Many problems were encountered during the construction of net oil sand isopach maps from electric logs in Wilmington field, California. These maps were to be used for secondary equity determinations, and required committee agreement on all details.
Problems arose in measuring sand thicknesses from electric logs, delineating exact fault-sand intercepts, and determining oil-water contacts. These problems were resolved with both old and new techniques through committee work.
Sand thicknesses were measured from electric logs by a modified spontaneous potential-short normal counting method. Log measurements were compared with all available core data to establish uniform counting techniques. Net oil sand log measurements from directionally drilled wells were corrected to vertical equivalent thicknesses before being placed on the isopach maps.
The omitted intervals caused by faulting were shown on the isopach maps by using zonal midpoint fault traces where the faults were within a major block, and by actually isopaching the fault wedge on block boundary faults.
The net oil sand was also isopached in the edgewater areas. Sands that were being encroached by edge water were assigned area cut figures representing the estimated per cent of water that a well completed in that interval would produce. From these figures the per cent of originally oil-saturated sand no longer capable of producing oil was calculated.
Average zonal oil gravities were assigned. These gravities were then converted to a value based on an historical average price range. Final secondary equities were calculated from the net oil sand isopach volumes penalized for water encroachment, oil gravity, production costs, and zonal differences.