Density Log Analysis in Geophysics

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Questions and Answers

What does the measurement of the photoelectric factor (Pe) indicate about the formation mineral?

It provides the porosity of the sedimentary rock.

It gives an indication of the average atomic number (Z) of the mineral. (correct)

It measures the water content of the formation.

It determines the permeability of the rock.

What is the primary method used to calculate porosity from density logs?

Statistical analysis of core density data

Calculation based on estimated rock properties (correct)

Direct measurement of pore size

Comparison with sonic logs

Which of the following statements is true regarding photoelectric factor values for minerals in sedimentary rocks?

Lithologies can be differentiated without considering porosity. (correct)

Photoelectric values are highly sensitive to changes in pore volume.

They only apply to igneous and metamorphic rocks.

They are the primary indicator of fluid saturation.

Which statement reflects the use of density logs in measuring porosity?

Density logs are used to estimate connected porosity. Signup and view all the answers

In the equation $ ho_eP_e = U$, what does U represent?

The capture cross-section of the formation. Signup and view all the answers

What is the significance of the equation for density of a mixture?

It indicates the density of a mixture is a linear function of its components' densities. Signup and view all the answers

How does the photoelectric absorption index compare to neutron porosity or density logs?

It is less sensitive to pore volume changes. Signup and view all the answers

From which method is the matrix density ( hoa) typically obtained?

Core analysis by statistical evaluation Signup and view all the answers

Which equation is similar in form to the density equation?

$U = (1 - \phi)U_{ma} + \phi U_{fl}$ Signup and view all the answers

For multimineral lithology, what is required to perform accurate density measurements?

Known values for each individual component Signup and view all the answers

What is the significance of the values obtained from the porosity logs in the given example?

They are used to estimate the rock's properties. Signup and view all the answers

Which mineral has the highest density according to the matrix list provided?

Anhydrite Signup and view all the answers

Which of the following statements about the sonic log is correct?

It provides an estimate of the connected porosity. Signup and view all the answers

What is the approximate percentage of limestone in the formation matrix?

20% Signup and view all the answers

Which mineral has the highest percentage in the formation matrix?

Sandstone Signup and view all the answers

What is the true porosity of the reservoir as mentioned?

19% Signup and view all the answers

What primarily influences the apparent fluid density ($ ho_f$)?

Mud filtrate density Signup and view all the answers

What is the gamma ray interaction that depends on the atomic number of the material?

Photoelectric effect Signup and view all the answers

In the measurement principle for density logs, what happens to gamma rays when they collide with electrons in the formation?

They lose energy Signup and view all the answers

Which of the following is a type of gamma ray interaction in earth formations?

Photoelectric effect Signup and view all the answers

What is indicated by the high energy gamma rays according to the density log measurements?

Bulk density Signup and view all the answers

Which two primary overlays are compatible for lithology identification and gas detection?

Limestone and Sandstone Signup and view all the answers

What is the expected lithology indicated in the typical log presentation?

Predominantly sandstone Signup and view all the answers

What density scale does the density track range between in the typical logging scheme?

1.9 to 2.9 g/cm3 Signup and view all the answers

What does the photoelectric absorption index (Pe) enable?

Lithological interpretation without prior knowledge of porosity Signup and view all the answers

What is the matrix density of limestone as stated in the content?

2.71 g/cc Signup and view all the answers

How is porosity estimated from porosity logs?

It is calculated from other perceived metrics. Signup and view all the answers

What characteristic is represented by the neutron porosity indicated in the typical logging scheme?

Scaled from positive to negative units in sand units Signup and view all the answers

What percentage represents the neutron porosity for sandstone on the typical log?

60 Signup and view all the answers

What is the standard unit used in the Photoelectric Factor (PE) Log?

Barns per electron Signup and view all the answers

How does the photoelectric absorption index of an atom change?

It increases as the atomic number increases. Signup and view all the answers

Which mineral has the highest photoelectric absorption index (Pe) according to the provided data?

Limestone Signup and view all the answers

What aspect does the Photoelectric Factor (PE) Log primarily indicate?

Aggregate atomic number of elements Signup and view all the answers

What does the photoelectric absorption index of shale range between?

3.00 - 4.00 Signup and view all the answers

What primarily affects Compton scattering?

Higher energy gamma rays Signup and view all the answers

Which mineral has an absorption index of 4.65?

Salt Signup and view all the answers

Why is the Photoelectric Factor (PE) considered an excellent indicator of mineralogy?

It is only slightly affected by pore volume and fluid/gas content. Signup and view all the answers

Study Notes

Density Log

The density log does not measure porosity directly, but rather provides information used to calculate it.
The density log is used to identify the type of fluid in the pore space of the formation, distinguishing between oil, gas, and water.
The density log also indicates the density of the formation itself.
The density log is a tool in understanding porosity by measuring the electron density of the formation.
The density log is influenced by the density of the fluid in the formation.
The density log is primarily used to calculate porosity, with higher density readings corresponding to lower porosity.
The density log measures the intensity of backscattered gamma rays, which are emitted from a放射性源 in the tool.
The density log uses two different energy levels of gamma rays: High energy for bulk density and low energy for lithology.
The density log is affected by the photoelectric effect, which results in the absorption of gamma rays by atoms in the formation. This absorption is more significant in elements with higher atomic numbers (Z).
The photoelectric effect is a tool used to identify lithology in the formation, without necessarily knowing the porosity.
Based on the photoelectric effect, a logarithmic scale called PE is also created.
The PE logarithmic scale is generally less sensitive to pore volume changes than either the neutron porosity or density logs.
The PE log is a specific measurement of the absorption of low-energy gamma rays by the formation, with units of barns per electron.
The PE value of a mineral is calculated by summing the individual elemental contributions based on weight.
In addition to bulk density, the log measures the photoelectric absorption index (Pe) of the formation.
The lithologies can be discriminated independent of porosity based on the PE measurements.
The PE values for sandstone, limestone, and dolomite in formation are 1.81, 5.08, and 3.14, respectively.
The higher the PE, the higher the atomic number (Z) of the element in the formation.
The Compton scattering is another gamma ray interaction, occurring over a wider energy range than the photoelectric absorption.
The PE log and denisty log are combined to gain insight into the mineralogy and density of the formation.

Porosity

Porosity is calculated from the density log, not measured directly.
The bulk density of a formation is a linear function of the densities of its constituents.
The formation matrix is often composed of various minerals depending on the lithology which can include sandstone, limestone, dolomite or anhydrite.
Density logs help determine the proportion of each mineral in the formation.

### Neutron Log

The neutron log is sensitive to the hydrogen content in the formation, which is mainly influenced by the hydrogen index (HI) of the formation fluid.
The neutron log measures the number of neutrons that are scattered back.
The neutron log helps determine the type of fluid in the formation, allowing for the distinction between oil, gas, and water.
The neutron log can identify the presence of gas by displaying a lower porosity reading.
The neutron and density logs are commonly used together to determine the porosity of a formation.
The neutron log is used to identify gas and determine porosity.
The neutron log can be influenced by both fluid content and lithology.
The neutron log is particularly sensitive to the hydrogen content of formation fluids.
The neutron log is generally used in conjunction with the density log to calculate porosity.
The neutron log typically utilizes a neutron source to analyze and quantify the hydrogen content in the formation.

### Sonic Log

The sonic log measures the travel time of acoustic waves through the formation.
The sonic log is used to determine the porosity of the formation, using the sonic transit time to measure the velocity of sound waves.
The sonic log is used in conjunction with the density log to calculate porosity and identify lithology.

### In General

The logs work by measuring the variation of a specific parameter such as the sonic wave velocity, the density of the formation, or the neutron interaction.
These measurements are then used to calculate the porosity of the formation, and to distinguish between different fluids.
The density log is a key tool for determining the type of fluid in the formation.
The neutron log and the sonic log are used alongside the density log to understand the characteristics of a reservoir.
Porosity, a measure of the void spaces within a rock, has a significant role in reservoirs.
Porosity is influenced by numerous factors, including lithology, grain size, and the presence of fluids.

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Description

This quiz covers the principles of density logging in geophysics, focusing on how it measures the density of formations and aids in calculating porosity. It explores the implications of density log readings for identifying fluid types and understanding the impact of gamma rays on measurements. Test your knowledge on this essential tool used in reservoir evaluation.