Why do isopachs of net oil thickness look like concentric lines between internal and external OWC locations?
The contours run parallel to the depth contours as they are controlled primarily by height above OWC. Inside the feather edge or ‘internal OWC’ the thickness of net oil is controlled more by the thickness of the sand. In many cases, the structural shape varies smoothly, and the rock thickness is also only slightly varying, so the net oil thickness varies smoothly, too – and gets shown as equally-spaced contour lines.
