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## How can I apply Passey’s DeltaLogR method in my IHS Kingdom project for quick-look source rock evaluation?

I know Passey’s DeltaLogR method for easy and very quick way for qualitative source rock evaluation, but how can I apply this method in IHS Kingdom? Can you give an optimum workflow for getting the DeltaLogR curve and the cross-plot right?

Thanks

Passey’s DeltaLogR technique is a simple, yet powerful technique for quick-look source rock evaluation, and it is fairly easy to do it by using EarthPAK. The hypothesis behind this approach is simply based on 2 major facts; 1) organic-rich sediments have higher formation resistivity readings compared to organic-lean sediments, 2) sonic transient time (Dt) decease across organic-rich sediments. This is important to remember because you may get DeltaLogR response across different rock compositions other than source rocks, i.e. carbonates, but a genuine DeltaLogR response would be where you have decrease in sonic transient time, and at the same time increase in resistivity. To see such a response requires using proper scaling, and also using other OH and mud logs, i.e. GR log as auxiliary curve which reads much higher values across organic-rich sediments, i.e. “hot shale”. Now let’s start the walk-through:

Calculating LogR:– Go to Logs> Calculations> Equations

– For “Equation Category”, select “Other” or you can create a new category by just typing a name

– Type “LogR= select Log10(x) function from the below list) and write (R) in parenthesis. At the end you should have LogR=Log10(R)

– Save this equation for later use

– You can optionally write “LogR calculation” as a description

– Click “Next” to define what would be the “R” in the equation

– Choose the deep resistivity log curve depending on you OH log data, i.e. Rt, LLD, ILD, AIT90. For later use, you may want to group the such resistivity curves under the same “alias”, i.e. DeepRes or FmRes

– Click “Assign” and go to the “Next” dialog window

– Choose a depth range, or zone, which would represent a stratigraphic column with various rock formations. I would recommend choosing the entire log interval

– Go to the “Next” step and “Finish” the calculation to create the new LogR curve

– This calculation will also generate LogR curves simultaneously for all the wells where it is applicable

Sonic transit-time (Dt) vs. Cross-plotting LogR:– Go to Logs> Facies Modelling> New

– Select the well you have just run the LogR calculation

– Select Dt curve, Y axis, and > to assign it

– If the Dt is not available other porosity logs can be used in place of Dt, i.e. Destiny log

– Select LogR curve, X axis, and > to assign it

– You may optionally assign GR curve to Z axis, but not necessary for this exercise

– When you click “OK”, you will have a new window to display the cross-plot. There are some screenshots you may have a look at this outside internet source.

– Passey uses a trend line for 50 microsecond/ft Dt vs. 1 decay Resistivity to represent the organic-lean shale clusters. This is a static line, but may not be a very good representative trend-line for all the different types of source rocks. Therefore, we need to modify it a little.

– While this window is active, go to Crossplot> Regression

– You would have two options to draw the best-fit line for your data; either “Numerical Regression” or “Digitize Regression”

– The first option draws a best-fit line automatically, and I don’t recommend you to use it

– Choose “Digitize Regression” as the method, and draw it by yourself. In this case, you will need to judge your own data much better. You need to draw the trend to determine low-resistivity shale line which is really the line to represent the organic-lean shale. This may be the most crucial step in the DeltaLogR method

– Once you draw this trend-line, it would give you the formula to represent the slope, i.e. Dt=103.8325-24.8742*LogR at the base of the cross-plot window

– You may adjust this line and later simplify the coefficients if you like

Calculating DeltaLogR (DLogR):– Go to Logs> Calculations> Equations

– Use the your formula to calculate the DeltaLogR curve for the entire log interval, i.e. “DLogR=103.83-24.87*LogR”, same as in the above steps for calculating LogR

– Don’t forget to save this equation

– This would generate the pseudo-sonic curve for your reference well

– By the way, this DLogR formula may be optimized by using the log curves of your other wells. I suggest optimizing it and using it for the same shale-play within the same basin, or at least a part of the basin. Decision may of course depend on your data coverage.

Plotting DLogR vs Dt in the same log-track:– Now, you need to overlay the two curves, DLogR vs. Dt, in the same log-track to see if we have any DeltaLogR response

– Select the well in the cross-section window

– Go to Logs> Settings

– I suggest putting the GR log in the log track-1

– Dt and DLogR in the log track 2

– Scale the both curves with same minimum and maximum values, i.e. 160-60

– Go to shading, and shade area where Dt is to the left of DLogR

– Put the resistivity curve in the log track 3

– You can add as many log curves as you like for your source-rock assessment, even total mug-log gas

Ideally, you should have a good separation between Dt vs. DLogR curves where you have good source rock (organic-rich sediments). Both curves should read on top of each other, or very close values where you have non-source rocks (organic-lean sediments)

This is a quite powerful technique because you can do bulk-calculation at once and display those curves in your cross-sections across your wells for different type of source rocks in a particular basin easily once you have all the formulas in place.

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LikeIs it also possible to calculate the TOC and display it alongside the DLogR vs. Dt logplot?

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LikePlease tell how to select base line of R and DT while calculating DlogR.

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LikeDo you mean the “shale line” for the DLogR calculation?

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LikeYes, it is possible to do that. You can use the formula below:

TOC=LogR*10^(2.297-0.1688*LOM)

LOM in the formula stands for “Level of Maturity”, and it is a bit tricky to get it right. If you have your source rock sampled, especially cored, it would be much better to optimize the TOC outputs.

Even though there are various ways of determining LOM and you may find many publications on it, I will give you only one method of using it in your TOC calculation.

First, you need to examine the relationship between LOM vs. Ro (Vitrinite Reflectance) with your lab-measured Ro values. This outside internet source has a good example. You need to scroll down for it. Then, get a good feeling about the range of LOM values to be used later in the calculation, i.e. 8.5 for gas shale-10.5 oil shale. At the end, you will need to pick a single value as the best representative number.

Calculating TOC curve:– Go to Logs> Calculations> Equations

– Let’s start with LOM=9

– Put “TOC=LogR*10^(2.297-0.1688*9)” formula and fill the areas with the all information asked

– Save the formula

– Follow the steps as it is given in above walk-through

– Plot the TOC curve, and examine it if the calculated curve satisfies the lab-measured values at the sample depths

– If not, go back to the formula. Chance the LOM value and run it again until you calibrate the TOC curve against the measured values

Once you calibrate the TOC curve for the sampled depth, you will have a continues TOC curve for the entire OH-logged internal.

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LikeCan the measured TOC values from the geochem report be plotted directly on top of the calculated TOC curve to facilitate calibration of the calculated curve?

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LikeYes, it is possible; however, it is not a straightforward data-import process. I mean, there is no direct way to import such “point” data. At least, I am not aware of it. This is true for other type of lab measured data like porosity, permeability, etc.

Here is what I can suggest to do as a work-around:

– Export your GR log just to get a LAS file to work on

– Open it in MS excel as “space delimited”

– Delete all the GR values

– Type your TOC values to the nearest depth points since you can change the sampling interval of the log

– Save it as “tab delimited” text file

– You now have a new LAS file with your TOC values only

– Now, open it in a text editor

– Edit the header as necessary, i.e. GR to TOC, etc.

– You will also see that this new LAS file will be limited to your data with NULL values where there is no entry

– Then, import it to your IHS Kingdom project as a LAS file

– You can display the TOC values as “discrete points” on the log track

Unfortunately, data points will plot really small, and there wouldnâ€™t be any option to change their size, color, etc.

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LikeAnother way of doing it, maybe simpler, is to prepare a text document within your depth range, but with all the depth values in the sampling interval, i.e. 0.5 ft. And then import it as a text file.

In case you may not see your data points after importing, or if you want to make changes on the data points;

– Go to Logs > Logs Table

– Select the TOC curve

– Make the changes and hit OK

– If you like to delete a row; highlight it and hit “Delete”. It is irreversible and you cannot add a row, as well.

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LikeWhat does it mean if your deltalogR doesn’t cross over your DT?

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LikeShales with relatively low resistivity readings can be considered as non-source rocks. Remember that DeltaLogR method is a visual method which needs to be assessed for a particular source rock within the same basin with consistent OH log data. Core-calibrated DeltaLogR crossovers are more reliable. However, in practice, organically-lean shales are likely to be no hydrocarbon bearing zones. Therefore, they wouldn’t give any DeltaLogR crossover.

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