(GP/GT) for Additional Water Supply in the Lower Rio Grande

section is greater than the displacement along the faults.
This
allows for a connection of the reservoir sands within the Marks and
Bond sand sequence (Fig. 3) (Collins, 1983).
The Marks sand (Fig. 4) occurs within an interval from
7,760'-8,230' in the southeast side of the area to 10,970-11,990'
in the west.
The sand continues to thicken and increase in depth
as it dips to the west toward the McAllen fault.
Because of
increased faulting r.e1ated to the main fault, the area of the
reservoir should be considered to be limited to the west before
the main fault is encountered (Fig. 3). To the northeast, the dip
flattens creating a high which lies along an axis which trends from
the northwest to the southeast along a line from Edinburg to Donna
(Fig. 2).
Within Reservoir Area C, average depth to the top of the Marks
sand is 9,881 feet. Average thickness of the sand is 409 feet, and
average net sand thickness is 245 feet, or an average of 63 percent
sand.
Average pressure at the top of the sand is 7,333 psi, with
an average pressure gradient of 0.73 psi/ft. Average temperature
(A.A.P.G. corrected) is 279°F.
The median temperature is, however,
nearer 300°F in the area near the center of the reservoir. Porosity
estimated from log resistivity and spontaneous potential averages
17 per cent, and permeability averages 14 md (mi11idarcies,
designated by the symbol K). The average KH (millidarcies x
average thickness of sands in feet) is 3,528.
These data are
summarized in Table 2.
No
core data were available for the Marks sand, but log
characteristics indicate the sand to be a fairly uniform shoreface
II-9