A determination of the volume of productive oil and/or gas sand under each of the 91 tracts of the Long Beach Unit is necessary for the establishment of equity assignments.

Sand thickness is determined using electric logs. Nearly all of the wells are directionally drilled so the sand thickness must be corrected for hole angle and formation dip. This results in a net vertical sand thickness which is assigned to the midpoint of each sand interval. Productive limits based on the oil-water interfaces are then determined for each sub-interval sand by using electric log and core data. Isopach maps are prepared after the addition of fault boundaries.

In 1977, under the direction of the participants of the Long Beach Unit, the Department of Oil Properties of the City of Long Beach and the Long Beach Operations staff of the California State Lands Commission began discussion with Dames & Moore to determine a computer mapping technique which could be used to process the data, prepare the isopach maps, and calculate the oil sand volume underlying each 1,000 ft grid square for each of the 91 Long Beach Unit Tracts. There was an abundant supply of computer programs which could plot isolines utilizing one algorithm or another; however, there was no system or set of programs readily available which would do all that was desired for this particular application.

Dames & Moore worked with the city and state representatives for about 9 months, developing the necessary techniques and programs which could: input the oil sand thickness data; plot a map of the well data points; draw an isopach map through the data points; input the isopach data in the pinch-out zone; and calculate and report volumes in 1,000 ft grid cells under each of the 91 tracts.

Standard programs in Dames & Moore's library were utilized to perform most of the functions and these programs were modified and linked into one system. However, a difficult problem became apparent when attempting to utilize grid cells to accurately calculate volumes in the pinch-out zone under each tract. Many procedures were tried and one was finally selected. The selected procedure combines the techniques of polygon overlay with those of area-moment calculations. In practice, the technique parallels the method previously utilized by the Long Beach Unit, thus permitting an accurate check. The procedure was tested for one sand interval and was found to vary less than 0.01% from the previous manual calculation.

At present, the system is performing accurately, efficiently, and in a timely manner for the participants of the Long Beach Unit.

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