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GCBorehole Imagery Resolves Channel Trend*
By
Connie Dodge Knight1
Search and Discovery Article # 40095 (2003)
*Adapted for online presentation from the Geophysical Corner column in AAPG Explorer, April, 2002, entitled “Borehole Images Can Identify Trends,” and prepared by the author. Appreciation is expressed to the author and to R. Randy Ray, Chairman of the AAPG Geophysical Integration Committee, and to Larry Nation, AAPG Communications Director, for their support of this online version.
1Consulting geologist in Golden, Colorado ([email protected])
Introduction
Borehole imagery
is one type of open-hole log
that provides high-resolution data for improved
reservoir characterization. Borehole images are used to:
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Characterize fracture and fault systems.
-
Interpret stratigraphic discontinuities.
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Quantify pay in thin-bed packages.
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Interpret environments of deposition.
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Resolve sandstone-body geometry and paleocurrent orientation.
Borehole imaging
tools have evolved from diplogs and dipmeters. Diplogs, which identify bedding
orientations, have been most commonly applied to structural analyses. Over the
past several years, borehole-imaging resolution, borehole coverage and
interpretive capabilities have improved significantly. Instruments such as the
Simultaneous Acoustic Resistivity Imager (STAR ImagerSM) provide a
vertical resolution on the order of 0.4 inches (1 cm). One application of
high-resolution borehole imagery is sedimentologic analysis
of reservoir
sandstone.
We present borehole images and core from the Frontier Formation on the Moxa Arch of southwest Wyoming. The core provides "ground truth" for sedimentary structure identification and gives us confidence in the technique of using borehole imagery to identify sedimentary structures. The borehole image also provides information about sedimentary strike and paleocurrent direction
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The
Frontier Formation contains both marine and non-marine gas reservoirs
that make up multiple stratigraphic sequences. Frontier gas reserves on
the Moxa Arch of southwestern Wyoming (Figure
1) exceed one trillion cubic feet of gas -- but due to reservoir
heterogeneity, Frontier fields contain scattered dry holes and marginal
producers. In order to reduce drilling and completion risks, borehole
imagery is being used to augment facies identification in a
Our case
study
Methodology and SedimentologyAll bedding structures were picked as dip vectors utilizing VisionTM software:
The
azimuths and dip angles of each vector population were then presented as
dip-vector plots and rose-frequency diagrams so that the paleo-flow
direction could be evaluated.
Figure 3 provides an overview of the sedimentologic Figures 4, 5, and 6 present core and interpreted "dynamic" borehole images of the channel reservoir. Borehole microresistivity data can be displayed as either static or dynamic images. Dynamic images have variable contrast applied to the data in a small moving window in order to show subtle details more clearly. Dynamic images are presented here because they were found to be more useful for identifying bedding features characterized by a limited resistivity contrast. On borehole images, planar bedding features that intersect the borehole are manifest as sine waves. Sine-wave amplitude is a function of bedding dip, such that steeper bedding dip angles correspond to steeper sine waves. Figure 4 shows a channel lag deposit overlying a basal channel scour. Referring back to Figure 3, we see that the basal scour dips at an angle of approximately 25 degrees. The pebbly nature of the channel lag is discernable from both the core and the borehole image. Figure 5 shows an image and core of carbonaceous material, crossbedding and a water-escape feature, all features characteristic of channel facies. In Figure 6, the core photo shows non-planar crossbedding, a channel lag deposit and upper mottled shale. The corresponding borehole image shows steeply dipping crossbeds, a channel scour, an overlying channel lag deposit and mottled shale. The steeply dipping crossbedding is also highlighted on Figure 3. Collectively all of these sedimentary structures are used to interpret paleocurrent flow direction -- but crossbedding dips are the best paleocurrent direction indicators. The rose-frequency histogram in Figure 3 shows that crossbeds dip to the south-southwest. Disturbed beds and channel scours dip generally perpendicular to the paleocurrent flow direction. The over-steepened crossbeds referenced earlier, which dip perpendicular to the crossbed dip direction, are interpreted as slumped material from cut-bank failure. In general, a southern paleo-flow direction is interpreted at this location.
Based on
outcrop studies and other oriented
SummaryHigh-resolution borehole imagery can be used to identify sedimentary structures and for defining paleocurrent trends. Sedimentary structures visible in Frontier core are clearly evident on new high-resolution borehole images.
In our
case study |