From loose grains to stiff rocks The rock-physics "life story" of a clastic sediment, and its significance in QI studies

Transcription

1 From loose grains to stiff rocks The rock-physics "life story" of a clastic sediment, and its significance in QI studies Prof. Per Avseth, NTNU/G&G Resources

2 Burial depth/temp. Elastic Modulus The rock physics life story of a clastic sediment a teaser: B A B A Constant Cement Contact Cement Age (M.yrs) Mechanical compaction 70 o C Chemical compaction Bjørlykke (2015) Tectonic uplift 1 K sat Friable Porosity 1 K mineral + Initial Sand Pack K + K fluid Vp/Vs Pore compressibility strongly affected by diagenesis! Deposition Sorting variability Mechanical compaction B HC trends Chemical compaction A Acoustic Impedance

3 Selected references: Helset et al., 2004: Combined diagenetic and rock physics modeling for improved control on seismic depth trends (EAGE Abstract) Brevik et al., 2011: Rock Physicist step out of the well location, meet geophysicists and geologists to add value in exploration analysis (The Leading Edge). Dræge et al. 2014: Linking rock physics and basin history Filling gaps between wells in frontier basins (The Leading Edge). Zadeh et al. 2016: Compaction and rock properties of Mesozoic and Cenozoic mudstones and shales, northern North Sea (Marine and Petroleum Geology). Avseth and Lehocki, 2016: Combining burial history and rock-physics modeling to constrain AVO analysis during exploration (The Leading Edge). 3

4 Reducing uncertainties through integration Geology Many parameters Ambiguities Variability Complexity Rock Physics Bridge & Bottle-neck Uncertainties QI prediction Geophysics Few observables Limitations/Resolution Noise/Artifacts More knowledge! Pit-falls More integration! More/Better data!

5 Case example from North Sea (Alvheim Field)

6 Sand and shale compaction trends in the North Sea

7 Elastic Modulus Alvheim well (Kneler discovery) Velocity jump in sst due to cementation Vclay Vp Porosity leg Constant Cement Contact Cement CCT Friable Initial Sand Pack Friable sst Porosity

8 Burial history and subsidence curves for top reservoir sst at Kneler well (schematic) 0 Deposition Paleocene -60 Continuous subsidence Tilting/Uplift Miocene Time (M.yrs before present) 70C = ca. 2km Upper Sst Unit Present day burial Lower Sst Unit Maximum burial Burial depth/temperature Time range of target > 70C (= cementation)

9 Avseth and Lehocki (2016) (Mahalanobis distance) AVO classification constrained by depth trends (Alvheim field, North Sea) Rimstad et al. (2012): (Bayesian classification) Gas Uncon. shale shale Unc. Gas Cem. Cemented

10 Are injectites on Volund cemented or not? From Schwab et al. 2015

11 Depth (m) Well log data from 24/ m 2000 Intra Balder (oil) 2050 Intra Balder (brine) 2100 Hermod Heimdal 2250m GR Velocity Acoustic impedance Vp/Vs GR Vs & Vp AI Vp/Vs

12 Elastic Modulus Vp (km/s) Rock Physics diagnostics of Paleocene sandstone units Heimdal Dvorkin-Nur contact cement model Hermod 3 Balder sst 2 Constant Cement 2.5 Contact Cement Unconsolidated sand model Balder sst 1 2 Initial 1.5 Sand 0.15 Pack Porosity Friable Porosity

13 Elastic Modulus Volume of Qz cement precipitated between time T1 and T2 Combined modeling of burial history and rock physics Mechanical compaction (Lander and Walderhaug, 1999) Sum of pore space: Chemical compaction (Walderhaug, 1996), when T > 70 C Porosity at Cement start Rock physics modeling Stable packing configuration Dep. porosity Volume of Qz cement at time T1 Molar weight of qz Initial volume of pore filling material Qz- density Constant Cement Effective stress Exponential decline factor Qz- precipitation rate Qz- surface area Heat rate Contact Cement Hertz-Mindlin (mech. comp) Dvorkin-Nur + Hashin-Shtrikman (chemical compaction) Friable Initial Sand Pack Porosity Slide 13

14 Combined burial and rock physics modeling of porosity versus P- wave velocity (sensitivity study)

15 Slide 15 Rock physics and AVO modeling constrained by burial history 1. Burial history 2. Diagenetic modeling (Walderhaug) 250 Deposition 0 Geologic Age (M.Yr) Cement volume Porosity Stø Fm Onset cement (Present day burial) Burial depth (m) Temperature (degrees C) Max. burial Depth/Temperature 3. Rock physics modeling (Dvorkin-Nur) Acoustic Imp. Depth/Temperature Vp/Vs + shale compaction and RP 4. AVO modeling (Zoeppritz) Gradient + Deposition - + Intercept Shale (Background) - Max. burial

16 16 A «typical» present day geo-section offshore Norway Sea water Quarternary sediments Miocene sediments Oligocene-Eocene Paleocene Prospect A Discovery Prospect B Prospect C Cenozoic uplift Increasing landward 70 C Cretaceous Syn-rift Jurassic/Triassic

17 17 «Restoring» geo-section to maximum burial. Have prospects been into the frying pan? The Frying Pan (T>70 C required for Qz-cem. )

18 Burial constrained AVO modeling at Discovery well. (Campanian sands w/oil give AVO class III) Burial curve Shale sand 70 C Shale Frying pan (chemical compaction/qz cementation) Compaction trend sst Fluid trend

19 19 Burial constrained AVO modeling at prospect A (Paleocene sst) -filled sst = AVO class I-IIp Burial curve Shale sand 70 C Shale Frying pan Compaction trend sst Fluid trend sst

20 Burial constrained AVO modeling at prospect B (Paleocene sand) -filled sst = AVO class III Burial curve Frying pan Shale sand 70 C Shale Compaction trend Fluid trend

21 Burial constrained AVO modeling at prospect C (Aptian sst) -filled sst = AVO class IIp Burial curve Shale sand 70 C Shale Frying pan Compaction trend sst Fluid trend sst

22 Gradient 22 RPT and AVO well 7319/12-1 Vp/Vs AI Vp/Vs shale heterolithics Porosity/Rock sand stiffness Gas Gas Limestone Gas (tight) Fluid trend AI Litho/Fluid classification Intercept

23 23 Burial analysis and simulated AVO signatures in Pingvin (7319/12-1) Uplift = 700m Gas 700 Max burial = 1250m Onset cementation = 70C Shale Net erosion map from N. Johansen (2017) sand Cem. sst Diag. Gas Cap-rock Reservoir Gas Unc. sand Sorting The reservoir sands in Pingvin has not been buried deep enough to be cemented! Hence, great fluid sensitivity! AVO class III expected for any HC-fill.

24 24 Eocene more deeply buried in Sørvestnaget Basin: Burial constrained AVO at well 7216/11-1S Onset cementation = 70 o C Gas

25 Wisting example Net uplift=1500m (Temp grad=42.5c/km) Gas

26 Summary: How burial controls AVO and fluid sensitivity Age Pingvin 70 o C AVO class III depth Eocene Prospect 1 AVO class III-IV Eocene Prospect 2 AVO class II-III AVO class II-IIp Jura prospect

27 Conclusions The present day seismic signatures will reflect depositional and burial history, and this knowledge should be included in AVO and QI studies. Rock physics modeling can be combined with burial modeling for uplift estimation, to constrain low-frequency trends, and to model expected AVO signatures. Normally, bright seismic amplitudes and class II-III AVO signatures are only associated with unconsolidated to poorly consolidated sandstones. There is likely a lot of «hidden» hydrocabrons (esp. oil) in consolidated sandstone reservoirs with stiff rock frame and reduced fluid sensitivity, that can be challenging to discover during AVO and QI studies.

28 Acknowledgements Thanks to Ivan Lehocki at Lehocki Geospace for contributions to burial modeling codes. Thanks to MCG for AVO data on Pingvin Thanks to FORCE for the invitation 28