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Explorer.
Stratton Field, South Texas: Example of Integration in Reservoir Characterization
By
Bob A. Hardage
Search and Discovery Article #40020, (2001).
Senior research scientist, Bureau of Economic Geology at the University of Texas at Austin ([email protected] ).
Adapted
for online presentation from two articles by same author, entitled
“Integration Aids Reservoir Effort” in Geophysical Corner, AAPG Explorer, December, 1997, and “Combining Data Aids
Interpretation” in Geophysical Corner, AAPG
Explorer, January, 1998. Appreciation is expressed to the author and to M.
Ray Thomasson, former Chairman of the AAPG Geophysical Integration Committee,
and Larry Nation, AAPG Communications Director, for their support of this online
version. For a somewhat expanded version of this B.E.G. study, the viewers are
referred to “3-D 
Seismic
Imaging and Interpretation of Fluvially Deposited
Thin-Bed Reservoirs,” by Bob A. Hardage, R.A. Levey, V. Pendleton, J. Simmons,
and R. Edson, in AAPG Studies in Geology No. 42, p. 27-33. This publication is
available from Bookstore at www.aapg.org.
This
article summaries a study of the Stratton Field, a large, Frio gas-producing
property in Kleberg and Nueces counties in South Texas. The stratigraphic
interval involved was the Oligocene Frio Formation – a thick, fluvially
deposited sand-shale sequence that has been a prolific gas producer in Stratton
Field and in several other fields along the FR-4 depositional trend (Figure
1).
This reservoir-characterization effort is an example of integrating geophysics,
geology, and reservoir engineering technologies to detect thin-bed compartmented
reservoirs in a fluvially deposited reservoir system. An
integrated interpretation of a complex reservoir system is made using 3-D
seismic, well log, and bottom-hole pressure data. Techniques
for thin-bed interpretations and recognition of reservoir heterogeneity are
essential elements in the interpretation.
Two
examples of seismic thin-bed interpretation in a fluvially deposited gas
reservoirs are shown, and these interpretations are supported with geologic and
reservoir engineering data. In these examples, the 3-D seismic data reveal
stratigraphic variations where reservoir pressure information implies that a
compartment boundary should exist. These examples illustrate that, although
fluvial deposition creates numerous compartment boundaries, determining which
seismically imaged stratigraphic changes are compartment boundaries requires
that geologic and reservoir-engineering data (particularly reservoir-pressure
data) be incorporated into the seismic interpretation.
In
this study it was particularly important to have an accurate and reliable way to
translate thin-bed stratigraphy (known in depth) into precisely defined seismic
time windows. VSP data, when properly recorded and processed, are the best
information to establish the detailed depth-versus-time calibration required to
seismically distinguish closely spaced thin-beds. The VSP calibration procedure
used was able seismically to distinguish thin-beds that are vertically separated
by as little as 4 ms.
Figure
1--Index map showing FR-4 depositional trend in which Stratton Field is located.
Figure
2--Map of Stratton field area, showing well locations and 3-D seismic grid.
Figure
3--Zero-offset image is spliced into a north-south vertical slice from the 3-D
data volume passing through the VSP well (Figure 2). Also shown is a graphic
representation of the stratigraphic column penetrated by the VSP well. The
F37 reservoir is about 20 feet above the F39 reservoir.
Figure
4--Continuous seismic reflection event imaged over the entire area by 3-D
seismic data, defining a geologic surface that corresponds to a fixed, constant
depositional time. Orange surface corresponds to C38 reservoir; green surface,
to F11 reservoir; and yellow surface, to F39 reservoir.
Figure
5--Reflection amplitude behavior on the F39 depositional surface shown in Figure
4.
Click here for sequence of Figures 5 and 9 (for comparison of F37 and F39).
Figure
6--Magnified view of this F39 surface in the vicinity of four key wells.
Figure
7--F39 reservoir pressure measurements that were acquired in all four wells. The
differences in these static pressures indicate that each well is in a different
F39 compartment.
Figure
8--Stratigraphic correlation section that displays the available geologic
control. The log curves infer that the F39 reservoir in each well was deposited
in a channel environment with some evidence of splay deposition.
Figure
9-- Reflection amplitude behavior on the F37 surface.
Click here for sequence of Figures 5 and 9 (for comparison of F37 and F39).
Figure
10--Enlargement of the meander features of F37.
Click
here for sequence of Figures 10 and 13 (seismic reflection event vs. model).
Figure
11--Log-based stratigraphic cross-section of the F37 reservoir.
Figure
12--Rapid F37 pressure decay in well 189.
Figure
13--Multi-discipline reservoir model, illustrating the proposed model for F37
reservoir.
Click
here for sequence of Figures 10 and 13 (seismic reflection event vs. model).
Thin-Bed Interpretation Procedure
Defining Chronostratigraphic Depositional Surfaces
The
study covered a 7.6-square-mile area (Figure 2) where 3-D seismic data were
acquired and where a large number of wells were used in making a geologic
analyses of the Frio reservoirs. Additional well data (with well locations
corresponding to the circled dots) were used to supplement the historic well
log, production and reservoir pressure databases; they consist of modern well
logs, cores, and various pressure tests.
Vertical
seismic profile (VSP) data were recorded in two closely spaced wells inside the
triangle shown near the center of the 3-D grid.
Thin-Bed Interpretation Procedure
The
seismic interpretation at Stratton Field was particularly challenging because
most of the Frio reservoirs are thin (<15 feet, or 5 meters), and they were
closely stacked--in some areas individually separated vertically by only 10-15
feet (3-5 meters). These conditions required precise calibration of
stratigraphic depth-versus-seismic travel-time to extract a depositional stratal
surface from the 3-D data volume that would reliably depict the areal
distribution of a particular Frio thin-bed reservoir. Zero-offset VSP data
recorded in one of the wells shown in Figure 2 were used to establish the
precise depth-versus-time control needed for the thin-bed interpretation. Figure
3 shows the zero-offset image spliced into a north-south vertical slice from the
3-D data volume passing through the VSP well. Also shown in Figure 3 is a
graphic representation of the stratigraphic column penetrated by the VSP well.
Only producing or potentially-producing Frio reservoirs are shown in this
diagram, and not all of the reservoirs are labeled by name. The top and base of
each reservoir are accurately positioned in terms of two-way VSP travel-time,
and because there is no difference in the VSP and 3-D time datum in this case,
the reservoirs are also correctly positioned vertically inside the 3-D seismic
data volume at the VSP well.
Using
these VSP travel-time control data, each thin-bed reservoir can be placed in the
correct reflection phase position in the 3-D seismic reflection wavefield at the
VSP well. This thin-bed calibration was extended away from the VSP well and
across the entire 7.6-square-mile area imaged by the 3-D data using the
interpretation principles of seismic stratigraphy.
Defining Chronostratigraphic Depositional Surfaces
The
fundamental assumption made in the seismic interpretation was that seismic
reflections follow chronostratigraphic depositional surfaces (Vail and Mitchum,
1977). Thus, a continuous seismic reflection even imaged over the entire
7.6-square-mile area by the 3-D seismic data defines a geologic surface that
corresponds to a fixed, constant depositional time. Two such areally continuous
reflection events were found in the Frio interval. These two surfaces are shown
on the east-west vertical section crossing the VSP well (Figure
4).
At
the VSP control well, the apex of the peak associated with the shallower stratal
surface (the orange surface in Figure 4) corresponds to the thick C38 reservoir
(Figure 3), and the apex of the peak at the deeper stratal surface (the green
surface in Figure 4) correlates with the F11 reservoir. Thus, the seismic time
surface following the apexes of all of the peaks of the orange event is assumed
to define the ancient topographic Frio surface at the time when the C38
reservoir sediments were deposited.
Similarly
the seismic time surface following the apexes of the peaks of the deeper green
event define the ancient depositional surface associated with the F11 reservoir.
Once the 3-D data volume was flattened relative to one of these two reference
stratal surfaces, it follows that any horizontal time slice in this flattened
data volume also followed an ancient Frio depositional surface – as long as
the seismic reflection character in the immediate neighborhood of the time slice
was time-conformable with the reflection character in the immediate vicinity of
the reference surface used to flatten the data volume.
This
interpretation is based on the assumption that the entire Frio section inside
the 7.6-square-mile grid was seismically conformable to one of the two seismic
reference surfaces. In this specific interpretation problem, with many closely
spaced (vertically) thin-beds, the VSP-defined position of a particular thin-bed
reservoir was rarely at the apex of a reflection peak or trough. Invariably,
each thin-bed of interest was positioned at some intermediate, commonly
non-descript phase point in the reflection waveform at the VSP control well.
To
create a seismic image that emphasizes the internal complex architecture of a
given thin-bed reservoir system, the migrated 3-D data volume was:
-
First time shifted so the proper pre-defined reference stratal surface was flat.
-
Then a horizontal time slice was made through this flattened data volume at the exact VSP-defined time for the targeted thin-bed, regardless of where that time slice is positioned in the reflection waveform at the VSP control well.
By
prior assumption, the seismic time surface contained in this horizontal slice
was the fixed depositional stratal surface where that thin-bed unit was
deposited, and any seismic anomalies seen on this surface would be related
directly to stratigraphic heterogeneities within the targeted thin-bed and, to a
lesser degree, would be related to stratigraphic variations in thin-beds
positioned immediately above and below the target thin-bed.
The F39 reservoir was the deepest Frio reservoir studied. (The depositional surface for the F39 reservoir is shown by the yellow horizon in Figure 4; the reflection amplitude behavior on the F39 depositional surface is shown in Figure 5.) The linear north-south trends near the center of the image are assumed to be residual effects from the deeper Vicksburg faults (a magnified view of this F39 surface in the vicinity of four key wells is shown in Figure 6).
F39
reservoir pressure measurements were acquired in all four wells (Figure
7), and
the differences in these static pressures indicate that each well is in a
different F39 compartment. The 3-D seismic image and the available geologic
control gave clues as to where the boundaries are that segregate the F39
reservoir into these distinct compartments.
Figure
8 displays the available geologic control. The log curves infer that the F39
reservoir in each well was deposited in a channel environment that shows some
evidence of splay deposition. The seismic image defines some possible
compartment boundaries. For example, the most likely cause of the compartment
boundary that separates well 197 from the other wells is the depositional
variation that created the red/blue (positive/negative) amplitude changes, which
trend north-south between crossline coordinates 130 and 140 (Figure
6).
Similarly,
a probable seismic indication of the compartment boundary that segregates well
75 from the other wells is the positive-to-negative (red-to-blue) amplitude
variations trending north-south between crossline coordinates 110 and 120. By
analyzing the seismic, geologic, and engineering data associated with the F39
reservoir, it is possible to detect F39 reservoir compartments seismically--at
least in the vicinity of wells 75, 175, and 197.
To
create this reservoir compartment model, it is essential that the seismic image
be interpreted with the assistance of reservoir-pressure data to infer which of
the many stratigraphic changes revealed in the seismic image are most likely to
be the compartment boundaries.
The F37 reservoir is approximately 20 feet (6 meters) above the F39 reservoir in the VSP calibration well (Figure 3). The two-way travel-time difference between F37 and F39 is only four milliseconds (4 ms). Using the thin-bed interpretation procedure above, a time slice was made through the flattened 3-D data volume 4ms above the F39 stratal surface. This F37 surface is displayed in Figure 9. Comparing this image with the F39 surface (Figure 5), red, linear north-south apparent channels in the central part of the F37 image are similar to those observed in the F39 image, implying that Vicksburg faulting was still controlling sedimentation in this part of the field.
However,
there is a significant difference in the southeast quadrant of the F39 and F37
images. Specifically, meander channel features occur at the F37 level but are
not present at the deeper F39 surface. (An enlarged plot of the meander features
in Figure 9 is shown on Figure
10.) A log-based stratigraphic cross-section of
the F37 reservoir across the meander features and extending southward beyond the
seismic grid was constructed (Figure 11). The depositional environment (either
channel or splay) at each well is an interpretation based on log-curve shape and
was made before the 3-D seismic data were recorded.
This
geologically-based interpretation of the F37 depositional environments indicates
that the meander feature seen in the F37 seismic surface is indeed a
depositional channel. Specifically, the log interpretation (Figure
11) implies
the F37 reservoirs found in wells 189 and 185 were deposited as channel fill,
and the seismic image shows these wells to be coincident with a meander feature.
The
interpretation of depositional environment for the extremely thin F37 reservoir
in well 211 is a splay (Figure 11). The 211 wellhead is approximately 300 feet
(91 meters) north of the meander feature (Figure
10). The log-based
interpretation of the F37 depositional environment at the 211 well is thus
supported by seismic evidence.
Pressure
histories recorded in several F37 reservoirs near these seismic meander features
were analyzed to determine if reservoir compartmentalization exists. These
pressure histories (Figure 12) show there are at least three, and perhaps
four, individual F37 reservoir compartments in this area of the field.
A
proposed reservoir model that honors all three databases--seismic, geologic, and
reservoir engineering--is shown in Figure 13. This model assumes that the F37
reservoir in the southeast quadrant of the 3-D grid is composed of three
intermeshed channels, labeled A, B and C, and a grid overlay of seismic inline
and crossline coordinates is provided so these channels can be correlated with
features in the 3-D seismic image.
The location of the F37 stratigraphic cross-section (Figure 11) is shown, but this geologic information defines channel locations along only a single 2-D profile of the model. The important information is the reservoir-pressure data, because without this engineering data there would be no reason to conclude that a three-channel model would be appropriate. Thus, the reservoir-compartment model places well 129 in Channel A and well 185 in Channel B; this allows these two wells to be in different F37 pressure regimes; i.e., in different compartments (channels).
Wells
127 and 161 are proposed to be in channel C, south of the 3-D seismic coverage.
Only one meander loop of this hypothesized channel C extends into the 3-D
seismic grid. The rapid F37 pressure decay observed in well 189 (Figure
12)
implies that this well is not in pressure communication with well 185, even
though both wells are in channel B. There may be an intrachannel compartment
boundary in channel B.
The
reservoir model in Figure 13 is hypothetical and may not yet be the correct
picture of the compartmentalized nature of these F37 reservoirs. However, the
F37 reservoir in this portion of Stratton Field is segregated into distinct
compartments, and this compartmentalization must be caused by the variable
elements of fluvial deposition, because the seismic data show no evidence of
faulting in these particular reservoirs.
The
proposed reservoir model honors all existing data that provide any information
about the F37 reservoir system. The seismic image in Figure 10 reveals not just
one meander channel system but at least three intermeshed thin-bed channels. By
using pressure histories it was possible to use 3-D seismic
images to define
where compartment boundaries most likely exist in the interwell space.
Raymond A. Levey, Virginia Pendleton, James Simmons and Rick Edson, all at the Bureau of Economic Geology at the time this work was done, assisted in the writing.