When is a seismic anomaly not anomalous? (1 of 3)

When was the last time you heard someone say that a prospect had a really robust ‘amplitude anomaly?’ Did you ask what was anomalous about it? What exactly is seismically anomalous anyway?! Geoscientists spend a lot of time looking at seismic ‘anomalies’ but how do we decide what is anomalous versus what is not?

My name’s Gareth and this is my first article on LinkedIn. My articles will aim to be both technical, and some thoughts I have on technical challenges we face in the oil and gas industry. I hope they create discussion and please don’t hesitate to comment on them.

Coming back to the title of the article, I looked up the term 'anomaly' in the dictionary and it gave me the following description;

"something that deviates from what is standard, normal, or expected"

This description implies that there should be some sort of benchmark, or sufficient information upon which to decide whether something is anomalous or not. When exploring for oil and gas, especially in frontier areas, we usually have very little prior information or data so how do we know what is anomalous? The answer may well be that we don't. Take the example below:

A seismic line in a frontier basin shows predominantly low amplitudes along a prospective stratigraphic interval but with 3 bright spots. Are these anomalous? In the context of this seismic line they probably are as most of the other amplitudes are dim. But we don’t just want to find bright amplitudes and it is what these amplitudes mean that is of most importance to us. 

If we were to drill one of these bright spots and found high porosity, water bearing sands, would the others still be described as anomalous? Using our criteria from above, yes they still are. But we now have a new data point and the context has changed. Given we are in the business of looking for hydrocarbons, not brine, I doubt the bright spots would continue to be considered 'anomalous'.

I would suggest that seismic anomalies be described within the context of the basin and the information and data available. What is the normal, standard or the expected view? Unless we know what this is and can differentiate from it, defining what is ‘anomalous’ will be challenging. How well we can do this will change depending on the amount of data we have. The less data we have, the worse our chance of defining what anomalous actually is. The more data we have, the better our view of what is ‘normal’, and the better chance we will have to identify true ‘anomalies’ that may be hydrocarbon bearing.

This article is a brief introduction to my next couple of articles*, where I will describe a method I have found useful in the past to understand which seismic amplitudes are anomalous in the context of searching for hydrocarbons. The first step is to understand what is ‘normal’ and then deviations from this norm can then be identified as potential hydrocarbon anomalies.

Please feel free to like, share, or comment on any of my material, and I will try to respond to any feedback or questions when I can. 

*My second article has now been published and can be found at the link here: How can we define true seismic anomalies?

Image Reference: Gated amplitude extraction from lower Congo slope showing highly sinuous amalgamated channels within incised slope canyons (Sikkema & Wojcik, 2001).

Obtained through: https://www.researchgate.net/publication/263569887_Controls_on_reservoir_distribution_architecture_and_stratigraphic_trapping_in_slope_settings