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.