October 31, 1975 - Gerald R. Ford Presidential Library and Museum

The Spraberry Trend oil field is located in Upton, Reagan, Irion,
Martin, Midland, and Glasscock Counties of West Texas, within the
Midland Basin.
The producing formations are the Upper and Lower Spraberry which are
C1earfork-Wichita equivalent of Permian age. The underlying Dean Sand
is Wo1fcamp, also of Permian age. The Upper Spraberry is generally
around 400 feet thick; the Lower Spraberry ranges from 500 to 600 feet
in thickness; and the Dean Sand is about 120 feet thick. Over most of
the area. about 90 percent of the thickness is sand, s·i1t. and shale
(mostly shale). The remaining 10 percent or so is limestone and
dolomite. The oil is indigenous to the formation. The reservoir rock
is extremely low in porosity and permeability, and is a siltstone.
Production is made possible by fractures which extend through both
the matrix siltstone and the shale. Depth to the top of the Upper
Spraberry varies from 6,000 to 8,000 feet.
The oil accumulations in the Spraberry are trapped stratigraphically.
The area is virtually devoid of folded or faulted traps. The structure
is a general monocline dipping from the eastern shelf into the depths
of the Midland Basin. The classic sections grade into limestones and
dolomites on the rims of the basin. The productive field, as defined
by the Texas Railroad Commission, covers 920,000 acres (about 1,400
square miles). The consultant firm also estimates that an additional
150,000 acres on the fringes of the field may be developed in the
future.
The crude oil at Spraberry Trend was initially undersaturated. However,
because of low permeability, pressure was quickly reduced in the
vicinity of producing wells causing release of gas from solution and
high producing gas/oil ratios. The primary producing mechanism was a
dissolved gas drive. Bottom water is not active. Oil drains from the
siltstone matrix into the fracture system and productivity depends on
the effectivel1ess of this fracture system. The normal spacing is 160
acres per well and over 3,900 wells are currently producing. About .
320,000 acres or 35 percent of the presently defined area of the field
have been under water flood.
Ultimate recovery by primary methods was determined by preparing
production decline curves for each lease. Primary ultimate recovery
is expected to average 500 barrels per acre. Secondary ultimate
recovery was similarly estimated by studying the production records
for the 34 water flood units. The contractor estimated that additional
secondary recovery will average 200 barrels per acre. These volumes
are considered proved. The secondary recovery reserves for 375,000
acres of future projects are reported under "Indicated Secondary
Reserves." The availability of production decline information and
the 80 percent extent of depletion of the proved volumes should cause
the reserves estimate to be quite firm.
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