American Oil and Gas Reporter - October 2015 - 158

SpecialReport: Gulf of Mexico Activity

Basement Tectonics Help Locate Pay
By Corine Prieto

HOUSTON-Establishing basement
tectonics should be regarded as the first
step in evaluating an international frontier.
More than two decades of data correlate
the relationship between oil and gas fields
and discovery wells to regional structural
basement characteristics in the northern
Gulf of Mexico, and demonstrate the rationale for utilizing regional basement
structure as a tool for evaluating exploration prospects.
Prospectivity continues to expand in
the northern Gulf of Mexico as drilling
technology reaches greater depths. However, the costs and risks associated with
deep- and ultradeepwater exploration and
drilling are increasing, and it has become
apparent that using regional basement
structure as an evaluation tool can help
reduce both cost and risk. The position
of oil and gas fields can be a vital parameter in predicting the viability of exploration sites, when the correlation between
producing and projected sites can be
identified.
Today's cost for acquiring seismic
data over an optimum areal extent is $15
million. Completion costs for a deeptarget drilling operation average $1 million
per 1,000 feet, or generally a minimum
of $30 million. New Wilcox and Norphlet
discoveries are at depths near 30,000
feet. At these prices, cost makes a strong
case for including all information, including the basement structure, in the
analysis and economic assessment of
prospective targets.
Based on the premise that a financial

risk reduction factor can be applied when
a correlation is made between the position
of an oil and gas prospect and the magnetically-based basement structure map,
a research study was conducted to correlate
the relationship between oil and gas fields
and discovery wells to regional structural
basement characteristics, as well as to
demonstrate the rationale for utilizing regional basement structure as an evaluation
tool. The study used data amassed in the
northern Gulf of Mexico over the past
two decades.
The study confirms that a financial
risk reduction factor can be applied when
a correlation between the position of an
oil and gas prospect and the magnetically-based basement structure map is made.
Many operators have found the result
has led to better decisions about how
best to use their drilling and development
budgets.
The Anticline Theory
Despite the technological explosion
of recent years, the most cost effective
concept defining the location of oil and
gas fields arguably is still the "anticline
theory," i.e., closed structural high at
reservoir level. Such a structural anomaly
must be located in the pathway of oil and
gas, which migrates both laterally and
vertically, in order to maximize its trapping
potential.
On the regional scale, a structural high
must be in close proximity to a structural
low (syncline, depocenter, or sub-basin).
From a basinwide perspective, high structural anomalies become the focal points
of migrating oil and gas.
Regional (low) structural anomalies

FIGURE 1
Jack & St. Malo Discovery Locations
Depth Range is 35,000 (Red) to 50,000 (Blue) Feet

158 THE AMERICAN OIL & GAS REPORTER

are the depocenters of generating
"kitchens." In most cases, basement anomalies underlie and cause regional basinwide
sedimentary structural anomalies. This
is the primary rationale underlying the
practical application of basement maps
as exploration tools.
Less accepted-or merely less known-
is that in any producing basin, oil and
gas are concentrated in specific areas. In
fact, 75 percent or more of the oil and
gas in a producing basin is contained in
only 25 percent or less of the total basin
area.
Further, structural positive (high) anomalies near structural negative (low) anomalies are the preferred targets. Typical
examples are the Jack (Walker Ridge
Block 759) and St. Malo (Walker Ridge
Block 678) discoveries, which were drilled
in ±7,000 feet of water and through salt
bodies greater than 8,000 feet thick. Significantly, each discovery can be correlated
to a basement high block on the depthto-basement structure map (Figure 1).
The basement depth at the Jack well location is 45,000 feet; the depth at the St.
Malo structure is 38,000 feet.

Methodology
The study's research objective was to
correlate the relationship between oil and
gas fields and discovery wells to regional
structural basement characteristics, and
to demonstrate the rationale for utilizing
regional basement structure as an evaluation tool.
Simply stated, the position of known
oil and gas fields can be a vital parameter
in predicting the viability of exploration
sites when the correlation between producing and projected sites can be identified. In the deepwater regime of the
northern Gulf of Mexico, the availability
of a reliable regional basement structure
map and a documented number of oil
and gas discoveries provides an opportunity to test the strategy.
Two types of databases are utilized
for the statistical analysis correlation:
* A magnetically-based basement
structure (depth and contours) map; and
* A tabulation of oil and gas field
discoveries from several published sources.
The basement map is the product of
two decades of diligent analysis of depth
estimates derived from various vintages
of aeromagnetic surveys. This offshore

American Oil and Gas Reporter - October 2015

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