brine

After Syed and Francis, 2001


Brine Saturated Reservoir Wedge Model

Brine case reservoir model with

laterally varying thickness



Brine Saturated Reservoir Relative Impedance Response



Relative Impedance Amplitude Response with TWT Thickness

0

0.2

0.4

0.6

0.8

1

1.2

1.4

0 5 10 15 20 25 30 35

Re

lati

ve A

mp

litu

de

TWT (ms)

Reference Amplitude = 0.7



Tuning Curve Model Relative Impedance Response

0

0.2

0.4

0.6

0.8

1

1.2

0 10 20 30 40 50 60

Seismic TWT Thickness (ms)

No

rmali

sed

Am

pli

tud

e

Peak tuning Amplitude

Amplitude decay at

greater thicknesses

Lack of Low F

User Defined

Normalisation Amplitude

Thin Bed

Limits


Overview of Connolly (2007)

• Connolly (2007) published a method of estimating

seismic net pay from seismic amplitude maps

– Extract thickness and amplitude between zero crossings

from relative impedance (coloured inversion) data

• Estimate Seismic Net Pay by simultaneously:

– Detune the amplitude using a modelled tuning curve

– Calculate apparent seismic Net-to-Gross

– Calculate apparent absolute time or z thickness of net sand


Amplitude Relation to N:G Different Time Thickness

0

0.2

0.4

0.6

0.8

1

1.2

0 10 20 30 40 50 60

Seismic TWT Thickness (ms)

No

rmali

sed

Am

pli

tud

e

Sand 2 Sand 1 Sand 3

Amplitude

N:G=0.48 N:G=0.41

N:G=0.91

The same amplitude corresponds

to very different N:G at different

thicknesses on the tuning curve


Seismic Net Pay or Detuning

• Connolly (2007) is constructed as a direct means of

calculating Seismic Net Pay and simultaneously

deals with both reservoir net/quality and detuning

• Excellent and elegant methodology but…

– Requires picking seismic zero crossings – time consuming

– 2D extracted thickness and amplitude maps only

– Requires well calibration

• There is considerable value in being able to rapidly

generate thickness/tuning corrected amplitudes

• Amplitude detuning only is a completely self-

consistent calculation that does not require wells


3D Seismic Detuning Earthworks DT-AMP™

• Earthworks has developed a method of removing

thickness related tuning effects from 3D seismic data

– Novel method developed over 2½ years with multiple client

trials in different geological environments

• DT-AMP detunes the 3D seismic trace volume in situ

• DT-AMP outputs significantly speed up amplitude

mapping and prospect generation from 3D seismic

• DT-AMP can extract and fit tuning curves from

windows or slices, without any horizon picking

– Significantly speeds up de-tuning analysis


Comments on Tuning Curves

• Tuning curves are independent of absolute

amplitude and so do not require well calibration

• Tuning curves are bandwidth dependent

– Very sensitive to low frequencies

• Tuning curves can vary both spatially and temporally

– Need to be able to generate in time window or slice

• There is a lot of valuable information in a tuning

curve

– Can be enhanced by plotting additional attributes

– Strat classes


High Frequency Response

Thickness

(seconds)

No

rma

lis

ed

tu

nin

g

cu

rve

Filter varying 5 – 30 Hz up to 5 – 77 Hz


Low Frequency Response

Thickness

(seconds)

No

rma

lis

ed

tu

nin

g

cu

rve

20 – 80 Hz

1 – 80 Hz


Sea Lion Field, Main Fan Original Coloured Inversion

Thinner Sand

Lower N:G

Thick Sand

High N:G

Bright Constant

Amplitudes Downdip

Well 14/10-4 Well 14/10-5

2100

2400

TW

T (

ms)


Sea Lion Field, Main Fan Detuned Modulated Amplitude

After Detuning, Downdip

Amplitudes Show N:G Change

Well 14/10-4 Well 14/10-5

2100

2400

TW

T (

ms)


0

1000

2000

3000

4000

5000

6000

7000

8000

0 10 20 30 40 50 60

Seis

mic

Am

plit

ud

e

Time Thickness (ms)

Before Detuning

N:G=70%

N:G=94%

Well 4 Well 5

Before Detuning:

Downdip Well 4 Higher Amplitude

But Lower Net:Gross


0

1000

2000

3000

4000

5000

6000

7000

8000

0 10 20 30 40 50 60

Seis

mic

Am

plit

ud

e

Time Thickness (ms)

After Detuning

N:G=70%

N:G=94%

Well 4 Well 5

Normalised Amplitude

After Detuning:

Downdip Well 4 Lower Amplitude

Consistent with Lower Net:Gross


3D Seismic Detuning Advantages to Interpreter

• Seismic sections from detuned output relate to

geology and property changes with the amplitude

effect due to thickness changes removed

– Rock property relations with amplitude will not be generally

valid without removing tuning effects


Sea Lion Field, Beverley Fan Time Thickness Map

From Francis et al, 2015


Sea Lion Field, Beverley Fan EEI -70 Amplitude

High amplitudes due

to thickness tuning



Sea Lion Field, Beverley Fan Detuned Net Sand

Genuine Sweet Spots

Revealed by detuning



3D Seismic Detuning Advantages to Interpreter

• Fast and reliable interpretation of amplitude

anomalies

– Sea Lion Field multiple turbidite fans took over 6 weeks to

pick manually and extract 2D grids

– Single event picking on 3D detuned output reduces picking

and analysis time to just a day or so per fan

• Same single event pick can be used to display

horizon slices of amplitude, detuned amplitude,

thickness, seismic net pay and other new and novel

attributes that arise from an analysis of tuning

responses on seismic

– Currently unpublished


Central North Sea Rotliegendes Sandstone

Original


Central North Sea Rotliegendes Sandstone

Detuned


Central North Sea Rotliegendes SSt Detuning

• After detuning the field outline is much more sharply

defined, with better closure separation to the NE

• The amplitude interpretation is different after

detuning:

– Bright events in north are either removed/de-emphasised or

in different locations

– Amplitudes to south are normalised and are now consistent

with known presence of Rotliegendes in well penetrations

– Interesting bright area in south revealed after detuning


York & Greater York DT-AMP™ Detuning Study

• The York Field and the Greater York Area

– UKCS southern gas basin gas fields and targets

• Gas in Rotliegendes and Carboniferous reservoirs

– Presence of gas is expected to brighten seismic amplitudes

• DT-AMP detuning was applied to the seismic and

automated, entirely self-consistent attributes were

extracted from a single existing interpretation pick

– TT0X TWT thickness of interval

– Top (and base) TWT of interval times

– Amplitude and detuned amplitude maps


Top Rotliegend TTOP (ms)

York

Rough


Rotliegendes TWT Thickness TT0X (ms)

York

Rough


Rotliegendes Original Amplitude Map

York

Rough


Rotliegendes DT-AMP™ Detuned Amplitudes

York

Rough


Rotliegendes Original Amplitude Map

Relative Amplitude Scaling


Rotliegendes DT-AMP™ Detuned Amplitudes

Relative Amplitude Scaling


York & Greater York

• The significance of detuning seismic amplitudes is

clearly demonstrated by the before and after

amplitude maps

• Thickness related tuning is by far the predominant

effect on seismic amplitudes

– Without detuning, amplitude interpretation is misleading

• Detuning significantly improves amplitude continuity

– Small thickness changes result in large amplitude changes

• After the detuning study, the Greater York acreage

was relinquished by all partners


Seismic Detuning Summary

• The largest physical effect controlling seismic

amplitude variation is thickness change

• Thickness related tuning can double seismic

amplitudes

• All seismic amplitudes should be detuned before use

• Amplitude detuning is a completely self-consistent

calculation and does not require well control

• 3D Seismic detuning extends existing 2D map based

detuning to detune seismic trace amplitudes in situ

on a 3D (or 2D) relative impedance volume


DT-AMP™ 3D Seismic Detuning Summary

• 3D Seismic detuning

– Facilitates the comparison of the possible effects of tuning /

no tuning directly on 3D seismic sections

– Removes one risk factor in assessing amplitude data

– Makes amplitudes more easily interpreted in terms of

geology and property changes

– Significantly speeds up amplitude analysis

– Introduces additional constraining attributes

– Allows direct computation of tuning data without seismic

horizon interpretation or picking


Acknowledgements for Data

• Centrica PLC

• Rockhopper Exploration PLC

• Premier Oil

• Also older examples from

– LASMO

– Acorn Oil & Gas/Tuscan Energy (Scotland) Ltd


http://www.earthworks-reservoir.com

de-risking seismic amplitude interpretation by 3d … may 2016 © earthworks environment &...

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