Time and velocity uncertainty is captured as part of seismic time to depth conversion and can be presented in the form of spatially variable uncertainty maps. Within the Cambo Field ±10 m was considered to capture most of the depth uncertainty ahead of our 2018 drilling campaign.

The pilot hole encountered reservoir sands within 3 m of prognosis and the horizontal side-track penetrated top reservoir within 1.2 m of prognosis which gave considerable confidence in the seismic time to depth conversion. However, drilling of the side track demonstrated that the deviation from depth prognosis became greater along the length of the wells. We estimate ~10 m off prognosis in only 320 m. While the depth errors are within the range predicted, the lateral variability occurred over a shorter wavelength than anticipated.

We will explore the likely reasons for this uncertainty and describe attempts to mitigate future depth errors of this type. The most likely candidate are polygonal faults in the overburden, the magnitude of which correlates almost one-to-one with the top reservoir prognosis error.


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