Geophysical Correlation. Seismic Stratigraphy Core/Well Logging

Transcription

1 Geophysical Correlation Seismic Stratigraphy Core/Well Logging 1

2 Spiess et al., 2003 Data collection Bottom profiler (3.5kHz) Source Air/Water guns ( Hz) Receivers (hydrophones) Single/multi-channel >240 m ( hyrdophones) 2

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4 Neogene Paleogene 4

5 Oi-1 c24n P-E MPBE K/T 5

6 E/O c24n P-E 6

7 Site 1263 Holes A,C,D P/E Boundary Cores C Hole A Hole D Hole -1.5 A Hole 1263 A 34 X Section C 14H Section D 4H Section 1 Depth (m from PE) C Hole D Hole Magnetic Susceptibility (Instrument Units) Seismic Stratigraphy acoustic properties of sediments - sound waves are generated and a reflection signal is recorded Not for well to well (hole) correlation, but extending stratigraphic information (time) from a single well (or several wells) over a large region Applications in subsurface Geology: 1. Facies geometry/architecture 2. Sediment deposition patterns 3. Optimization of drilling (scientific or oil exploration) 7

8 Basic Principles Acoustic properties of sediments Acoustic energy (sound vibrations) Frequency- cycles/sec Hertz (Hz) Velocity (V) - increases w/ density Sediments km/sec Discontinuity (bedding planes/uncomformity) - reflects a portion of the energy % reflected depends on a reflection coefficient acoustic impedance (I) - resistance to energy transmission by rocks on either side of the reflector. I=pV p= density (& rigidity) 1) porosity 2) grain density 3) pore fluid composition bedding surface or unconformities produce reflectors AI (m/s * g/cm 3 ) = pv 8

9 Refraction & Reflection Reflection better? More data processing Superior lateral resolution Deeper penetration with less equip. Better suited for marine applications 2 methods (1) Refraction - Basic Principles velocity of seismic energy as a function of depth is calculated from refracted energy (2) Reflection - from boundaries separating rock layers w/ different physical properties travel times T direct = 2X/V 1 X = distance between source and receiver (geophone) V 1 = velocity of the media 9

10 Sediment Density and Velocities T=2 sec, V 1 =5 km/sec T direct = 2X/V 1 X = T direct V 1 /2 X = 5 km Lithology V(km/s) Shale 3.6 Sandstone 4.2 Limestone 5.0 How does porosity affect velocity? Seismic Penetration/ Resolution Low frequency waves (20 Hz, typical ~200 to 500 Hz) Deep penetration (~10 km) low resolution (cannot resolve beds <50 m) High frequency waves (3.5 khz) Shallow penertration (~50 m) higher resolution (~0.5 m or thinner) Reflection Amplitude Density contrast Bed spacing/thickness Fluid content 10

11 High Frequency (3.5 khz) Reflection Seismology: Stratigraphic Interpretation Reflector configuration Parallel, divergent, prograding, chaotic, blank Reflector continuity Continuous, discontinuous (top/base discordant) Reflector amplitude Density contrasts, bed spacing Reflector frequency & velocity Bed spacing, fluid & gas pressure, porosity 11

12 Great Barrier Reef (Glenn et al., 1993) Fildani & Normark,

13 Fildani & Normark, 2004 Neogene Lower Turbidite System (LTS) Fildani & Normark, 2004 Late Quaternary Upper Turbidite System (UTS) 13

14 Reflection Data Observed vs. Synthetic Core and Downhole (Well) Logging 2 means of obtaining information - Discrete sampling outcrop cores or mudlogs Downhole/Well/Borehole logging Boreholes - tying discrete sample measurements (cores or mud logs) to seismic reflection profiles 14

15 Geophysical and Geochemical Logging Benefits: High resolution regional stratigraphic correlation Provides first order lithologic variations (2 or 3 component systems) Speed/ Resolution (±1000yr) essential for interpreting seismic reflection data Downhole Logging 15

16 Geophysical and Geochemical Logging 3 categories 1. Electrical Logs 2. Radioactivity Logs 3. Sonic Logs Logging Tools Resistivity (phasor Dual Induction Tool [DIT]), bulk density (Hostile Environment Litho-Density Sonde [HLDS]), gamma radiation (Hostile Environment Natural Gamma Ray Sonde [HNGS]), and porosity (Accelerator Porosity Sonde [APS]) components with two additional LDEO tools that measure high-resolution gamma radiation (Multi-Sensor Gamma Ray Tool [MGT]) and highresolution temperature, acceleration, and pressure (Temperature/Acceleration/Pressure [TAP]. Formation MicroScanner (FMS)-sonic tool string, consisting of microresistivity (FMS), sonic velocity (Dipole Sonic Imager [DSI]), gamma radiation (Scintillation Gamma Ray Tool [SGT]), and orientation/acceleration (General Purpose Inclinometer Tool [GPIT]) 16

17 1. Electrical logs measure the electrical properties of rocks and fluids Passive or dynamic methods: Passive: naturally produced currents caused by differing potentials Dynamic: strength of an induced current after it has passed through a known portion of the rock S.P. Logs - (passive) Spontaneous Potential between borehole fluid and formation fluid. permeability salt content of fluids Resistivity logs - (dynamic) electrical current carried to the rock unit via an electrode in the tool. current needed to maintain a constant voltage drop over a fixed interval of rock function of the inverse square root of porosity (conductivity) Formation microscanner (FMS) - array of 16 electrode buttons pressed against borehole wall ODP Site 1207, Shatky Rise, NW Pacific Age: Cretaceous - Cenozoic Lithology 17

18 measure the nuclear properties of rocks and fluids natural gamma ray 2. Radioactivity logs scintillation detector - gamma rays emitted by naturally radioactive isotopes lifted up the bore hole K-40, Th, U in most clay minerals U in organic matter ODP Site 1207, Shatky Rise, NW Pacific Age: Cretaceous - Cenozoic Lithology: Pelagic Limestone, Chalk (low GR) Chert stringers Black Shales -U Clays - K and Th 18

19 Leg 189, Tasman Sea 19

20 Regional correlation of major lithostratigraphic units, using total gamma ray data from Leg 189 Spectral gamma-ray data from Hole 1161, showing high Th values in a mudstone unit 20

21 Spectral gamma-ray data from Hole 1172D, showing high U values in an organic-bearing claystone unit between ~ mbsf Spectral gamma-ray data from Hole 1171D, showing high K values due to the presence of glauconite 21

22 2. Radioactivity logs induced gamma ray spectroscopy (GST) (14 MeV neutron generator) (bombardment by sum-atomic particles (neutron activation) 1. Density log gamma rays collide w/ electrons in minerals and scatter. Counters located further down the tool record scattered rays which reflect density. 2. Neutron log emit fast moving neutrons, slowed by H + in water slow neutrons then enter the nuclei of atoms in minerals, destablizing the atoms which emit gamma rays Detectors-256 channel energy spectrum- measure either the: returning neutrons (fluid volume or porosity) gamma rays (elemental composition) relative concentrations of Si, Ca, Fe, S, H, Cl Log ~ Depositional Facies Organic rich, clay layer high U high K, Th low density low neutron porosity 22

23 Depositional Facies Bethany Beach, NJ 23

24 Log - Depositional Facies Bethany Beach, NJ 3. Sonic logs Digital Sonic Tool (DIT) What? lithologies & rock porosity How? transmitters & receivers sound waves (compressional)mirgrate through a unit of rock along the borehole wall travel time (velocities) varies as a function of lithification & porosity Use in combination with gamma ray logs 24

25 ODP Site 1207 Limestone, Chalk, Chert stringers Core Logging Direct measurement of sediment properties (cores) multisensor track: 1. Grape 2. P-wave 3. Magnetic susceptibility 25

26 1. GRAPE - Gamma ray attenuation porosity evaluator bulk density (function of porosity) Gamma rays absorbed or scattered by electrons - proportional to bulk density. density is determined from attenuation of gamma rays within a specific energy. attentuation increases with increasing density I = I o e µd! " ρ β = bulk density in g/cc, I = beam intensity which penetrates the absorber I o = source intensity µ = mass attenuation coefficient (cm 2 /gm) d=thickness or diameter of sample (cm)! " = ln I o I ( 1 µd ) 2. P-Wave - Compressional Velocity density/porosity transmits a 500 khz compressional wave pulse - repetition rate of 1 khz 3. Natural Gamma decay of radioactive elements (mainly K, Th, U). 26

27 4. Magnetic Susceptibility magnetizable material - 2 types ferromagnetic (magnetite, hematite, iron titanium oxides, pyrrhotite, maghemite, greigite and goethite) paramagnetic clay minerals (chlorite, smectite and glauconite) iron and manganese carbonates (siderite, rhodochrosite) ferromagnesian silicates (olivine, amphiboles, pyroxenes, etc.), mm internal diameter sensor loop 27

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30 Spiess et al.,

31 d/ t= 125m/6 m.y. d/ t= 20.8 m/m.y. 31

32 0-100 mbsf mbsf mbsf mbsf ODP Leg 208 Sites Walvis Ridge 32

33 ODP Leg 208, Walvis Ridge PE Boundary Depth Transect %CO m Zachos, Kroon, Blum et al. (2004) Paleodepths (~55 Ma) 3500 m 33

34 Leg 208: CCD/Climate Excursions (50-56 Ma) Age (Ma) ELMO (ETM-2) PETM (ETM-1) 56 Shallow Deep ELMO/ETM

35 Turnover of Planktonic Foraminifers Paleocene Foraminifera Cretaceous Foraminifera 35

36 Depth (meters) Cretaceous/Tertiary Boundary (66 Mya) SIte Magnetic Suscept A B C D E Compute Sedimentation Rates (SR) for this core! Spec. FA(Ma) A B C D E LA is same for A,B,C SR = d/ t SR = ( m)/( Ma) SR=22.4 m/m.y. What is the dominant cycle period? p= t/#cycles p=210 k.y./10 = 21 k.y. 36