Tertiary clastics

In most deltaic settings the compaction-driven velocity behavior of sand and shale is very similar and does not create distinct velocity boundaries that would justify setting up a layer-cake model. Depth conversion in such monotonous compacting clastics can best be done with the V_avg = V₀ + k * t, a single-step depth conversion method which tends to best approximate the vertical velocity increase. Applicability may be reduced if the prospective interval contains lenses of slow shales, but usually lateral variations in overburden velocity can be captured well in a laterally varying V₀ grid. For assistance in deriving k and V₀ values, you can contact the JewelSuite Subsurface Modeling support desk.

You need to define a base surface time horizon in order to use this single step depth conversion. A meaningful base surface may be an eventual ‘Top Overpressures’ time horizon. Otherwise, you may set up an arbitrary time horizon with time being the maximum 2-way time of the seismic volume:

1. Copy any existing time 2D grid of appropriate grid dimensions.

2. Use the Property Calculator to modify its time position:

   • Make certain that the new 2D grid is selected in the JewelExplorer
   • Type the desired time value in the Property Calculator expression field
   • Choose Time as output property, and click Update property
3. Rename the modified 2D grid in the JewelExplorer to reflect its new function.

Next steps would be to map V₀ values as a new property onto the time horizon and to define an appropriate velocity value for ‘Underburden’, even though this may never be used if the velocity layer extends to the base of the seismic volume.

For water depths exceeding 20 m, it may be appropriate to insert a separate water layer into the velocity model, with seabed (time) as surface horizon. For the water layer, an interval velocity can be defined using the ‘Water Velocity Profile Avg’ option or a constant velocity value.