imagery is one type of open-hole log that provides high- resolution
data for improved reservoir characterization.
images are used to:
- Characterize fracture
and fault systems.
- Interpret stratigraphic
- Quantify pay in thin-bed
- Interpret environments
- Resolve sandstone-body
geometry and paleocurrent orientation.
imaging tools have evolved from diplogs and dipmeters. Diplogs,
which identify bedding orientations, have been most commonly applied
to structural analyses.
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).
of high-resolution borehole imagery is sedimentologic analysis of
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
image also provides information about sedimentary strike and paleocurrent
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
to reduce drilling and completion risks, borehole imagery is being
used to augment facies identification in a sequence-stratigraphic
framework, and to delineate sandstone-body geometries and paleo-transport
study well (figure 1) was drilled along
the western limit of commercial Frontier gas production.
2 shows a gamma ray log and environments of deposition interpreted
using borehole imagery. This well produces gas from upper shoreface,
foreshore and fluvial sandstones, but the primary reservoir (highlighted
with a red bar) is a channel sandstone deposited on a lowstand event
of core and high-resolution borehole imagery were used to better
understand the reservoir geometry of this 15-foot thick reservoir.
structures were picked as dip vectors utilizing Vision™
- The regional structural
dip was determined.
- Bedding structures
were interactively classified using DipInt interpretation software.
- Structural dips were
rotated out for sedimentologic analyses.
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.
3 provides an overview of the sedimentologic analysis. Three
categories of color-coded dip vectors are shown. The vertical positions
of borehole image/core displays presented as figures 4 to 6 are
4 through 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.
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.
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.
5 shows an image and core of carbonaceous material, crossbedding
and a water-escape feature, all features characteristic of channel
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.
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.
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.
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.
imagery will be used in this portion of the field to further evaluate
reservoir geometry and extent.
borehole imagery can be used to identify sedimentary structures
and for defining paleo-current trends. Sedimentary structures visible
in Frontier core are clearly evident on new high-resolution borehole
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.