Factors controlling velocities in carbonate sediments and rocks

Transcription

1 Workshop: Geophysical Applications to Carbonate Reservoirs, 22 SEG Annual Meeting, Salt Lake City, Utah. Factors controlling velocities in carbonate sediments and rocks Gregor T. Baechle, Flavio S. Anselmetti2, Gregor P. Eberli, Michael L. Incze3, and Guido Bracco Gartner () Comparative Sedimentology Laboratory, University of Miami, 46 Rickenbacker Cswy, Miami, FL 3349 (2) Geological Institute, Swiss Federal Institute of Technology (ETH), Sonnneggstr, Zurich, 892, Switzerland (3) DIVNPT, Naval Undersea Warefare Center, Introduction A seismic section is the image of a physical response of an acoustic signal to the rock. The results are based on experimental studies of the University of Miami Petrophysics Group. The velocimeter was built by Karl Coyner (Verde Geoscience) and contains a transducer system that simultaneously measures a single compressional wave and two independant orthogonally polarized shear waves. is measured under both variables confining pressures and pore pressures. Confining pressures can be increased up to MPa and pore Distribution of acoustic impedance & geometries define physical responses Acoustic impedance is defined as the product of velocity and density Impedance of carbonates: mainly controlled by variations in velocity Processes which control velocity distribution in carbonates: Depositional environment? Diagenesis? Pore Type? Knowledge is important to interpret sonic logs and seismic data. Unconsolidated carbonates Petrophysical response to change in facies: Grain size distribution as a function of environment Tidal Channel Lagoonal Sediments exposed beach Great Bahama Bank tidal channel Florida Bahamas bank interior Bank Interior tidal bar patch Patch Reef/Tidal Bar interior lagoon back channel Back Reef Channel/ Low Energy Platform Interior low energy tidal bar/lagoon Lagoonal/Low Energy Tidal Bar Grain size distribution: generally high spatial homogeneity at Bahama & Florida samples the higher the energy the more homogeneous is its mean grainsize the higher the energy regime, the better the sorting Mineralogy / velocity: % Total Porosity Porosity: some environments show considerable overlap inverse correlation between porosity and energy regime weak correlation Porosity ooid levee margin 6 Reef Margin Energy 4 2 Florida Margin Grain Size

2 Workshop: Geophysical Applications to Carbonate Reservoirs, 22 SEG Annual Meeting, Salt Lake City, Utah. How diagenesis and carbonate clay ratio effect velocity Unconsolidated shallow water carbonates: Coralgal Ooids Experiment: and porosity as a function of pressure and matrix content. Components: matrix = aragonite mud with 7% porosity grains:. ooids (3 microns) 2. coralgal ( microns) Porosity as a function of matrix content as a function of matrix content Acoustic Coralgal Ooid % Porosity 2 2 MPa 3 MPa CORALGAL % % 2% 4% 8% % 3% 6% % Extensive fracturing of coralgal grains (Halimeda) POROSITY (Coralgal) 4 6 MPa ACOUSTIC VELOCITY 4 8 % Total Porosity as a function of matrix and pressure CONFINING PRESSURE (MPa) Critical porosity at very low matrix content (2%) Samples of high matrix content show significantly velocities at high pressures Maximum velocity achieved at low matrix content <2% Significantly decreasing velocity at matrix content > 46% Early diagenesis: Evolution of pore type during early diagenesis: Increase in total cement Change in primary porosity Decrease in total porosity (excluding vugs) Transition from primary to secondary pores Effect on velocity: Lithified samples demonstrate higher variability in velocity Higher velocities at equal porosities due to constructive diagenesis of unlithified mud samples are indicating that porosity reduction due to compaction has a minor effect on velocity Carbonate rocks: Pwave velocity Pwave velocity vs insolubles content (MPa) Pwave velocity vs carbonate content (3MPa) Gargano Last Chance Canyon Insolubles (%) Carbonate content (%) Pure carbonates have a wide range of velocities. Clay decreases velocity above a threshold of % porosity.

3 Workshop: Geophysical Applications to Carbonate Reservoirs, 22 SEG Annual Meeting, Salt Lake City, Utah. Porosity evolution during diagenesis velocity a function of pore type Microporosity Moldic porosity Densely cemented Interparticle/ Intercrystalline porosity Lessons : Early diagenesis strongly influences velocity in carbonates, overriding burial and age as controlling factors in carbonates. in carbonates is a product of porosity and pore type. scatter is due to different pore types that are created by diagenesis. Constructional diagenesis overprints compaction effect. Combined effect of lithology & diagenesis on pore type & velocity Clino Diagenetic Pore Units Type Facies deeper margin d 2 3 D 4 E mbmp B C D 2 E F Pliocene upper Density and fore Densit Pleist c C and Pliocene Facies a b 3 e g f deeper margin G Miocene mbmp Pleistocene Mineralogy Density Unda Diagenetic Pore Units Mineralogy Type H h (ft/s) 2 (g/cm3) Miocene F/G (ft/s) Platform top (Unda): fairly good correlation between lithology and pore type velocity controlled by pore type Slope (Clino): Connected Molds 2 (g/cm3) Vuggy to Cavernous is the dominant factor to create acoustic impedance contrast in carbonates. Seismic Sequence Boundaries correlate to surfaces of subaerial exposure, changes in facies, mineralogy and/or diagenetic units. Dolomite wt.% Diagenetic Zone Microporosity Calcite wt.% Diagenetic Zone 2 Mixed connected/isolated molds Aragonite wt.% Diagenetic Zone 3 Intercrystalline weaker correlation between lithology and diagenetic units velocity controlled by the combined effect of lithology and diagenetic overprint No correlation exists between dolomite content and velocity!

4 Workshop: Geophysical Applications to Carbonate Reservoirs, 22 SEG Annual Meeting, Salt Lake City, Utah. Permeability trends derived from pore type Deviation log Quantitative image analysis of thin sections Observation: Assumption: Pore structure of round pores are more likely to represent low permeable samples. Two dimensional pore shape gives an indication about connectivity of the pores. Porosity vs Permeability Permeability (md) Permeability (md) Pore shape vs Permeability image MICROPOROSITY MOLDIC POROSITY INTER/INTRAFRAME Endmember Conclusions High diagenetic alteration potential distinguishes carbonates from other No correlation exists between dolomite content and velocity. Shallow water carbonates have higher average velocity values than carbonates from the deeper shelf, or basin due to lack of clay. The velocitydeviation log can provide continuous pore type record and display trends in permeability. Pore shape parameter obtained by image analysis of thin sections show positive correlation with permeability.

5 ical Applications to Carbonate Reservoirs, 22 SEG Annual Meeting, S Predicted velocities using the Gassmann theory Pwave velocity 4 Gassmann sample sat sample dry Gassmann sample 38 sat sample 38 dry Pressure Porosity (MPa) (%) Gassmann sample 9 sat sample 9 dry Pressure (MPa) Gassmann sample 3 sat sample 3 dry Velocities in saturated samples are larger than dry sample velocities Predicted saturated velocities tend to be either over or underestimated Measured dry and saturated velocities at 4MPa effective pressure pvelocity dry svelocity dry pvelocity sat svelocity sat deviation between measured and predicted values Difference in pwave velocity deviation between measured and predicted values Difference in swave velocity Predicted pwave velocities show +/ 6% deviation from measured data, whereas swave velocities show even higher deviations. Vp dispersion Vp vs effective pressure Pressure [MPa] Velocities measured at.mhz normalized to MHz velocities. increase of up to 3% A percental decrease in No uniform pressure sensitivity velocity of up to.% is observed, as frequency is reduced from MHz to.mhz.