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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:
-
Characterize fracture and fault systems.
-
Interpret stratigraphic discontinuities.
-
Quantify pay in thin-bed packages.
-
Interpret environments of deposition.
-
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,
Our case
study well (Figure
1) was drilled along the western limit of commercial Frontier gas
production.
Figure 2 shows a gamma ray log and environments of deposition
interpreted using
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 analysis. Three
categories of color-coded dip vectors are shown. The vertical positions
of
Figures
4, 5, and
6 present core and interpreted "dynamic"
Dynamic
images are presented here because they were found to be more useful for
identifying bedding features characterized by a limited resistivity
contrast. On
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
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 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 well data, the overall regional
Frontier paleo-flow direction has been interpreted to be from west to
east. A southward flowing Frontier channel system at this location is
significant, because it may represent an isolated depositional system
that is yet not drained by existing development wells.
Summary
High-resolution In our case study well, a southerly flow direction is interpreted that predicts an unexpected trend of local Frontier reservoir facies. Interpretation results will help identify future infill drilling locations to improve reservoir drainage. |