Abstract

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 commenced discussion with Dames & Moore to determine if a computer mapping technique existed which could be used to process data, prepare isopach maps, and calculate oil sand volume underlying each 1,000 foot grid square and/or for each of 91 Long Beach Unit Tracts. There was an abundant supply of computer programs which could plot isolines utilizing various algorithms, however there was no system or set of programs readily available which would do all that was desired for this particular application.

Apparent sand thickness is determined from electric logs. Nearly all of the wells are directionally drilled, hence the sand thickness must be corrected for hole angle and formation dip. These adjustments result in a net vertical sand thickness which is assigned to the mid-point 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. Net oil sand isopach maps are prepared after the addition of fault boundaries.

Dames & Moore worked with the city and State representatives for about 20 months developing the necessary techniques and programs which could, (1) input the oil sand thickness data, (2) plot a map of the well data points, (3) draw isopach contour lines through the data points, (4) input the isopach data in the pinch-out zone, and (5) calculate and report volumes in 1,000 foot grids and under each of the 91 tracts.

Standard programs in Dames & Moore’s “software” library were utilized to perform most of the functions and these programs were modified and linked together into one system. However, difficulty arose when attempting to utilize grid cells to accurately calculate volumes in the pinch-out zone. The solution for this problem combines the techniques of polygon overlay with those of area-moment calculations. In practice, the technique parallels the hand calculations 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 percent from the previous manual calculation.