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by Robert Gardham and Suhattaya Kaewla-iad

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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by Robert Gardham and Suhattaya Kaewla-iad

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  1. Amplitude versus Offset (AVO) Processing and Interpretation in The Greater Pratu Tao Area of The S1 Concession by Robert Gardham and Suhattaya Kaewla-iad

  2. 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

  3. 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.

  4. 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.

  5. GPTO 3D seismic survey

  6. 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.

  7. 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.

  8. W 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

  9. 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.

  10. NE SW

  11. 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.

  12. 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

  13. 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.

  14. 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.

  15. 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.

  16. Far offset Near offset Sn S3 S2 S1 R1 R2 R3 Rn Reflector CMP

  17. AVO Response of The Seismic Amplitudes in The GPTO Area (con’t) AVO Processing: • Near and Far EPSI Full Blanking Near Blanking Far Blanking

  18. AVO Response of 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

  19. 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.

  20. E W N In-line 280 Oil in Top K Fm.

  21. Near Stack Far Stack Normal Stack RC + RC -

  22. RC - RC + RC + RC - Amplitude extraction from far stack Amplitude extraction from near stack Amplitude extraction from far-near difference. RC + RC -

  23. 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.

  24. E W N In-line 370 Oil in Lower Pratu Tao Fm.

  25. Near Stack Far Stack Normal Stack RC + RC -

  26. RC + RC - RC + RC - Amplitude extraction from far stack Amplitude extraction from near stack RC - RC + Amplitude extraction from far-near difference

  27. 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)

  28. E W N Anomaly In-line 341

  29. Far Stack Near Stack Normal Stack RC + RC -

  30. RC + RC - RC + RC - Amplitude extraction from far stack Amplitude extraction from near stack RC + RC - Amplitude extraction from far-near difference

  31. 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.

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