# Stratal Slicing

## Contents

## Stratal Slicing

Stratal slicing is a proportional and linear slicing between at least two reference points (usually horizons) in seismic. This is done to adjust for changing thicknesses, to improve visual understanding and extraction of otherwise unclear or muddied horizons, and to overall create a volume that identifies a geologic-time series. It is a common method employed by petroleum geologists in 3D seismic interpretation.

This method is based upon the assumption that normal deposition occurs relatively evenly throughout a determined area without large, local discrepancies. Additionally, using this method assumes that the area of interest has not been altered drastically (i.e. unconformities, erosion, etc). Even if your data contains some discrepancies or unconformities, this method may still be useful for analysis in some areas.

### Pros and Cons of Using Stratal Slicing

This technique is especially good for several reasons and on certain occasions. First, the accuracy and precision of stratal slicing are great depending on the thickness of the section or thickness between reference points (also the frequency of the data).^{[1]} Depending on the frequency of the data and the scale of the area of interest, this method can be a great tool for seismic interpretation from small to large scale areas.

Second, this method is great for understanding paleogeomorphology. This is especially true when identifying channels and delta systems. Structural dip or other structures in a system can often distract from overlying or big-picture morphology. Through stratal slicing, one is able to more fully identify channels or overall paleogeomorphology.^{[1]}

There are, however, some challenges to using stratal slicing. One main challenge to stratal slicing includes dealing with unconformities and discontinuity surfaces. Along that note, another difficulty deals with human error and poor picking of reference points or horizons. Without a careful approach, it can become easy to pick an invalid reference point or to confuse references that may not be laterally traceable. Another challenge is that stratal slicing requires several assumptions such as constant thickness, porosity, velocity, etc.^{[2]} ^{[3]}

## History and Background

Vail et al. (1977) invented the stratal slicing method in order to look at genetic depositional surfaces and units.^{[4]} In the 1970’s Vail was working on sections in the Gulf Coast Tertiary when he penned the assumption that seismic reflections follow chronostratigraphic patterns.^{[1]} Further research showed that this assumption could be made across larger scale sections and still be accurate. Later, these techniques were used to study depositional facies and paleogeomorphology.^{[5]} ^{[6]}

## Process

To make a stratal slice, a few steps need to be made.^{[1]} This process and flow can be seen in the *Figure 1* to the right.

- Assess the seismic data and select at least two reference horizons or partial horizons. These reference points should be clear, distinct, and laterally continuous with significant acoustic impedance contrasts.
- Evenly spaced linear slices are then made between the two or more reference points. These lines show changes in thickness that can be properly seen in the next step.
- If desired, a flattened model or volume is made adjusting for changing thickness by using the evenly spaced slices.
- Using the new flattened model, new understandings are able to be made depending on your reason for using stratal slicing.

## Application in Petrel

It is possible to make simple stratal or proportional slices in seismic software such as Petrel (example seen in *Figure 2*). To do so, some steps must be taken and the flow should occur as follows:

- Two horizons should be picked. Ideally, these horizons should not be very far apart and should contain the same characteristics as have been mentioned (i.e. clear, distinct, laterally continuous, significant impedance contrast).
- These horizons need to then be made into surfaces.
- Then it needs to be determined how many stratal slices are desired.
- The calculator function is then employed by right clicking on one of the two surfaces and then left clicking on the calculator function.
- In the calculator, some factors need to be accounted for. The following function needs to be typed in:
- (The following function is calculating for 9 slices):
- =0.9*HORIZON_1+0.1*HORIZON_2
- =0.8*HORIZON_1+0.2*HORIZON_2
- =0.7*HORIZON_1+0.3*HORIZON_2
- continue entering in the functions until the reverse of the first function is entered in (=0.1*HORIZON_1=0.9*HORIZON_2)

- To do a different amount of slices, the same type of function would be typed in adjusting for some differences.
- For example, if 19 slices are desired, the function would be:
- =19/20*HORIZON_1+1/20*HORIZON_2

- The equations would continue until the reverse equation is entered in.

- For example, if 19 slices are desired, the function would be:

- (The following function is calculating for 9 slices):
- Once the calculations are complete, there should be new horizons showing the newly made stratal slices that can then be displayed.

## External Links

Stratal Slicing: Benefits and Challenges

Stratal Slicing in Northern Kutei Basin

## References

- ↑
^{1.0}^{1.1}^{1.2}^{1.3}Zeng, H, (2010), "Stratal slicing: Benefits and challenges," The Leading Edge 29: 1040-1047. - ↑ Zeng, H., M. M. Backus, K. T. Barrow, and N. Tyler, 1998a, Stratal slicing, part I: realistic 3D seismic model: Geophysics, 63, no. 2, 502–513.
- ↑ Zeng, H., S. C. Henry, and J. P. Riola, 1998b, Stratal slicing, part II: real seismic data: Geophysics, 63, no. 2, 514–522.
- ↑ Vail, P. R. R. G. ToddJ. B. Sangree, , and , 1977, Seismic stratigraphy and global changes of sea level, Part 5: Chronostratigraphic significance of seismic reflections, in C. E. Payton ed., Seismic Stratigraphy: AAPG Memoir 26, 99–116.
- ↑ Posamentier, H. W. , G. A. Dorn, M. J. Cole, C. W. Beierle, and S. P. Ross, 1996, Imaging elements of depositional systems with 3D seismic data: A case study: Gulf Coast Section SEPM Foundation, 17th Annual Research Conference, 213–228.
- ↑ Tipper, J. C. , 1993, Do seismic reflections necessarily have chronostratigraphic significance?, Geological Magazine, 130, no. 1, 47–55.

This page is currently being authored by a student at the University of Oklahoma (Kurt Crandall). This page will be complete by December 1, 2018.