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Lecture Presentation Fundamentals of Well Logging

Natural Gamma-Ray Logs and their Interpretation Carlos Torres-Verdín, Ph.D. Professor Department of Petroleum and Geosystems Engineering

The University of Texas at Austin

Objectives: 1. To understand the physical principles behind the operation of spontaneous gamma-ray logging tools, 2. To learn how to interpret gamma-ray logs in terms of clastic lithology, shale content, grain size, and some other petrophysical properties, 3. To conceptually understand when and when not gamma-ray logs are indicative of shale/clay concentration, 4. To introduce the concept of spectral gamma-ray logs, and 5. To understand what environmental corrections are customarily applied to gamma-ray logs.

IMPORTANT REMARKS: 1. Clay/shale can substantially affect petrophysical properties of rocks such as porosity, irreducible water saturation, capillary pressure, relative permeability, absolute permeability, and permeability anisotropy. 2. It is necessary to diagnose the specific distribution of clay/shale in the pore space, the type of clay, and the volumetric concentration of clay/shale in order to quantify the petrophysical properties of rock formations. 3. Presence of clay/shale affects practically ALL well-log measurements. 4. Presence of clay/shale can cause electrical, permeability, and elastic anisotropic behavior.

Examples of Turbidites: Bouma Sequences

Where are the Shales?

Relationship between Grain Size Distribution, Pore Size Distribution, Throat Size Distribution, and Tortuosity: Influence on Permeability and Capillary Pressure

Turbidite Deposits / Submarine Fans

Geological/Depositional Model

Analogous Example: Offshore Nigeria, Niger Delta Slope From Pirmez et al., 2000

Dr. Galloway’s model

Geological/Depositional Model

Shale, Silt, and Clay

FACTS: 1. Clays are naturally radioactive (they spontaneously release gamma rays). 2. Most clays contain Th, U, and K. Clay/shale concentration increases with [Th, U, K] concentration. 3. In siliciclastic rocks, grain size often correlates with presence of clay/shale. 4. Warning I: there are some rocks which have no clay/shale but do exhibit abnormal concentrations of [Th, U, and/or K]. 5. Warning II: Drilling mud can contain K.

RADIOACTIVITY: THE BASICS

NATURAL RADIOACTIVITY OF ROCKS, NATURAL GAMMA RAY ACTIVITY

GAMMA RADIATION

DEFINITION OF HALF LIFE

QUESTION:

Why are shales/clays naturally radioactive?

Natural Element Abundance in the Earth’s Crust

(After Darwin Ellis)

What do clay and silt (shale) have to do with natural gamma ray activity?

What is a clay? Example of Clay-Coated Sand Grains

What is a clay? Example: Chlorite

What is a clay? Example: Pore-Filling Kaolinite

What is a clay? Example: Pore-Bridging Illite

Smectite

Kaolinite

Chlorite

Illite

Glauconite

Where are the Shales?

Examples of Turbidites: Bouma Sequences

Thick-bedded turbidite sands

Thick-bedded turbidite sand with discontinuous shale-clast horizons

Where are the Shales?

Where are the Shales?

Where are the Shales?:

The case of naturally radioactive sands

Where are the Shales?

Warning!

Some Evaporites are Naturally Radioactive

Where are the Shales? Sequence Boundary

Dolostone Bed

Cycle Top

nic c Bento Li m es

g E lon

ate

lays

tone

u nod t r e d ch

les

cat (sili

es)

Where are the Shales?

Calibration of Gamma Ray Detectors

SCINTILLATION COUNTERS

Logging Tools RESISTIVITY

LATEROLOG

40 cm

NEUTRON RADIOACTIVITY

GAMMA RAY DENSITY

ACOUSTIC

SONIC MICRO RESISTIVITY

RESISTIVITY

MICROLOG DIPMETER

250 cm

200

150

100

80 cm

50

30 cm 20 cm

RESOLUTION

80 cm

INDUCTION LOG

60 cm 5 cm 2 cm 0 cm 0 cm

DEPTH OF INVESTIGATION

TYPICAL GAMMA RAY RESPONSES

SPECTRA FOR K, Th, and U

TYPES OF MEASUREMENTS

INTEGRAL GAMMA-RAY MEASUREMENT

GAMMA-RAY API VALUES OF MINERALS

GAMMA RAY LOG EXAMPLE

SPECTRAL GAMMA RAY LOG

Bed-Boundary Effects

DEFINITION OF VOLUME OF SHALE Rock = Liquids and Gases + Solids (Matrix)

ROCK

SOLIDS LIQUIDS AND GASES

Shale Solid Component of the Rock Volume of Shale =

Volume of Shale Total Rock Volume

Clay and Sandstone

After Rabaute et al. (2003)

Core Vshale: Example

EXAMPLE: Synthetic gamma-ray logs

Tide Influenced Delta Frewins Castle Sandstone Belle Frourche Member Frontier Formation Cretaceous (Cenomanian) Tisdale Mountain Anticline, Wyoming

Photo by Rob Wellner

Definitions • Shale can be dispersed, laminated or structural • Shale structure is not critical in computing hydrocarbons-in-place. It is important in determining producibility. • Shale structure can only be determined from core or with image logs, like the FMI.

Laminated Shale

2 feet

• Shale laminae occupy both pore space and grain space

• φe = φss - VshL φss • These laminae are at the density resolution limit. (sand grains not to scale)

Dispersed Shale

2 mm

• Dispersed shale occupies only pore space

• φe = φss - VshD • φss or PHIMAX is the maximum clean sandstone φ

Structural Shale

2 mm

• Structural shale occupies grain space

• φe = φss

EXAMPLE 40 35

Porosity (%)

30 25

STRUCTURAL

LAMINATED DISPERSED

20 15 10 5 0

0

10

20

30

40

50

60

Csh (%)

70

80

90

THOMAS-STIEBER PLOT

100

Example: Thin Bedded Pay

Static Normalized Images Static - Equal Increments - Linear 64 color X Scale = 1:4.2 Y Scale = 1:10.0

0

90

180

•

Deep water facies thin bedded pays appear as organized bands and have moderate dips.

•

This facies makes good reservoirs, even with a very high Vsh and low interval average PHIE.

•

They are hard to detect due to low resistivity and high GR

270

ESTIMATION OF SHALE CONTENT

“Clean Sand” Baseline Volume of Shale (Vsh) Computation: Empirical Technique Ish 1

γ

0

γmin

γmax

“Shale” Baseline

ESTIMATION OF SHALE CONTENT (I)

ESTIMATION OF SHALE CONTENT (II)

EXAMPLE M-10

M-Series Sands

M-20

(

M-30

where

M-40 M-50 M-60

)

Vsh = 0.083 * 23.7 I GR − 1

I GR =

GR − GRclean GRsh − GRclean

CLAY MINERAL IDENTIFICATION

MINERAL IDENTIFICATION

Some Review Questions (Part I): 1. Why is a rock naturally radioactive? What is the definition of half life of a radioactive substance? 2. What is a gamma ray? Why do gamma rays come in different (quantum) energies (frequencies)? 3. What is a clay? 4. According to well-log practitioners, what is the definition of shale? 5. Why are precisely the gamma-ray spectral signatures of Th, U, and K used in well logging as indicators of shale concentration (amount of shale per unit volume)? 6. When will the gamma-ray spectral signatures of Th, U, and K not be indicative of shale concentration? 7. Are gamma-ray logs sensitive to the solid (matrix) or to the fluid component of a rock, or to both? 8. Are gamma ray logs sensitive to mud-filtrate invasion?

Some Review Questions (Part II): 9. What is the radial length of investigation of natural gamma ray tools? Does it matter if the rock is denser than normal? 10. Why are spectral gamma-ray logs often used to assess types of lithology? 11. List at least five different geological situations that will entail the use of spectral gamma ray logs for the assessment of shale concentration. 12. What are typical values of gamma-ray readings in a carbonate sequence? 13. What are the typical values of gamma-ray readings in a siliciclastic sequence? 14. Why are tuffaceous sands naturally radioactive? 15. What are the environmental corrections that are applied to gamma ray logs? 16. Unaccounted presence of barite in the mud, will it cause a sand to appear shalier or cleaner? Explain your answer.

Acknowledgements:

• Baker Atlas • Schlumberger