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Stepwise multiple discriminant function analysis of detailed geochemical data on 57 crude oil samples from six closely spaced sandstones in the "shallow oil zone" (Pliocene) of the Elk Hills U.S. Naval Petroleum Reserve No. 1, California, shows that 77 percent of the crude oils may be assigned to their proper stratigraphic unit on the basis of their chemistry.
Crude oil assignment is performed in two parts. The first part assigns a crude oil to one of the three local, economic stratigraphic units: (1) the Wilhelm sandstone, (2) the sub-Mulinia sandstone, or (3) four stratigraphically higher sandstone units. This assignment is based on seven chemical properties in crude oil: one gas chromatographic measurement of the saturated hydrocarbons, three additional gas chromatographic measurements of the aromatic hydrocarbons, and three trace-element measurements (Be, Fe, and V). If a crude oil is assigned to the group of four stratigraphically higher sandstones on the basis of these properties, a second assignment is performed to assign the crude oil to one of these four units. This assignment is based on one chromatographic measurement of the satu ated hydrocarbons and two trace-element measurements in ash (Co and V).
Crude oil differences, particularly in the chromatographic properties, are most marked among the three stratigraphic units used in the first part of the assignment (the Wilhelm, the sub-Mulinia, and the upper four sandstone units, collectively). Because these distinctions reflect, in part, lower percentages of distillate in the deeper productive areas, they may have resulted, in part, from separation-migration of a common crude oil parent. Weaker differences among the upper four sandstones may represent a weak natural chromatographic separation during secondary migration.
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