What does a gamma ray log do?
Gamma ray (GR) logs measure the natural radioactivity in formations and can be used for identifying lithologies and for correlating zones. Shale-free sandstones and carbonates have low concentrations of radioactive material and give low gamma ray readings.
What type of log Do we use for lithology identification?
Neutron and density logs combined. Neutron and density logs each react to both lithology and porosity, so by analyzing the two logs together, one can begin to distinguish lithology from porosity.
How does gamma ray tool work?
Unlike all other nuclear tools (and, in fact, all other logging measurements), it is completely passive. It emits no radiation. Instead, it simply detects incoming gamma rays from the formation and (unfortunately) the borehole. Gamma rays are electromagnetic radiation, generally in the energy range 0.1 to 100 MeV.
What is relationship between grain size and gamma ray reading?
The obvious relationship being suggested when gamma-ray log shapes are used in facies analysis, is that gamma-ray changes, besides indicating clay content, mirror grain size- changes. Low log values indicate a coarse grain size while high values indicate fine grain size (Fig. 2).
What does gamma tool measure?
Gamma ray tools record naturally occurring gamma rays in the formations adjacent to the wellbore. This nuclear measurement indi- cates the radioactive content of the forma- tions. Effective in any environment, gamma ray tools are the standard device used for the correlation of logs in cased and openholes.
Why is gamma rays high in shale?
The higher levels of radiation in shale are caused by absorption of thorium by clay minerals, the potassium content of clay minerals (principally illite), and uranium fixed by associated organic material.
What is a lithology log?
Lithology logs A log of the natural radioactivity of the formation along the borehole, measured in API units, particularly useful for distinguishing between sands and shales in a siliclastic environment.
What is the difference between stratigraphy and lithology?
See the discussion of Lithology Types Tables for more details….How Stratigraphy is Modeled.
| Lithology | Stratigraphy |
|---|---|
| Is often the first step in entering borehole rock types | Is often the second step in entering borehole rock types. |
| Can contain repeated sequences (sand, clay, sand, clay) | Cannot contain repeated sequences. |
What tools use gamma rays?
Gamma rays are widely used in medicine and specifically in the area of oncology to treat malignant and cancerous tumors during a process called gamma knife surgery. In this type of treatment, concentrated beams of gamma rays are directed at tumors in order to kill cancerous cells.
What is porosity log?
The “Porosity” logs. There are three types of logging tools that are used to estimate the amount of pore space in a rock: the neutron, density, and acoustic velocity (or sonic) tool. So, the log is mainly a measure of hydrogen concentration (mostly contained by the pore fluids of the formation).
What are the shapes of a gamma ray log?
The shape of a gamma ray (or SP) log through a sand body is often thought of as a grain size profile. Three basic log shapes are recognized: funnel (coarsening upward), cylinder (blocky), and bell (fining upward) (Figure 1).
Is there high radioactivity on gamma ray log?
In most cases, a high radioactivity on the Gamma Ray log is typically associated with shales, however this is not always the case. When interpreting Gamma logs it’s important to always remember that what is displayed is either the count of radioactive isotopes, or the individual elemental contributions of each isotope.
Can a gamma ray log be used in a shale?
Radioactive minerals in sands, especially K-feldspar, zircon, and mica, can raise sand readings as high as adjacent shales. Gamma ray logs may be useless in immature sands derived from basement terranes. However, beach placers rich in zircon may be valuable correlative markers if not mistaken for shale.
How are well-log curves used to interpret facies?
3. Log curve shape: predictive tools for facies interpretation Selley (1978) [4] considered the shapes of well-log curves as basic tool to interpret depositional facies because shape of log is directly related to the grain size of rock successions.
