1. Why Is That ???
Recall: Reflections are generated where there is a change in acoustic properties (z =  * v)
Consider: Where would sharp changes in impedance occur?
horizontally as lithofacies change?
vertically across stratal boundaries?
Brushy Canyon Formation, West Texas
SLIDE 13
Why Is that???
Reflections are generated where there is a change in acoustic properties (z =  * v)
Where would sharp changes in impedance occur?
horizontally as lithofacies change?
vertically across stratal boundaries?
Consider this West Texas outcrop 1200 ft thick
This section consists of primarily offshore shales with some nearshore silts and sands
The sandy layers appear as white ledges – more resistant than other layers
Let’s say I walked one of these sandy ledges for 5 miles and took a sample every 300 ft
The first sample might have 80% sand, then 78% sand, then 77%, then 74%, then 75%, then 73%, then 71%, then 74%, then 72%, etc.
The point is that sand % changes slowly, with an overall trend to lower sand as I move towards the basin (right in this case)
Now, one of you can repel down the cliff and sample the outcrop every 10 ft – I’m too old for that
Your first sample has 5% sand, then 18%, then 12%, then 47%, then 23%, then 82%, then 22%, then 7%, etc.
Vertically there is more lithologic variation over a shorter spacing as we sample different beds, bedsets, and parasequences
You would also notice stratal surfaces across which there are abrupt lithologic breaks
There are very gradational lateral changes in physical properties
But there can be abrupt vertical changes in physical properties especially at parasequence boundaries
The abrupt impedance contrasts that generate seismic reflections are the abrupt vertical changes in physical properties at larger-scale stratal unit boundaries
Very Gradational Lateral
Changes in Physical Properties
There Can Be Abrupt Vertical
Changes in Physical Properties
Especially at PS Boundaries
Courtesy of ExxonMobil
L 11 – Stratigraphic Analysis