The Northwest Ten Section accumulation discovered in 1958 is between the Canal and Ten Section oil fields in Secs. 23 and 24, T. 30 S., R. 25 E., approximately 14 miles southwest of Bakersfield in Kern County, California. Shell Oil KCL 15X-24, a 15,739-foot basement test, is credited with finding two new Stevens oil accumulations; however, the first producing well in the pool was a follow-up well, Shell KCL 84-23. An upper Stevens accumulation was indicated in 15X when a formation test

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of the interval, 8,206-8,248 feet, recovered 68 barrels of 35° gravity oil in 4 hours, a 420 B/D rate. Side-wall samples in the lowermost Stevens sand interval, 9,475-9,664 feet, indicated this 125-foot thick net sand section to be oil stained but the interval was untested because of the washed-out condition of the hole. In the follow-up well, 84-23, this lower Stevens sand was found to be 120 feet thicker than in 15X-24. The lower Stevens interval from 9,455 to 9,545 feet was perforated and pump-tested and yielded an average of 100 B/D of 32° gravity oil, cutting 10% during an 8-day test period. Production from the upper Stevens zone was then commingled with the lower Stevens zone and flowed oil at an average of 450 B/D during the first month of production.

The upper Miocene-Stevens is a prolific oil-producer in many fields of the southern part of the San Joaquin Basin. The estimated ultimate is over 800,000,000 barrels. The Stevens sequence in this area is over 1,500 feet thick and as the many shale breaks are thin and facies changes abrupt, correlation in this unit is difficult even where wells are closely spaced. Recent oil discoveries in the lower Stevens have focused attention on the fact that localization of structural trends and stratigraphic changes at this horizon are commonly different from upper Stevens prospects. The Stevens sand is an arkose containing abundant acid igneous rock fragments and a kaolinitic matrix. The upper Stevens sands are typically medium- to coarse-grained and the lower Stevens sands are typically fine- t medium-grained with permeabilities averaging 200 md and 50 md, respectively. Individual sands within the Stevens may appear massive on the electric log but a study of cores reveals that these sands are made up of numerous, usually thin, graded beds. The lower Miocene and Eocene sandstones lack sufficient permeability and porosity to provide an economic reservoir.

The basement wildcat was planned to explore the Stevens (upper Miocene), the Media sands (middle Miocene), the Vedder sands (lower Miocene), and Eocene sands. Stratigraphic changes in the lower Stevens section and Eocene section were also considered in locating the 15X well. Eocene sands were topped at 15,540 feet with the base at 15,639 feet. A formation test of the Eocene in the interval, 15,560-15,650 feet, for 18 hours recovered water at 140 B/D rate. A formation test of the lower Miocene-Vedder in the interval, 13,090-13,165 feet, for 12 hours recovered water with a trace of oil at a 55 B/D rate. Basement was topped at well depth 15,718 feet. It should be pointed out that the planned total depth of the well was 15,000± feet although our seismologists advised that they were p cking the uppermost possible basement reflection and that a second or third possible basement event might be more reliable. Prior to drilling, a depth of 15,800 feet was considered the maximum depth to basement. Similar experiences 2 miles north-northeasterly in Shell's Posuncula 1 at Strand dictated that this uppermost reflection event would probably be basement; however, this was not the case at 15X-24.

Several points of interest concerning the upper Stevens accumulation are: (1) the oil-water contact appears to be near the same level as in the Canal oil field, and (2) the low hydrostatic pressure which is approximately 1,000 pounds below virgin pressure suggests these sands are in fluid communication with the producing measures in Canal or Ten Section, or both.

This oil find is an example where detailed seismic shooting and study have defined a small, structurally closed accumulation in a densely prospected area. The actual areal extent and size of the oil reserves will need definition by development wells. Current estimates of the upper Stevens pool are that the accumulation will cover 300-500 acres with maximum height of oil column, 150 feet, and oil reserves of the order of 5 million barrels. There are insufficient data to predict reserves from the lower Stevens.

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