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The AAPG/Datapages Combined Publications Database
Houston Geological Society Bulletin
Abstract
Abstract: Grayson Field, Jurassic Smackover Reservoir, Columbia County, AR:
A Case Study Using Leading Edge Reservoir Characterization
Seismic
Processing of 3D
Seismic
Data




By
1 Hill Geophysical Consulting, Shreveport, LA
2 Anderson Oil and Gas, Shreveport, LA
The discovery well at Grayson Field, Columbia County,
Arkansas, was drilled on a four-way dip closure defined by
three 2D seismic
lines. Investors were hoping to find a
maximum of 100 feet of pay. After the discovery of 158 feet of
pay at 8000 feet (measured depth) in the Jurassic Smackover
limestone in January 1993, the participants decided that a 3D
seismic
program was needed. The objective of the 3D
seismic
program was to define the structural and stratigraphic limits of
the new field.
Specific processing and interpretive tools will be illustrated with
many different seismic
displays. Evidence will be presented that
1) relative amplitude of the Smackover reflector does not
define the reservoir's stratigraphic parameters, 2) attributes of
the acoustic impedance data (inversion) show good statistical
correlation to key reservoir parameters, 3) AVO shows a
hydrocarbon indicator over the reservoir, and 4) reservoir
characterization data (petrophysical volumes generated with
Hampson-Russell Emerge software) generated with the 3D
seismic
data delineates the production.
The Grayson field Smackover reservoir was originally divided
into two zones. The upper 35 feet of the reservoir is dolomitic
lime with high porosity (20-25%) and moderate to low spotty
permeability of 10-100 md. The main pay zone is approximately
40 feet below the top of the Smackover. It has primary
oolitic porosity of 17-20% with permeabilities into the darcies.
The separation between the pay zones ranges from 1 to 10 feet.
Through the use of this fully integrated sub-surface well information
and a 3D seismic
data set, the thinner upper pay interval
can be discerned from the thicker main pay. Horizontal well programs
have been designed using the 3D
seismic
data to exploit
the best porosity and permeability in the upper pay interval.
Vertical
wells adequately drain the main, lower pay interval.
In July 1995, gas injection was begun in the field. In February
1998, a water flood program replaced the gas injection.
In-fill drilling was needed to optimize the production.
3D seismic
data was needed for this work, but the resolution of
the reservoir was not really clear enough on the original processing.
Reprocessing of the data set with "state-of-the-art"
parameters such as detailed editing of each shot record,
pre-stack time migration~ and post-stack inversion (acoustic
impedance), AVO, and
petrophysical cubes was
needed. The full integration
of the entire suite of
logs from every well in
the area into the poststack
processing of the
3D data yielded multiple
data volumes. The bandwidth
of the reprocessed
data is 15--90 hertz, with a
dominant frequency of 43
hertz. With the broad
Unnumbered Figure. Smackover structure map with location of Smackover fields in Columbia County, Arkansas. Map courtesy of Geological Consulting Services. @GCS 2002.
End_Page 13---------------
bandwidth and integration of the well log information, the new data volumes show the stratigraphic components of the reservoir very clearly.
The tuning thickness of the wavelet at the Smackover level is
9 milliseconds (approximately 54 feet). Identification bf the
upper and lower pay intervals is very difficult because the upper
zone is thinner than the tuning. The two pay zones appear to
merge into one thick, high-amplitude seismic
event. The sonic
and density logs from the wells were used to calibrate and generate
an acoustic impedance data volume. This multi-linear
regression technique yields a data volume that better defines the
two layers within the reservoir. The acoustic impedance (velocity)
of the upper 100 feet of the Smackover was evaluated with
different attributes to see if there was a correlation to the overall
reservoir. Cross-plots of certain
seismic
attributes exhibit a good
statistical fit with the reservoir's porosity and pore-volume maps
generated from well information. The Grayson field Smackover
reservoir is a low-velocity zone encased within high-velocity
rocks. This is a classic Type III AVO case.
Unnumbered Figure. North-south dip line from 3D seismic
data showing water saturation
data volume calculated with Archie's equation.
Unnumbered Figure. The average of the top 100 ft. of the Smackover indicates that there may be some low velocity areas on the west end of the structure. However, low velocity was only indicative of porosity and the permeability was virtually absent on the western end. Only Grayson field is seen on this map. The oil wells in the upper right corner are Barlow Branch field.
Unnumbered Figure. Comparison of "porosity" 3D seismic
data volume (top) and "water saturation" 3D
seismic
data volume (bottom).
The "water saturation" volume properly defines the stratigraphic limits of the reservoir.
End_Page 15---------------
A positive AVO P*G (primary times gradient) response is seen at the producing wells, whereas no AVO P*G response is observed in the non-producing areas.
The difficulty of identifying the thin upper pay interval was
overcome by making deep induction and gas effect "seismic
"
volumes that could be used in conjunction with the relative
amplitude, AVO, and acoustic impedance volumes. The
seismic
data quality was very good and a sufficient population of well
bores, with the target attributes, was available. Hampson-Russell
Emerge software was used for the reservoir characterization data
generation. This is an artificial neural network algorithm that
successfully computed the deep induction, density porosity, and
neutron porosity volumes.
Cross-plotting the density porosity and neutron porosity data
volumes generated a gas effect "seismic
" volume. The results of
this data volume were outstanding. The thick pay interval at
Grayson field as well as a thin stratigraphic pay at Barlow Branch
field stood out extremely well.
The hydrocarbon saturation and water saturation can be calculated
using Archie's equation with the "seismic
" deep induction
and density volumes. The data show the upper and main pay
intervals as well as a possible separate deeper zone. The deeper
zone has produced in two wells, and it was originally thought to
be connected to the main pay zone. However, the new processing
shows this zone to be separated from the main pay zone by at
least 15 feet throughout the field. Further evaluation of this
interval is ongoing.
The acoustic impedance data show low velocity (porosity) extending outside of the known producing limits of the field whereas the oil and water saturation volumes show the definitive limits of the pay interval. The oil and water saturation volumes also more dearly define the separation of the three different pay zones within the reservoir.
Conclusion
3D seismic
data played a significant role in the development of
Grayson field. The 3D
seismic
data allowed Petro-Chem
Operating Company to drill the best structural locations within
the field. Reprocessing the 3D
seismic
data brought out the
stratigraphic nuances of the field. The relative amplitude
strength of the top Smackover reflector does not define any
reservoir parameters. The acoustic impedance data volume
shows a good statistical correlation to the gross reservoir parameters
of the upper 100 feet of the Smackover. Multi-attribute
inversion using an artificial neural network algorithm (Emerge)
successfully computed the deep induction, density porosity, and
neutron porosity volumes. "
Seismic
" volumes of gas effect,
water saturation, and hydrocarbon saturation dearly delineate
the reservoir. New wells drilled using these 3D
seismic
volumes
greatly increased the production rate and ultimate recoverable
reserves in the field. Recent drilling proves that the 3D
seismic
effort and expense were well worth the money.
Unnumbered Figure. Relative amplitude of the top Smackover reflector with subsurface and
seismic
depth contour overlay. Arb line represents the
seismic
line
seen on page 15.
Unnumbered Figure. "Gas effect" calculated by taking the density porosity 3D seismic
volume minus the neutron porosity 3D
seismic
volume.
End_of_Record - Last_Page 17---------------