How to depth convert using the “Salt Wedge Model 2” formula

Overview

The “Salt Wedge Model 2” function allows for depth conversion of a layer containing two mixed lithologies, a predominant layer and a subordinate layer (or layers) that may be variable in distribution.  The layer is defined as usual by a top and a base grid, but is unusual in that an isochron grid (i.e. a time thickness map) containing the distribution of one of the lithologies is also provided as an input to the function.

Extra Inputs

  • Isochron grid of one of the lithologies (matching the extents of the layer defining girds)
  • Velocity of the predominant layer (grid or constant)
  • Velocity of the mapped subordinate lithologies (grid or constant)

Example

A typical scenario is in the Zechstein formation in NW Europe.  In the North Sea, this consists of fast velocity anhydrite rafts within a slower, more halitic layer.

In this illustrative section, the anhydrite layers have been picked by top & base using 4 different interpretation horizons.

zechstein-formation

 

If there are multiple rafts stacked vertically, more horizons may be required.

The isochron of the anhydrite layer can be computed by subtracting the base from top anhydrite surfaces for each horizon pair, and then summing up & gridding.  The isochron should be zero where no anhydrites exist, and the cumulative sum of the anhydrite time thickness layers vertically.

  • Import this grid into the Velit model into a slot (I’ll assume General 01) for the appropriate layer.
  • On the Surface display, open the pin-open the “Depth” fly-out, select the “Parameters” tab and set the “Formula” drop-down to “[General]SALT WEDGE MODEL 2” (& ensure “From” field is set to “User”).

output-velit

You can see the inputs are for “hardrock isoc”, the usual “last time”, “time”, “depth” and “error” slots, and the “VS” and “VH”. VS and VH are velocities for the primary & secondary lithologies respectively.  In the Zechstein example, the slow velocity halites & faster anhydrites respectively.

Typically, the secondary (VH) velocity may be a constant (in this case I will use a very high 5900 m/s), but the VS velocity may be a constant or a grid.  If you have good well coverage, you can deselect wells that don’t intersect with any of the secondary lithology and use the Wizard mapping function to map out the well velocities to create this grid.

  • Copy the interval velocity grid to e.g. a General 02 slot, then turn the other wells back on as you may wish to perform residual error correction.
  • Fill out the depth conversion dialog as follows (many of the parameters will be defaulted for you). Remember the first time you do a depth conversion, set the “Error” value to zero.  The depth conversion computes the residuals which you can then grid and apply on a second depth conversion should you require.

depth-conversion-dialog

 

  • Press the “Apply” button and the outputs will be computed.  The isopachs for primary and secondary layers are put into the General 03 & 04 grids, but you can change these (but be careful not to overwrite your input isopach or velocity grids in General 01 & 02).
  • If required, grid up the “XYZ > Error” values into an “Error” grid, then use this for the second conversion to tie the depth surface to the well markers.