Attribute Illumination of Basement Faults
Examples from Cuu Long Basin, Vietnam
Ha T. Mai, Kurt J. Marfurt, University of Oklahoma, Norman, Oklahoma, USA
INTRODUCTION
• Faults and fractures play an important role in forming effective fracture porosity
for hydrocarbon traps in fractured basement reservoirs in many places, such as
Venezuela, USA, Morocco, Brazil, Libya, Algeria, Russia ... and Vietnam.
• Since this type of reservoir is very complicated, lack of stratified, coherent
reflectors, illumination of basement faults/fractures is more problematic than
illumination of faults within the sedimentary column.
• In order to address these problems, it is important to carefully analyze different
seismic attributes.
• Seismic attribute is any measure of seismic data, related to target objects, such as
time, velocity, amplitude, frequency, phase, energy … that helps us better
visualize or quantify features of interpretation interest (which is faults/fractures
surface in our case here).
• Using proper set of seismic attributes can greatly help interpreters to delineate
sweet spots, characterize fractured basement reservoirs, and better aid drilling.
In this research, we present several computation methods to enhance the signature
of faults/fractures within basement zone, by taking seismic amplitude data, we
generate the following seismic attributes:
- Apparent dip
- Amplitude gradients
- Curvature
These seismic attribute volumes will then be rotated in different pre-defined
directions, in order to better show the location or development of faults /fracture
system in fractured basement.
METHODS
A planar surface such as dipping horizon or faults can be presented by its true dip
azimuth q and strike . The true dip  can be presented by apparent dips x and y
along the x and y axes
Having apparent dips at any two direction, we can reverse the calculation, and find
true dip or apparent dip at any direction.
With that in mind, we expect to see planar or steeply dipping features more
distinctly by viewing them perpendicular to their strike.
Base on this property, we choose to do directional analysis on three attributes:
• Dip angle of reflection surface
• Amplitude gradient
• Curvature of reflection surface
DIRECTIONAL APPARENT DIP ANGLE AND AMPLITUDE GRADIENT
DIRECTIONAL CURVATURE
Dip angle is the angle of most coherence signal in a direction of analysis
Amplitude gradient is the lateral gradient of coherent amplitude along a direction of analysis
Curvature in 2-D is defined by the radius of a circle tangent to a curve. In 3-D, we need to fit two
circles tangent to a surface. The circle with minimum radius is the maximum curvature (kmax) and
the circle with maximum radius is the minimum curvature (kmin).
Based on inline and crossline dip components, we calculate maximum, minimum curvature, and
minimum curvature azimuth.
From these three attributes volumes, we compute the apparent curvature of reflection surface for
any pre-defined direction .
- Take post-stack 3-D seismic data as input
- Apply filter and smoothing methods to reduce noise and increase the coherency in the data.
- Calculate dip angle and amplitude gradient for inline and crossline direction
- Calculate dip angle and amplitude gradient for any defined direction.
Curvature in two dimensions
Chopra and Marfurt 2007
Inline
dip/grad
cube
Seismic
cube
Directional
dip/grad
cube
Crossline
dip/grad
cube
Max
curvature
cube
Inline
component
AASPI
processing package
p  px cos(   )  p y sin(    )
APPLICATION
AASPI
processing package
Crossline
component
Curvature in three dimensions
Directional
curvature
cube
Min
curvature
cube
Min
curvature
azimuth
cube
k  k max sin 2 (   )  kmin cos 2 (   )
SOFTWARE
In order to demonstrate the mention methods, we will present some results from fractured basement of Cuu Long basin
in Vietnam.
Taking 3-D post-stack depth migrated seismic data from Cuu Long Basin as an input, using our AASPI Attribute
Processing Software, we calculate directional attribute volumes, then present the results for directional dip angle,
apparent amplitude gradient, and curvature at calculation directions of 0, 30, 60, 90, 120, 150 from North.
Depth slices at 2750m is being shown. This is the depth right bellow basement surface.
The results will show that different features will show up better in few analysis direction, and some other features will
show up better in other analysis directions. Scanning though the different analysis directions, we can extract more
information from seismic data.
 =0
 =30
 =60
 =90
 =120
 =150
Attribute
Volumes
Seismic
cube
3-D Seismic Volume
AASPI Attribute Software
http://geology.ou.edu/aaspi/
Seismic attributes such as structural dip,
gradient, structure oriented filter, energy,
curvatures, …
Attribute Illumination of Basement Faults
Examples from Cuu Long Basin, Vietnam
Ha T. Mai, Kurt J. Marfurt, University of Oklahoma, Norman, Oklahoma, USA
CONCLUSIONS
A. DIRECTIONAL APPARENT DIP
CUU LONG BASIN - INTRODUCTION
1000m
The structure of Pre-Cenozoic basement of the Cuu Long Basin is very complex, and is
mainly composed of magmatic rocks. Under the influence of tectonic activity, the
basement was broken into a suite of fault systems.
1000m
Several modern attributes, including volumetric computation of structural dip and azimuth,
structural curvature, amplitude gradients, and amplitude curvature, are multi-component in nature
and are thus amenable to visualization from different user-controlled perspectives.
Precomputing every desired azimuthal view results in consumption of significant disk storage.
However, through the use of ‘fast-batch’ spreadsheet-like attribute calculators available in several
3D interpretation software packages, such manipulation can now be put under user control.
Eventually, we envision generating truly interactive azimuthal visualization software, thereby
enabling the interpret to extract as much information from the data as possible.
1000m
 =0
 =30
 =60
2
2
2
2
2
 =0
1
2
2
1
This faulting provided favorable conditions for hydrocarbons from a laterally deeper
Oligocene-Miocene formation to migrate and accumulate in the basement high. The
basement is un-layered granitic rocks, such that the seismic signal appears to be very
weak and noisy.
We apply our workflow to enhance the faults signatures will aid our seismic
interpretation, with the ultimate goal of estimating fracture location, density, and
orientation
We will focus on depth slice 2750m, which is right bellow top of basement, and
examine the directional at 0, 30, 60, 90, 120, 150 from North.
2
2
1000m
1000m
1000m
 =90
 =120
 =150
2
2
2
 =90
2
ACKNOWLEDGEMENTS
2
We would like to thank PetroVietnam and Cuu Long JOC for providing seismic data, and allowing
us to publish the results used in this report.
- The attribute seismic results are generated by a processing package developed by AASPI group
at University of Oklahoma.
- The rotation of the images was achieved through the use Stanford’s SEPlib mathematic utility
-The slice images are generated using of Schlumberger’s Petrel.
-The Rose diagrams were generated using Dr. R. J. Holocombe’s GEOrient software.
B. DIRECTIONAL AMPLITUDE GRADIENT
1000m
1000m
1000m
1000m
1000m
1000m
 =0
 =30
 =0
 =60
 =30
 =60
2
2
2
2
2
2
1
2
2
 =0
2
2
1
REFERENCES
1000m
Dip Magnitude with basement top
Seismic with basement top
1000m
1000m
1000m
1000m
1000m
 =90
 =120
 =90
 =150
 =150
 =120
2
2
2
2
2
 =90
C. DIRECTIONAL CURVATURE
Most negative curvature
Most positive curvature
1000m
1000m
1000m
1000m
1000m
 =0
 =30
 =0
kmax
1000m
 =60
 =30
 =60
2
kmin
seismic
Most negative curvature
Inline Dip
1000m
Inline amplitude gradient
1000m
1000m
1000m
1000m
1000m
2
 =90
 =90
2
2
2
kmin azimuth
Variance
Most positive curvature
Crossline Dip
Crossline amplitude gradient
 =120
 =120
 =150
 =150
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