American Oil and Gas Reporter - May 2015 - 69

SpecialReport: Study In Geophysics
northwest-to-southeast across the survey
area and has a highly irregular nature. The
irregularity of the salt edge suggests that
salt dissolution created the edge rather than
salt deposition, with dissolution occurring
during the Late Jurassic and Early Cretaceous.
Two of the proposed mechanisms for
salt dissolution include compaction-driven water migration in the Lyons formation
and basement tectonics. Based on the 3D seismic patterns observed, basement tectonics was the likely control on the location of the salt edge, but additional analysis is required to determine the role played
by the Lyons formation.
The Permian salt also has an effect on
the overall structure of the Silo Field.
Where the salt is absent, a thickened
Dakota-Sundance interval compensates for
the lack of salt. This compensation creates
a structural monocline in the strata overlying the salt edge as a result of differential compaction. It is possible that this
thickened Dakota-Sundance section is
the cause of high water production from
the Niobrara in the Silo Field.
However, differential compaction is not
the only cause for the monocline. Examining the basement horizon shows there are
possible basement faults that also contribute to the location of the structural monocline within Silo Field. It also is likely that
basement faults control the location of the
salt edge.
Analyzing FMI logs within and outside
of the main field area demonstrates that the
orientation of natural fractures changes rapidly over short distances. In the southern
part of the field, the dominant fracture orientation is northwest-to-southeast, similar
to the underlying basement structure.
Less than five miles south of the field, the
fracture direction changes to a more
north-to-south orientation. This has implications for future development of the
field, and careful consideration is needed
when attempting to develop outside the
main field area.
Understanding the nature of the Permian salt edge in Silo also could have important implications for understanding Permian salt dissolution and its impact in other
Rocky Mountain fields.
Silo Field Geology
Silo Field is located approximately 17
miles northeast of Cheyenne in Laramie
County, Wy. Since its discovery in 1981,
the field has produced more than 11.4 mil-

lion barrels of oil and 9 billion cubic feet
of gas from the fractured Niobrara formation at depths ranging from 7,100 to
8,800 feet. Developing the field with
horizontal wells and multistage hydraulic
fracturing stimulation suggests much
greater future production. Initial oil production from new horizontal wells ranges
from 500 to 2,000 barrels a day.
Silo is on the eastern flank of the
Denver Basin, where the beds dip gently

to the west. The basin is Laramide-age and
the majority of the sediments are composed
of sandstones, shales and carbonates. The
deposition of the Niobrara occurred during the Late Cretaceous, after a major
transgression joined the seaways and created the Western Interior Cretaceous Seaway. The thickness of the Niobrara ranges
from 100 feet in South Dakota to greater
than 1,800 feet in western Wyoming.
Within the Silo Field, average Niobrara

American Oil and Gas Reporter - May 2015

Table of Contents for the Digital Edition of American Oil and Gas Reporter - May 2015