Depth Conversion

To understand the procedure for Velocity conversion of horizons in the Seismic VSP tie for the formations post.

Shallow horizons (above the Top Heimdal reflector) were depth converted using interval velocities derived from wells in the area. The Top Heimdal Formation was depth converted using the linear formula:

V_{z}=V_{o}+kZ

• V_{z} = velocity at depth Z
• V_{o} = initial Velocity
• k = compaction factor
• Z = Depth (subsea)

This linear velocity function can also be visually understood

The linear velocity function which minimizes the depth difference between the observed and the estimated time/depth data is based on the selection of k parameter.

Based on analyses from 11 wells in the 15/9 block (including 15/9-19 SR), the minimum point for depth residuals occurred at k= 0.346, whereas the minimum for a standard deviation of V_{o} occurred at k= 0.71.

This infographic explains the discrepancy in Vo that leads to different velocity trends for thin formations. (Image 3) The compaction factor (K) is considered constant for thin formation, so a value of K that leads to the minimum error between the actual and expected depth is selected.

Image 3: Process to convert from time to depth domain for thin formations^​1​

A similar analysis using the 5 vertical wells on the Gamma High (15/9-9, -11, -13, -15, -16 and -17) resulted in minimum depth residuals at k= 0.456. Bearing in mind that the well 15/9-19 SR is highly deviated, less weight is given to that well than to the vertical wells on the Gamma High. Also, larger k-value gives less standard deviation on the V_{o} values. Therefore k= 0.456 is chosen for the Theta Vest area as for the rest of the Gamma High.

1. 1.

   .. Velocity Modeling Course. Schlumberger; 2009.