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