Making sense of seismic amplitude variation (3 of 3)

In my previous articles on seismic amplitudes I asked the questions When is a seismic anomaly not anomalous? and then, How can we define 'true' hydrocarbons anomalies? Once we think we have identified a 'true' seismic anomaly at a prospect, the next step is usually to make sense of, and communicate the amplitudes we see/observe. We'd like to develop confidence in our prospect so in my final article on seismic amplitudes I’ll discuss a method that I have found useful to do this, with the help of a hypothetical prospect and a simple visual matrix.

My hypothetical prospect is imaginatively called Prospect A, and shown above in figure 1. It is an amplitude driven stratigraphic trap with low, medium and high strength amplitudes at top reservoir. How might we view and communicate this variation in amplitude across the area to confirm the validity of Prospect A?

In the example, I’ll assume that we have some good regional well and seismic data, and that our modelled seismic responses discussed in the previous article show high amplitudes (red) to be ‘anomalous’, and medium (blue) and low (grey) to be the brine and background shale (‘normal’) amplitudes respectively. The amplitude variation across figure 1 needs explaining in the context of our geological model/understanding of Prospect A, so using the 3 locations above, and a ‘should see’ – ‘shouldn’t see’ matrix (figure 2) that was introduced to me by an ex-colleague Brian Taylor, I’ll attempt to do this. The matrix essentially compares what we expect to see from our models to what we do see in the seismic.

Figure 2.

‘Should see’ – ‘Shouldn’t see’ matrix for describing amplitude driven/supported prospects   

Starting at location 1, we have an anomalous amplitude where we would expect in our stratigraphic trap, up dip of a possible Gas Water Contact (GWC). Here, our modelled seismic gas response from well data matches what we observe in the seismic (the top left of the matrix). This is a good start, but to gain more confidence in Prospect A we would also like to be able to see on the amplitude map where we expect the down dip brine leg to be (below the possible GWC at location 2). Here, we shouldn’t see a gas amplitude and we don’t, this is the lower right part of the matrix. We have now identified areas on the map that show gas and brine responses in locations where we expect from our geological understanding of Prospect A. This gives us greater confidence in the validity in prospect A. The top right and lower left of the matrix however, are also potential cases that need to be considered. 

The amplitudes at location 3 are a slightly trickier case but always need explaining. I have found that this is quite a common case in amplitude driven/supported prospects and occur when we have amplitudes indicative of gas in a place we don’t necessarily expect gas (e.g. down dip of where we think our GWC might be). Cases like this usually require further work and might be the result of lithology variation or of higher porosity sands. Amplitude responses like location 3 are a lot easier to explain (dismiss/ignore?!) when spatial constraints such as amplitude conformance to depth structure or flat spots are observed. Without such constraints, amplitudes like those at location 3 will usually (and should) increase the risk on prospects.

The final, top right part of the matrix, although not really applicable to Prospect A, is explained in figure 3. It shows two discoveries on 4-way closures with observed gas responses. Our well based modelling implies we should see a gas response at the 4-way closure at Prospect B in the seismic but we do not. If this was the case, we may question the validity of Prospect B, and look closer at (for instance) the risk on charge, migration and retention of hydrocarbons into the structure given discoveries have been made on other closures in the area.

Figure 3.

Hypothetical depth map of Prospect B and adjacent gas discoveries. Given there is a 4-way dip closure at prospect B we would expect to see a gas response but do not.   

I hope you have found some of my thoughts on seismic amplitudes interesting and thank you for your prior and future comments. Amplitudes are a broad topic and there is much I haven’t covered but if anyone has any further views of their own that they’d like to share, please comment below or email me directly as I’d be interested in your experiences.

In my next article, which I plan to post in a few weeks time, I will change course slightly and provide some thoughts I have on seismic interpretation. Is there a right or wrong way to interpret our seismic? Or are there just better and worse ways? As with a lot of topics in Geoscience I don't think there is a clear answer to those questions, but I will discuss a few things that I have found useful, and have helped me throughout my career interpreting seismic.

As always, 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. If you have not yet read my 2 previous articles on seismic amplitudes you can access them at the links below:

1 of 3 When is a seismic anomaly not anomalous?

2 of 3 How can we define 'true' hydrocarbons anomalies?