De-risking Seismic Amplitude Interpretation by 3D Seismic Volume Detuning

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Godfrey Robbins
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2016 Ashley Francis 24 May 2016 Earthworks

2 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 24 May 2016 Earthworks Environment & Resources Ltd. All rights reserved 2

3 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 24 May 2016 Earthworks Environment & Resources Ltd. All rights reserved 3

4 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 24 May 2016 Earthworks Environment & Resources Ltd. All rights reserved 4

5 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 % So we expect bright negative amplitudes are gas Except thickness changes in the same interval can change amplitudes by 100%! 24 May 2016 Earthworks Environment & Resources Ltd. All rights reserved 5

Reflection Coefficient Kadanwari Field Sand AVO / Fluid Model 0.1500 Soft Shale/ Brine Sand 0.1000 0.0500 0.0000-0.0500-0.1000-0.1500-0.

6 Reflection Coefficient Kadanwari Field Sand AVO / Fluid Model Soft Shale/ Brine Sand Soft shale / Gas Sand GW C Hard Shale / Brine Sand Hard shale/soft sand Gas is about 25% brighter than brine Hard Shale / Gas Sand Sin 2 q After Syed and Francis, May 2016 Earthworks Environment & Resources Ltd. All rights reserved 6

Brine Saturated Reservoir Wedge Model Brine case

After Syed and Francis, 2001 24 May 2016

Brine Saturated Reservoir Relative Impedance Response After Syed and Francis,

9 Relative Amplitude Relative Impedance Amplitude Response with TWT Thickness Reference Amplitude = TWT (ms) After Syed and Francis, May 2016 Earthworks Environment & Resources Ltd. All rights reserved 9

Normalised Amplitude Tuning Curve Model Relative

6 Thin Bed Limits Peak tuning Amplitude Amplitude decay

2 User Defined Normalisation Amplitude 0 0 10 20 30 40

10 Normalised Amplitude Tuning Curve Model Relative Impedance Response Thin Bed Limits Peak tuning Amplitude Amplitude decay at greater thicknesses Lack of Low F User Defined Normalisation Amplitude Seismic TWT Thickness (ms) 24 May 2016 Earthworks Environment & Resources Ltd. All rights reserved 10

11 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 24 May 2016 Earthworks Environment & Resources Ltd. All rights reserved 11

Normalised Amplitude Amplitude Relation to N:G Different Time Thickness 1.2 1 0.8 The same amplitude corresponds to very different N:G at different thicknesses on the tuning curve 0.6 0.

12 Normalised Amplitude Amplitude Relation to N:G Different Time Thickness The same amplitude corresponds to very different N:G at different thicknesses on the tuning curve Amplitude N:G=0.41 N:G=0.48 Sand 1 Sand 2 Sand Seismic TWT Thickness (ms) N:G= May 2016 Earthworks Environment & Resources Ltd. All rights reserved 12

13 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 selfconsistent calculation that does not require wells 24 May 2016 Earthworks Environment & Resources Ltd. All rights reserved 13

14 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 24 May 2016 Earthworks Environment & Resources Ltd. All rights reserved 14

15 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 24 May 2016 Earthworks Environment & Resources Ltd. All rights reserved 15

Normalised tuning curve Low Frequency Response 1 80 Hz 20 80 Hz Thickness

18 TWT (ms) Sea Lion Field, Main Fan Original Coloured Inversion Well 14/10-4 Well 14/ Thick Sand High N:G Thinner Sand Lower N:G 2400 Bright Constant Amplitudes Downdip 24 May 2016 Earthworks Environment & Resources Ltd. All rights reserved 18

19 TWT (ms) Sea Lion Field, Main Fan Detuned Modulated Amplitude Well 14/10-4 Well 14/ After Detuning, Downdip Amplitudes Show N:G Change 24 May 2016 Earthworks Environment & Resources Ltd. All rights reserved 19

Seismic Amplitude Before Detuning 8000 7000 6000 Before Detuning: Downdip Well 4 Higher Amplitude But Lower Net:Gross 5000 4000 3000 N:G=70%

20 Seismic Amplitude Before Detuning Before Detuning: Downdip Well 4 Higher Amplitude But Lower Net:Gross N:G=70% Well 4 Well 5 N:G=94% Time Thickness (ms) 24 May 2016 Earthworks Environment & Resources Ltd. All rights reserved 20

Seismic Amplitude After Detuning 8000 7000 6000 After Detuning: Downdip Well 4 Lower Amplitude Consistent with Lower Net:Gross 5000 4000 Normalised Amplitude

21 Seismic Amplitude After Detuning After Detuning: Downdip Well 4 Lower Amplitude Consistent with Lower Net:Gross Normalised Amplitude N:G=70% N:G=94% Well 4 Well Time Thickness (ms) 24 May 2016 Earthworks Environment & Resources Ltd. All rights reserved 21

22 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 24 May 2016 Earthworks Environment & Resources Ltd. All rights reserved 22

Sea Lion Field, Beverley Fan EEI -70 Amplitude

Francis et al, 2015 24 May 2016 Earthworks

Sea Lion Field, Beverley Fan Detuned Net Sand Genuine Sweet Spots Revealed by detuning From

26 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 24 May 2016 Earthworks Environment & Resources Ltd. All rights reserved 26

Central North Sea Rotliegendes Sandstone Original 24 May 2016

29 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 24 May 2016 Earthworks Environment & Resources Ltd. All rights reserved 29

30 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 24 May 2016 Earthworks Environment & Resources Ltd. All rights reserved 30

Top Rotliegend TTOP (ms) York Rough 24 May 2016

Rotliegendes Original Amplitude Map York Rough 24 May 2016

37 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 24 May 2016 Earthworks Environment & Resources Ltd. All rights reserved 37

38 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 24 May 2016 Earthworks Environment & Resources Ltd. All rights reserved 38

39 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 24 May 2016 Earthworks Environment & Resources Ltd. All rights reserved 39

40 Acknowledgements for Data Centrica PLC Rockhopper Exploration PLC Premier Oil Also older examples from LASMO Acorn Oil & Gas/Tuscan Energy (Scotland) Ltd 24 May 2016 Earthworks Environment & Resources Ltd. All rights reserved 40

Text... The giant Johan Sverdrup Field was discovered in 2010, and improvement in seismic imaging played an important role in it's discovery.

0 Text... The giant Johan Sverdrup Field was discovered in 2010, and improvement in seismic imaging played an important role in it's discovery. Only one year before the discovery, PGS acquired on behalf