Do you have you well tops correlated yet? If so you can make two maps using them (top and bottom) and build an isochore/thicknees map between them. You could also sample total sand thickness (if you have painted/caluclated discrete logs) into a welltop attribute. Then you can use this for mapping/intepolation.
hope this helped. Pop these keywords into the help and you should get there!
good luck!
