Podcast
Questions and Answers
What does a Gamma Ray Log measure?
What does a Gamma Ray Log measure?
Natural gamma ray emissions from subsurface formations
In which type of holes can measurements of Gamma Ray Logs be made?
In which type of holes can measurements of Gamma Ray Logs be made?
- Cased holes
- Open holes
- Only in cased holes
- Both open and cased holes (correct)
Which of the following are applications of Gamma Ray Logs? (Select all that apply)
Which of the following are applications of Gamma Ray Logs? (Select all that apply)
- Measuring temperature
- Identifying shale zones (correct)
- Evaluating shale content within a formation (correct)
- Correlation between wells (correct)
Gamma Ray Logs depend on the resistivity of the mud used.
Gamma Ray Logs depend on the resistivity of the mud used.
The half-life time is the time required for a radioactive isotope to lose _____ of its activity.
The half-life time is the time required for a radioactive isotope to lose _____ of its activity.
What are the three main isotopes contributing to natural gamma radiation?
What are the three main isotopes contributing to natural gamma radiation?
Match the radioactive elements with their associated minerals:
Match the radioactive elements with their associated minerals:
What type of radiation do gamma rays represent?
What type of radiation do gamma rays represent?
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Study Notes
Gamma Ray Logging
- Invented by Lane Wells in 1936.
- Similar to an SP log but does not depend on Rm, Rmf, and Rw.
- Measures natural gamma ray emissions from subsurface formations.
- Can be run in open and cased holes, and in water and oil base muds.
Log Presentation
- Presented in track 1 on a linear grid scaled in API units.
- Readings increase from left to right.
- Scale may use a primary and back-up scale.
Gamma Ray Log Applications
- Correlation between wells.
- Identifying shale zones.
- Determining bed boundaries.
- Evaluating shale content within a formation.
- Mineral analysis (type of clay minerals).
- Depth control for side-wall coring or perforating.
- Tracking movement of radioactive tracers.
GR Principles
- Radioactive decay is a transition from one nuclear energy state to another, lower state.
- Excess energy is shed by the nucleus by means of α, β or γ radiation.
- Unstable elements emit:
- ɣ radiation (no mass, no charge, but has energy)
- β radiation (very small mass, negative charge)
- α radiation (mass equivalent to 2He4)
Gamma Ray
- Highest energy form of electromagnetic radiation.
Radioactivity
- Gamma rays are electromagnetic radiation or photons emitted by a nucleus in an excited state.
- The half-life of a radioactive isotope is the time it takes for the isotope to lose 50% of its activity.
- Radioactive materials are originally found in igneous rocks and rarely in sedimentary rocks.
- Radioactive elements are often restricted to shales (clay minerals) and other fine sediments.
- Carbonates have a very low radioactivity level but during the formation of secondary porosity or dolomitization, radioactive elements may deposit by formation water, which could show higher radioactivity.
Natural GR Principle
- The estimated age of the earth is around 4 * 10^9 years.
- Three isotopes have half-lives of that magnitude or greater.
- The most common sources are:
- Potassium-40, found in mica, feldspar, and clay minerals.
- Thorium, associated mostly with clay minerals.
- Uranium-series elements deposited over geological time.
- Potassium and Thorium are associated with most shales.
Main Contributors
- K40 with a half-life of 1.3 * 10^9 years.
- Th232 with a half-life of 1.4 * 10^10 years.
- U238 with a half-life of 4.4 * 10^9 years.
Sources
- K40: Feldspar, mica, illite.
- Th232: Heavy minerals, clays.
- U238: Organic material.
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