Expanded Abstract Example
Expanded Abstract Example
Expanded Abstract Example
John Eastwood* and David Johnston, Exxon Production Research Company; Xuri Huang, Ken Craft, and Ricky
Workman, Western Geophysical
The Exxon 1983 data shows amplitude artifacts associated ume was re-gridded to be coincident with the Exxon 1993 3D
with acquisition along the east west direction whereas the volume.
Western 1995 survey shows a very pronounced acquisition
artifact in the north east - south west direction. As well a Stage 1: Post Stack - Post Migration
polygon is outlined on both maps indicating a low fold low 1. Global cross equalization
amplitude artifact associated with the offshore platform for the 2. Global cross equalization + long window trace amplitude
1995 Western survey. equalization
3. Long window trace amplitude equalization + Global
Field Description Weiner-Levinson filtering
4. Long window trace amplitude equalization + Global
Geologic Setting Weiner-Levinson filtering + local event alignment
The Lena Field (Mississippi Canyon Block 281) is located 5 5. Spatially variant Weiner-Levinson filtering
miles south of the modern Mississippi delta in 1,000 feet of 6. Reverse migration - global Weiner-Levinson filtering -
water. The field is situated on the western flank of a salt dome migration
within a fault-bounded intraslope basin. Hydrocarbon pro-
duction is from six Pliocene-age sands, the deepest of which The long window trace equalization between surveys is a nec-
(B80) is the subject of this study. essary step to provide amplitude consistency between the two
surveys since marked differences were observed during the
The B80 reservoir is located about 10,000 feet below SL at initial quality control analysis of the data sets. The single
about 3 sec seismic TWT. The interval is interpreted as a low- match filter for steps 3 and 4 was designed over a sub-volume
stand fan systems tract representing deposition in distributary of the survey composed of 5,000 traces downdip and above
lobes composed of amalgamated and channelized turbidities. the B80 reservoir. The dip of the reflectors in this portion of
The updip limit of the sands lies about 2,000 feet west of the the section is only a few degrees. Prior to the match filter
salt flank and the reservoir thickens basin-ward to the west. calculation, the events in the analysis window were aligned
The average porosity of the B80 is 27% and the permeability between surveys to remove time shifts. A match filter was
ranges from 30-200 md. The gross average reservoir thick- designed for each trace pair between surveys and then aver-
ness is 100 feet with a net-to-gross of 47%. aged to create a single filter operator. The characteristics of
this single filter, convolved with the 1995 data set, are shown
B80 Production History in Figure 2.
Oil production in the B80 began in 1987. The reservoir has
been depleted by a combination water drive and gas cap ex-
pansion supplemented by gas injection. The reservoir pressure
was initially 5,000 psi and is now about 3,500 psi, near the
bubble point. Numerous down structure wells have watered
out and also have high GOR production from over running in
the low dip beds of this reservoir.
Seismic Data
A pre-production 3D seismic survey over the Lena Field was
acquired by Exxon in 1983. It was shot with a single source
and a single 3000 m streamer. The field bin size is 12.5x50 m
with 60 fold. Western Geophysical acquired a regional 3D
spec survey covering the Lena Field in 1995, after 8 years of
production. The survey was shot with dual sources and dual
4,000 m streamers. The field bin size is 12.5x40 m with 52
fold. The 1983 survey was shot in an east-west direction and
the 1995 survey was acquired in a N58oE direction. The ma-
jority of the reservoir is at tuning frequency with an average Figure 2: Weiner-Levinson match filter derived from
isochron of 25 ms. sub volumes of the two 3D surveys.
Processing Scenarios Stage 2: Remigration and Pre-stack analysis
1. Migrate both data sets using the same velocity, aperture,
A stepwise approach was taken regarding the processing of the and algorithm
two data volumes. The processing strategy was separated into 2. Process both surveys with consistent statics, wavelet
two distinct stages relating to the degree of sophistication of processing, amplitude handling and velocities. Design,
the processes. The first stage represents a less expensive rapid quantify, and apply cross equalization filters. Time mi-
analysis methodology, while the second stage represents a grate with identical algorithm, aperture, and velocity field
more rigorous, expensive, and time-consuming methodology.
Prior to analysis in the first stage, the Western 1995 3D vol-
Processing for robust time-lapse seismic analysis
For each processing scenario the differences are calculated for The authors would like to thank the Exxon USA New Orleans
both the reservoir zone (relative to the seismic reservoir hori- Production office for their cooperation in providing the data
zon) and for the seismic volume in the vicinity of the reser- and support, and the management of both Exxon Production
voir. It was found that the largest incremental improvement in Research Company and Western Geophysical for permission
stage 1 was achieved with the application of global Weiner- to publish this work.
Levinson filtering, long window trace equalization and local
event alignment.
For the B80 reservoir (which has an average dip of 20o) the
range of time lags to maximize correlation was 10 - 50 ms,
with the magnitude of correction increasing as the reservoir
dip increased. This implies that the spatial positioning of
these dipping reflectors in the independently migrated data
volumes are also not aligned by as much as several seismic
bins in the dip direction. This problem is best addressed by re-
migrating both data sets with the same migration velocity field
and algorithm in the second stage of our study.
Processing for robust time-lapse seismic analysis
B80 Reservoir
Figure 3: Representative cross section for seismic surveys after match filtering. Large differences are observable for the dipping
reflectors.
B80 Reservoir
Figure 4: 4D seismic attributes based on local event alignment. A significant residual amplitude anomaly associated with the
B80 reservoir is apparent.