Tutorial: good practice in well ties

Well ties are a very important part of the interpreter’s trade. They provide a means of 1. correctly identifying horizons to pick, and 2. estimating the wavelet for inverting seismic data to impedance. Just as well ties are paramount in the calibration of a seismic interpretation, so too they are the cornerstone of using seismic amplitudes in impedance and AVO inversion, and ultimately of inferences fed into the risking process. Figure 1 shows an example where the geologically relevant amplitude information is highly focussed in a particular seismic loop (the blue loop in this case). The wavelet is ~30 degrees rotated from symmetry and its main loop is 24 ms deeper in time than time zero. This is nominally minimum phase data displayed with European polarity, i.e. with negative values corresponding to compressions and plotted as troughs. Given this convention, together with the fact that the red loop is closest to the checkshot time, some interpreters might expect that the red pick is the one to make. If you are after the amplitudes that map out the sedimentology, then it clearly isn’t. Once you know the wavelet, it is evident that the wavelet energy is concentrated in the blue loop and this loop is delayed relative to the checkshot time. Readers are referred to our previous tutorial article (Simm and White 2002) for a discussion of phase, timing, polarity and the interpreter’s wavelet and their impact on seismic interpretation. What is important in this example is that both the wavelet shape and timing were estimated without making assumptions about the wavelet (i.e. what the wavelet should look like) or the timing (i.e. which loop represents the top of the reservoir). The subject of this paper then is the process by which wavelets are estimated through a well tie procedure that results in quantitative measures of synthetic to seismic goodness-of-fit and likely wavelet accuracy. In the authors’ view this is a good practice approach, which should form at the very least the initial stages of a well tie study. Activities such as stretch and squeezing a synthetic to fit a seismic trace are unscientific and definitely not good practice! Regrettably, while ‘stretch and squeeze’ is a common feature of well tie software, q.c. features are at best rudimentary.