Assignment Quick-Look Interpretation
You are the petrophysicist responsible for the well log analysis on the final appraisal well before a field development decision. The well log analyses of the earlier field wells revealed that the total gamma ray reading was the most indicative of the formation’s shaliness in the reservoir formation. Describe an approach that you would use to determine the density and neutron responses for both clean reservoir formation and pure shale intervals, using a single plot.
Solution
The Z plot is a useful technique for establishing the density and neutron well log responses for both clean reservoir formation and pure shale using a single plotting technique. The function of a Z plot is to map the distribution of three variables simultaneously. This is accomplished by indicating the scaled average of the third variable, Z, at each X-Y cell address.
For this log analysis, the density-neutron crossplot is used as the base X-Y framework. The natural gamma ray readings are represented by the Z variable, scaled so that the least radioactive levels with the lowest gamma ray readings are assigned a value of 0 and the most radioactive levels are assigned a value of 10. The density and neutron well log responses for a clean reservoir formation are indicated where the Z variable is from 0 to 1. The density and neutron log responses for a pure shale are indicated where the Z variable is from 9 to 10.
Assessment Quick-Look Interpretation
1. What are histograms particularly useful for?
A .Establishing the bottomhole temperature
B .Verifying that well log curves are on depth with each other
C .Determining the formation water resistivity
D .Comparing log data from a new well with an existing field well database
2. What is usually the main objective of using crossplots?
A .To establish the costs of the well logging program
B .To establish a petrophysical parameter needed for quantitative log analysis
C .To save the time spent conducting a full petrophysical analysis
D .To put all the acquired well log data on a single plot
3. A histogram application used in a heavily fractured environment enables analysts to do what?
A .Compare the strike direction of fractures indicated on borehole imaging devices with those from identified faults
B .Determine the permeability of the fractures
C .Establish the fracture filling materials
D .Identify whether fractures are open or closed
4. Which type of crossplot shows the number of occurrences of a particular pair of log data points?
A .Diagnostic plot
B .Number plot
C .Particular plot
D .Frequency plot
5. Which of the following are applications for histograms? (Select all that apply.)
A .Establishing the bottomhole temperature of a well
B .Checking the validity of computed results
C .Picking data minima and maxima
D .Rescaling old and poorly calibrated logs
6. What are essential features of overlays? (Select all that apply.)
A .Density logs must always be included.
B .Relative deflection and separation between curves indicate a formation property of interest.
C .Curves being compared are compatibly scaled.
D .Composite display includes at least two curves
7. What is the primary application for quick-look well log analysis?
A .Saving the work of conducting a complex evaluation
B .Saving the money spent on comprehensive well log interpretations
C .Real-time petrophysical evaluations to facilitate well operational decisions
D .As a training exercise for early career petrophysicists
8. What type of overlay is useful for detecting hydrocarbons in clean formations?
A .Density porosity versus sonic porosity
B .Ro versus Rt
C .Sonic porosity versus density porosity
D .Gamma ray versus SP
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