American Oil and Gas Reporter - October 2015 - 161

SpecialReport: Gulf of Mexico Activity
highs adjacent to a deep basement structural low become attractive sites for oil
accumulation. Structural highs located
between two adjacent basement lows
offer special attraction for migration pathways from both sides. Regional low areas
become the sites for increased sediment
deposition with a preferred content of
oil/gas source beds (depocenters).
Regional highs become the sites for
reservoir development, especially on the
flanks where sand units pinch out against
existing topographic and structural highs.
Finally, oil and gas that is generated thermally in regional structural lows migrates
updip to regional structural highs.
Logical Connection
For many skeptics, the absence of a
logical connection, or rather the lack of
data concerning a connection between
basement structure and the required structural situation at the level of deeper Jurassic, Cretaceous and Eocene source beds,
is critical. Such a connection is required
to assign a higher exploration value to
the basement structure map.

Unfortunately, there are no maps generated with confidence from these deeper
levels today. Therefore, analyzing prospective acreage blocks must be restricted to
the relationships observed between the
magnetically-based basement structure
location and distribution of productive
acreage blocks, and to discovered oil/gas
reserves.
Basement structures in the northern
Gulf are the result of multiple deepseated tensional and shear faulting at the
basement level. Subsequent basement
motions, as well as the impact of topographical differences at the level of sedimentation during Mesozoic and Lower
Tertiary times, originally led to the development of regional lows and regional
highs. The quality of basement structure
mapping has improved continuously with
higher quality data, an abundance of data,
and more refined interpretation techniques.
Deepwater Gulf of Mexico discoveries
have verified that the basement structure
has influenced oil and gas migration. In
July 2013, Shell Oil Co. announced that

its deepwater Mississippi Canyon Block
393 Vicksburg discovery encountered
more than 500 feet of pay at a depth of
26,385 feet (water depth was at 7,446
feet). The Vicksburg discovery was a
follow-up to the Shell/Nexen Appomattox
discovery (February 2012), which encountered approximately 150 feet of oil
pay at a total depth of 25,851 feet.
The press release said a contingent
recoverable resource of 215 million barrels
of oil equivalent was identified in the
Jurassic Norphlet formation in the northeast fault block of the Appomattox structure. The well is located in Mississippi
Canyon Block 348 in 7,257 feet of water.
The complexity of the basement structure in the immediate area surrounding
the wells is shown in Figure 2. This
author has not been privy to the seismic
interpretation, and can describe the basement environment only from an interpretation based on a high-resolution aeromagnetic survey and refraction velocity
data. The basement depth at the Vicksburg
well location is 48,000 feet. The depth at

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American Oil and Gas Reporter - October 2015

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