Difficult Equity Net Pay Determination by Computer

Don Clarke

The East Wilmington oil field is currently undergoing an equity determination. The net pay must be determined for 92 tracts as accurately as possible. This will be done on the computer by following a detailed procedure.

This procedure involves the construction of a computer model that bounds an enclosed volume. The largest uniquely mappable segment must be defined and mapped. The segments are added together at a later stage for the final volume determination. Our smallest definable segment is bounded by two nonvertical faults, two structural surfaces, an oil-water contact and a boundary truncation.

Grids of the structural surfaces and nonvertical fault surfaces are constructed. Vertical barriers or faults are then defined.

The fault grids and barriers are combined with the structural grids. This is accomplished through a series of operations that result in a single grid based on each structural horizon. All unwanted grid data above or below the intercepts are dropped. Next, all values of the grid below the oil-water contact are set to the value of the contact. This results in a bounding grid that represents all but one side of the segment to be mapped. Two stratigraphically adjacent boundary grids are then subtracted.

The resulting grid represents the enclosed volume between the two grids. This grid is the net pay isochore unless there are enclosed shales. The shales can be taken out by multiplying this grid times a percent-shale grid.

All of the individual pieces are constructed for a given interval. The pieces are then added together horizontally to form a complete grid over the area of study. Some overlaps result along nonvertical fault boundaries. These are vertically added as part of the horizontal assembly. This grid can now be contoured and have volumetrics determined.

This procedure is no more than a series of geometric manipulations, but the results are easily applicable to other petroleum problems.

AAPG Search and Discovery Article #91024©1989 AAPG Pacific Section, May 10-12, 1989, Palm Springs, California.