Amplitude versus Offset (AVO) Processing and Interpretation in The
Greater Pratu Tao Area of The S1 Concession
byRobert Gardham and Suhattaya Kaewla-
iad
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Contents
• Introduction• Objectives• Geological Setting• Seismic in The Greater Pratu Tao Area
– Acquisition– Processing
• AVO Response of The Seismic Amplitudes in The Greater Pratu Tao Area
• Conclusions
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Introduction
• The Greater Pratu Tao (GPTO) area is part of the S1 Concession within the Sukhothai and Phitsanulok provinces.
• Six exploration wells were drilled in this area with somewhat disappointing results, which are mainly due to off-structure drilling (interpretation based on 2D seismic data).
• 3D seismic survey was acquired in 1983/84 to improve the structural resolution.
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Introduction (con’t)
• In 1999/2000, re-processing review was applied in GPTO area in order to improve subsurface image and to obtain more structural information.
• The AVO studies in Phase I presented the preliminary evaluation and qualitative results.
• The Studies in Phase II would be quantitative interpretation combined petrophysical evaluation.
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GPTO 3D seismic survey
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Objectives
• To perform preliminary evaluation AVO processing and qualitative interpretation as a direct hydrocarbon indicator.
• To indicate constraints in AVO interpretation.
• To provide potential area for drilling exploration well, Wat Taen BA.
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Geological Setting
Depositional Environment and Stratigraphy:
• Pre-Tertiary– Most of Pre-Tertiary rocks are meta-
sediments, metamorphics, volcanics and igneous basement litologies.
• Tertiary– The predominant depositional
environments were open lacustrine, fluvial plain and basin margin alluvial fans.
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PLIOCENE-RECENT
LATE
MIDDLE
EARLY
OLIGOCENE
EOCENE
PALEOCENE
CRATACEOUS
JURASSIC
TRIASSIC
PERMIAN
CARBONIFEROUS
MIO
CE
NE
HimalayanOrogeny
Asia / IndiaCollision
Indosinian IV
Shan Thai /IndosinianBlocks Collision
PhitsanulokGroup
Fluvio Lucustrine
STRATIGRAPHYOROGENIC
EVENTS
?
Khorat Group
MarineSediments
NotDifferenciated
10
20
30
40
50
15
5 IV TRANSPRESSION
BasalticRhyolitic Volcanism
III TRANSTENSION
TRANSPRESSION
TRANSPRESSION
II EXTENSION-
I EXTENSION
W E
PING FM
YOM
FM
PRATUTAO FM
CHUM SAENG
FM
SARABOP
FM LKU
FM
KHOMFM
NONGBUA
FM
137.10 Y
~15.10 Y
~17.10 Y
6
6
~30-23.10 Y6
6
Seismic Horizon
Fig.2
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Geological Setting (con’t)
Structural Framework:• The riedel fault patterns are dominated
by N-S and NW-SE trending faults.• Faults control the occurrence of
hydrocarbon accumulations via the formation traps (juxtaposition and clear smear) and their influence on basinal sedimentation.
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SW
NE
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Seismic
Acquisition:• The 1983/84 GPTO 3D seismic survey
was acquired by Companie Generale Geophysique (CGG) using swath/split spread technique.
• A total of twenty swaths covered a rectangular area of 447.53 km2.
• Shot spacing was 60 m in the cross-line direction and 360 m in the in-line direction.
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Seismic (con’t)
Acquisition:• Spread configuration:-
– Minimum offset 42.4 m– Maximum offset 2358 m– Normal coverage 12 fold (3 fold in X; 4 fold
in Y)– CDP sampling 30 m x 30 m
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Seismic (con’t)
Processing:• GSI Processing Sequence
– The survey was originally processed by Geophysical Services International (GSI Singapore) in 1985.
– The original processing objective was to enable an accurate structural and stratigraphic interpretation of the GPTO area, especially the deep target area.
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Seismic (con’t)
Processing:• Processing review by Shell Geoscience
Services– The re-processing of seismic survey in
1999/2000, using powerful high resolution processing SIPMAP software and EPSI technology, were to improve the structural image and to enable AVO studies to lower the exploration risk.
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AVO Response of The Seismic Amplitude in The
GPTO Area
Introduction to AVO:• Amplitude versus Offset is performed
on seismic data to determine the reflection amplitude behaviour with changing offsets between shot and receiver.
• This can indicate the difference between a hydrocarbon- and water-filled reservoir.
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Reflector
Sn S3 S2 S1 R1 R2 R3 Rn
Near offset
Far offset
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AVO Response of The Seismic Amplitudes in The
GPTO Area (con’t)
Near Blanking Far Blanking Full Blanking
AVO Processing:• Near and Far EPSI
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AVO Response of The Seismic Amplitudes in The
GPTO Area (con’t)AVO Interpretation:
Plane-wave reflection coefficients at the top of each Rutherford and Williams (1989) classification of gas sand.
Class IV sands, not discussed by Rutherford and Williams, have a negative normal-incidence reflection coefficient, but decrease in amplitude magnitude with offset.
Lan Krabu Sands
Yom/PTO Sands
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AVO Response of The Seismic Amplitudes in The
GPTO Area (con’t)AVO Interpretation:• AVO effect in Top K Formation surrounding Wat Taen A01 (WTN-A01) area.
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N
In-line 280
E W
Oil in Top K Fm.
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Normal Stack
Near Stack
Far Stack
RC +
RC -
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Amplitude extraction from near stack
Amplitude extraction from far stack
Amplitude extraction from far-near difference.
RC +
RC -
RC +
RC +
RC -
RC -
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AVO Response of The Seismic Amplitudes in The
GPTO Area (con’t)AVO Interpretation:• Limitation of AVO studies due to poor offset sampling in Lower Pratu Tao Formation surrounding Nong Tum A01 (NTM-A01) area.• Prospect is deeper than maximum offset.
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In-line 370
N
E W
Oil in Lower Pratu Tao Fm.
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Normal Stack
RC +
RC -
Near Stack
Far Stack
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Amplitude extraction from near stack
RC +
RC -
Amplitude extraction from far stack
RC +
RC -
Amplitude extraction from far-near difference
RC +
RC -
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AVO Response of The Seismic Amplitudes in The
GPTO Area (con’t)AVO Interpretation:• The first well to potential prove this concept in fluviatile sediments of the Pratu Tao Formation
will be drilled in 2000, Wat Tean BA (WTN-BA)
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In-line 341
N
E W
Anomaly
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Normal Stack
RC +
RC -
Near Stack
Far Stack
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Amplitude extraction from near stack
RC +
RC -
Amplitude extraction from far stack
RC +
RC -
Amplitude extraction from far-near difference
RC +
RC -
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Conclusions
• AVO studies allow the prediction of hydrocarbon fill in the prospect of GPTO area.
• The concept will be confirmed by further drilling programme.
• Success in deeper prospects may be constrained by limited offset in land seismic data.
• Further studies in petrophysical evaluation would be continued in Phase II.