American Oil and Gas Reporter - May 2015 - 70

SpecialReport: Study In Geophysics
thickness is approximately 300 feet.
The Niobrara formation shares conformable contacts with the overlying
Pierre Shale and the underlying Carlile
Shale. In the Denver Basin, the Niobrara
is composed of the lower Fort Hays limestone member and the upper Smoky Hill
member. The Smoky Hill is further subdivided into the A, B and C chalks and
marls, where the A units are the highest
stratigraphically. In the Silo Field, the B
chalk is split into the B1 and B2 because
of an additional marl unit, and are the primary targets of horizontal drilling.
Seismic data for this study were provided by Global Geophysical Services
from a 30 square-mile 3-D seismic survey it acquired over the Silo Field. Eight
horizons were interpreted to aid in analyzing the Niobrara and the Permian
salt. From deepest to shallowest, the horizons used included the Precambrian
basement, the Wolfcamp (base salt), top
Permian salt, Sundance, Dakota, Niobrara, a major reflector in the Pierre, and
an arbitrary horizon called the "shallow
horizon."

Figure 1 is a comparison of the timestructure maps from the shallow horizon
to the basement horizon. All maps show
a fault-bound syncline in the northwestern part of the survey area. The basement
and Wolfcamp horizons have a northwestto-southeast trending anticlinal feature, but
there is a distinct change in the structure
at the Permian salt level. A structural monocline is present in all of the horizons
above the Wolfcamp. The edge of the
structure and all of the horizons above the
salt orient northwest-to-southeast, demonstrating the northwest-to-southeast trending structural monocline.

Attribute Extractions
Similarity, the most negative, and
most positive, curvatures were the three
seismic attribute extractions used for
Niobrara fault interpretation. All horizons
in the attribute extractions showed a pair
of faults bounding a syncline in the northwestern part of the survey. The faults are
near-vertical and penetrate from the basement through the Niobrara. These features
have been described as wrench faults, and

they do fit many of the criteria for wrench
fault classification, but only a small portion of the faults is displayed within the
survey, and no piercing points are present
in the survey area.
Figure 2 shows an arbitrary seismic line
oriented southwest-to-northeast across
the faults. They appear to be wrench
faults, but the lack of piercing points makes
it difficult to determine any wrench movement. Because of the complex nature of
the feature, it is likely that there are
splays coming off the main faults
At the Niobrara level, the seismic attributes illustrate a fault system in the
south-central area of the survey. The
faults are layer-bound to the Niobrara and
the orientations appear to be relatively random, suggesting that this could be a
polygonal fault system. Because the orientations are not completely random, it is
possible that basement structure is exerting some control on the faulting.
A comparison of the interpreted faults
to a previously performed surface lineament analysis demonstrates that basement control is a likely cause for fault ori-

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American Oil and Gas Reporter - May 2015

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