Characterization and occurrence of opal-reflectors

Transcription

1 Characterization and occurrence of opal-reflectors T. Pedersen*, T. Throndsen**, F.T. Lysell***, Å.H. Drottning**** and T. Rautakorpi** EAGE Madrid 16. June 2005 *Institute for Energy Technology, Norway **Querqus AS, Norway ***Globex Norway AS, Norway ****NORSAR, Norway

2 Introduction Opal A->opal CT->quartz transitions cause density and rigidity increases The increased accoustic impedance may produce cross-cutting reflectors These may be misinterpreted as Direct Hydrocarbon Indicators or Bottom Simulating Reflectors (gas hydrates)

3 Fylla prospect offshore West Greenland A presumably Opal-CT->Quartz transition is observed at 2110 mrbb with A moderate increase in density and velocity After GHEXIS (2001)

4 Objectives of this study To provide a better understanding of the occurrence and characteristics of the opal transitions. This may have beneficial implications for petroleum exploration in two ways: Improved identification of false DHIs and thus improved risk assessment in areas where opal-reflectors occur Understanding the occurrence of the opal-reflectors may provide important control parameters for depositional environments, tectonic evolution and maturity reconstructions (e.g. temperature)

5 Methodology Literature survey Theoretical studies Well observations Seismic interpretation Seismic modelling

6 Important variables in silica diagenesis After Kästner (1985) and Behl and Garrison (1994)

7 Temperature and time 1,0 OPAL-A FRACTION 0,8 0,6 0,4 0,2 0,0 50 C 30 C 10 C 1st order reaction A= /y E=16 kcal/mol (Source: Mizutani, 1970) TIME (MY)

8 Fractions of opal-a, opal-ct and Q versus depth 1,0 FRACTION PHASE A, CT, Q 0,8 0,6 0,4 0,2 OPAL-A 1st order reaction A1 (A->CT)= /y A2 (CT->Q)= /y E1=E2=16 kcal/mol Source: Mizutani (1970) OPAL-CT QUARTZ V=300 m/my dt/dz=30 C/km 0, DEPTH (M)

9 Temperature scale for silica diagenesis Derived for the Monterey Formation by Keller and Isaacs (1985)

10 Very pure opal transformed at extremely low tempertures After Bohrmann (1994)

11 Calculated depths for BSRs Water depth, m 30 degc/km 40 degc/km 50 degc/km 60 degc/km

12 Porosity versus depth Data the Point Pedernales, Monterey Formation, California. After Compton (1991)

13 Sonic and density log in well 30/ Density log, g/cm Depth, mrkb opal-a/opal-ct transition zone Sonic log, µs/ft

14 Sonic and density log in well 6704/ Density, g/cm Depth, mrkb opal-a/opal-ct transition zone Sonic log, µs/ft

15 Depth beneath seabed to the opal-refl. 30/3-3 31/2-1 31/2-2 31/2-3 31/2-5 34/7-1 34/7-6 35/3-2 35/3-4 36/1-1 36/ / / / / / /9-1 Up 7117/9-1 Lo UK 214/4-1 WG Qulleq-1 Present day depth Neogene net uplift Depth beneath seabed, m

16 Opal transition temperatures* 30/3-3 31/2-1 31/2-2 31/2-3 31/2-5 34/7-1 34/7-6 35/3-2 35/3-4 36/1-1 36/ / / / / / /9-1 Up 7117/9-1 Lo UK 214/4-1 WG Qulleq-1 Present day temperature Maximum temperature Depths m Temperature, o C *Corrected for uplift (Throndsen et al, 2003)

17 Seismic display of opal anomaly 6704/12-1 Possible opalanomaly Transparent low-frequency layer Layer with small-scale faulting

18 Seismic interpretation results Both conventional and seismic attribute displays were generat used in the seismic interpretation The most useful seismic attributes were found to be : relative acoustic impedance instantaneous frequency instantaneous phase apparent polarity Other seismic attributes were also used in addition to the original seismic trace representing the real part of the analysis signal, f(t).

19 Seismic modelling parameters

20 Survey description

21 Seismic modeling results

22 Conclusions I Common phenomena Transition temperatures depend on lithology Wells analysed in this study cluster at about 42 C The seismic response generally easier to identify offshore Mid-Norway than in the North Sea Mostly in the Brygge Formation offshore Mid-Norway and in the Hordaland Group in the North Sea Seismic modelling shows that for small offsets the seismic response of opala-ct and GWC almost identical, whereas for larger offsets the responses will be significantly different

23 Conclusions II Small-scale faulting and chaotic internal seismic facies that may be due to shrinkage of the sediments. This has become one of the most important seismic criteria in order to identify, interpret and distinguish opal-anomalies from seismic responses of direct hydrocarbon indicators (DHI), boundaries caused by lithofacies, and gas hydrates. Seismic sequences with opal-anomalies usually show passive basin-fill configurations. Up-dipping or down-dipping opal-reflectors may indicate lateral variations in geothermal gradients or structural modifications post-dating the opal- formation.

24 Conclusions III The discontinuous and relatively weak reflectors associated with opalanomalies are mainly due to internal complexity of the host rock, including small-scale faulting, and often also due to seismic interference. True direct hydrocarbon indicators show in most cases more continuous and undisturbed seismic reflections, and occur over a wider depth range (including higher temperatures) than opal-anomalies. The occurrence of opal-anomalies are limited by relatively low maximum temperatures. Seismic modeling may provide useful in separating DHIs and OPTs

25 Acknowledgements Characterisation and occurrence of opal-reflectors This presentation is based on the multi-client report Throndsen et al. (2004) Opalreflector