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Amplitude versus Offset (AVO) Processing and Interpretation in The Greater Pratu Tao Area of The S1 Concession. by Robert Gardham and Suhattaya Kaewla-iad. Contents. Introduction Objectives Geological Setting Seismic in The Greater Pratu Tao Area Acquisition Processing

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Amplitude versus Offset (AVO) Processing and Interpretation in The Greater Pratu Tao Area of The S1 Concession

by

Robert Gardham and Suhattaya Kaewla-iad


Contents
Contents in The Greater Pratu Tao Area of The S1 Concession

  • 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


Introduction
Introduction in The Greater Pratu Tao Area of The S1 Concession

  • 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.


Introduction con t
Introduction (con’t) in The Greater Pratu Tao Area of The S1 Concession

  • 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.


GPTO 3D seismic survey in The Greater Pratu Tao Area of The S1 Concession


Objectives
Objectives in The Greater Pratu Tao Area of The S1 Concession

  • 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.


Geological setting
Geological Setting in The Greater Pratu Tao Area of The S1 Concession

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.


W in The Greater Pratu Tao Area of The S1 Concession

E

5

IV TRANSPRESSION

PING FM

Basaltic

OROGENIC

Rhyolitic Volcanism

EVENTS

STRATIGRAPHY

6

10

137.10 Y

YOM

III TRANSTENSION

PLIOCENE-RECENT

FM

TRANSPRESSION

LATE

E

6

Phitsanulok

~15.10 Y

15

N

PRATU

Group

II EXTENSION-

E

MIDDLE

C

TAO FM

Himalayan

TRANSPRESSION

O

I

Orogeny

Fluvio Lucustrine

6

M

~17.10 Y

20

EARLY

I EXTENSION

?

CHUM SAENG

OLIGOCENE

Asia / India

Collision

FM

EOCENE

PALEOCENE

30

SARABOP

CRATACEOUS

Khorat Group

LKU

FM

Marine

JURASSIC

FM

Indosinian IV

Sediments

TRIASSIC

Shan Thai /

KHOM

Indosinian

Not

FM

40

PERMIAN

Differenciated

Blocks Collision

CARBONIFEROUS

NONG

BUA

FM

6

~30-23.10 Y

50

Seismic Horizon

Fig.

2


Geological setting con t
Geological Setting (con’t) in The Greater Pratu Tao Area of The S1 Concession

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.


NE in The Greater Pratu Tao Area of The S1 Concession

SW


Seismic
Seismic in The Greater Pratu Tao Area of The S1 Concession

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.


Seismic con t
Seismic (con’t) in The Greater Pratu Tao Area of The S1 Concession

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


Seismic con t1
Seismic (con’t) in The Greater Pratu Tao Area of The S1 Concession

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.


Seismic con t2
Seismic (con’t) in The Greater Pratu Tao Area of The S1 Concession

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.


Avo response of the seismic amplitude in the gpto area
AVO Response in The Greater Pratu Tao Area of The S1 Concessionof 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.


Far offset in The Greater Pratu Tao Area of The S1 Concession

Near offset

Sn S3 S2 S1 R1 R2 R3 Rn

Reflector

CMP


Avo response of the seismic amplitudes in the gpto area con t
AVO Response in The Greater Pratu Tao Area of The S1 Concessionof The Seismic Amplitudes in The GPTO Area (con’t)

AVO Processing:

  • Near and Far EPSI

Full Blanking

Near Blanking

Far Blanking


Avo response of the seismic amplitudes in the gpto area con t1
AVO Response in The Greater Pratu Tao Area of The S1 Concessionof The Seismic Amplitudes in The GPTO Area (con’t)

AVO Interpretation:

Lan Krabu Sands

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.

Yom/PTO Sands


Avo response of the seismic amplitudes in the gpto area con t2
AVO Response in The Greater Pratu Tao Area of The S1 Concessionof The Seismic Amplitudes in The GPTO Area (con’t)

AVO Interpretation:

  • AVO effect in Top K Formation surrounding Wat Taen A01 (WTN-A01) area.


E W

N

In-line 280

Oil in Top K Fm.


Near Stack W

Far Stack

Normal Stack

RC +

RC -


RC W-

RC +

RC +

RC -

Amplitude extraction from far stack

Amplitude extraction from near stack

Amplitude extraction from far-near difference.

RC +

RC -


Avo response of the seismic amplitudes in the gpto area con t3
AVO Response Wof 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.


E W

N

In-line 370

Oil in Lower Pratu Tao Fm.


Near Stack W

Far Stack

Normal Stack

RC +

RC -


RC W+

RC -

RC +

RC -

Amplitude extraction from far stack

Amplitude extraction from near stack

RC -

RC +

Amplitude extraction from far-near difference


Avo response of the seismic amplitudes in the gpto area con t4
AVO Response Wof 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)


E W

N

Anomaly

In-line 341


Far Stack W

Near Stack

Normal Stack

RC +

RC -


RC W+

RC -

RC +

RC -

Amplitude extraction from far stack

Amplitude extraction from near stack

RC +

RC -

Amplitude extraction from far-near difference


Conclusions
Conclusions W

  • 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.


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