Geology Reference
In-Depth Information
feet below the surface. Flow of the salt (deformable and
less dense than overlying sedimentary rock) occurs due
to lithostatic pressure differences in the rock column. As
a salt dome flows and punches upward through the
overlying sediments over millions of years, it develops
a cap rock of sulfate and carbonate minerals that
forms from impurities in the salt. The cap rock is sev-
eral hundred feet thick on top of the salt and drapes
over the edge and the upper sides of the dome. Cap
rocks may be covered by several hundred feet of sand
and mud/shale layers in the Houston dome area.
Sulfur, formed from bacterial alteration of the sul-
fate minerals (anhydrite and gypsum), is a resource
extracted from the cap rock by the Frasch process (sulfur
is pumped out after being melted by superheated steam).
In addition to the salt that is removed by solution mining
from the dome, oil and natural gas are extracted from
traps in steeply dipping strata that tenninate at the edge
of the salt dome. Salt domes are also used for storage of
oil and gas in cavities that have been solution mined in
the salt (e.g., U.S. Strategic Petroleum Reserve).
Extraction of sulfur from the cap rock has pro-
duced
trough subsidence
(gentle downwarping over
the extraction zone) and
collapse sinkholes
(smaller,
circular, steep-walled depressions). Natural subsidence
over salt domes is indicated by depressions and saline
lakes over dome crests. When a cavern is formed by
extraction of salt or sulfur, the cavern roof may fail,
with collapse of rock into the cavern. As more of the
overlying rock and sediment fails, upward migration
of the broken zone occurs.
Damage from subsidence following salt dome
resource extraction includes loss of the flooded land,
damage to roads and buildings, loss of well casings
(greatest risk), and loss of drilling equipment. In one
instance after sulfur extraction had been completed at
a site, an attempt to recover some of the well casing
caused the almost immediate collapse of the land sur-
face to a depth of 70 feet and loss of the rig.
At the Orchard Salt Dome in Fort Bend County,
Texas (Figure 9.6, topographic map), subsidence
related to removal of sulfur has been documented
(Mullican, 1988). The dome is about 7,000 feet in diam-
eter, covering an area of 620 acres as shown by the
occurrence of subsurface structural contours on the salt
surface. Structural contours outline the shape of a rock
unit in the subsurface (see Figure 9.6). Minimum
depths to the cap rock and the salt are 285 feet and 375
feet, respectively. Production of sulfur was from the
dome margin in the zone 1000-3000 feet below the
surface, from 1938 to 1970. Maps and photo surveys
document the change beginning with the first sink-
hole (100 ft in diameter) that developed near the
southwest (SW) margin in 1941. By 1952 subsidence
troughs and circular sinkholes in the northeast part of
the dome began, followed by other failures there
(Figure 9.7). After sulfur extraction terminated in 1970,
high-altitude U-2 photographs in 1979 showed addi-
tional subsidence in the northeast part of the dome.
TABLE 9.2 Subsidence at Bench Mark P7 in San Jose
Year Leveled
Total Subsidence (ft)
1912
0.0
1920
0.3
1934
4.6
1935
5.0
1936
5.0
1937
5.2
1940
5.5
1948
5.8
1955
8.0
1960
9.0
1963
11.1
1967
12.7
1969
13.0
1988
13.0
1990
13.0
c. Did decreased pumpage, increased precipitation, or
water imports have the greatest role in causing the water
pressure surface (artesian head) to rise from the levels of
the mid-1960s?
19. Between 1970 and 1980 the depth to water below land
surface had risen to an average of 90 feet; between 1990 and
1995, the depth to water had risen to an average of 45 feet.
What happened to the subsidence rate as a result of the
change in the water-pressure surface?
20.
Would subsidence resume if imports of water were reduced
and pumping of groundwater was increased?
PART C. SULFUR EXTRACTION
FROM ORCHARD SALT DOME, TEXAS
Natural and mining removal of salt from salt domes,
sulfur from their cap rocks, and oil and gas from adja-
cent strata can result in subsidence features on the land-
scape. In this exercise we look at the collapse processes,
landforms, and damages that occur with the extraction
of sulfur from salt domes in the Houston dome area of
Texas. Salt domes in this area are roughly cylindrical
masses of rock salt (halite) formed by the upward
migration of salt from Jurassic evaporites thousands of
