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

Thai Shell 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

Thai Shell 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.

Thai Shell 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.

Thai Shell

GPTO 3D seismic survey

Thai Shell 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.

Thai Shell 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.

Thai Shell PLIOCENE-RECENT LATE MIDDLE EARLY OLIGOCENE EOCENE PALEOCENE CRATACEOUS JURASSIC TRIASSIC PERMIAN CARBONIFEROUS M I O C E N E Himalayan Orogeny Asia / India Collision Indosinian IV Shan Thai / Indosinian Blocks Collision Phitsanulok Group Fluvio Lucustrine STRATIGRAPHY OROGENIC EVENTS ? Khorat Group Marine Sediments Not Differenciated IV TRANSPRESSION Basaltic Rhyolitic Volcanism III TRANSTENSION TRANSPRESSION II EXTENSION- I EXTENSION W E PING FM YOM FM PRATU TAO FM CHUM SAENG FM SARABOP FM LKU FM KHOM FM NONG BUA FM Y ~15.10 Y ~17.10 Y 6 6 ~ Y 6 6 Seismic Horizon Fig. 2

Thai Shell 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.

Thai Shell SW NE

Thai Shell

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 km 2. Shot spacing was 60 m in the cross-line direction and 360 m in the in-line direction.

Thai Shell 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

Thai Shell Seismic (con’t) Processing: GSI Processing Sequence –The survey was originally processed by Geophysical Services International (GSI Singapore) in –The original processing objective was to enable an accurate structural and stratigraphic interpretation of the GPTO area, especially the deep target area.

Thai Shell 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.

Thai Shell 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.

Thai Shell CMP Reflector Sn S3 S2 S1 R1 R2 R3 Rn Near offset Far offset

Thai Shell AVO Response of The Seismic Amplitudes in The GPTO Area (con’t) Near BlankingFar Blanking Full Blanking AVO Processing: Near and Far EPSI

Thai Shell 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

Thai Shell 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.

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

Thai Shell Normal Stack Near StackFar Stack RC +RC -

Thai Shell Amplitude extraction from near stack Amplitude extraction from far stack Amplitude extraction from far-near difference. RC +RC - RC + RC -

Thai Shell 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.

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

Thai Shell Normal Stack RC +RC - Near Stack Far Stack

Thai Shell Amplitude extraction from near stack RC + RC - Amplitude extraction from far stack RC + RC - Amplitude extraction from far-near difference RC + RC -

Thai Shell 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)

Thai Shell In-line 341 N E W Anomaly

Thai Shell Normal Stack RC +RC - Near Stack Far Stack

Thai Shell Amplitude extraction from near stack RC +RC - Amplitude extraction from far stack RC +RC - Amplitude extraction from far-near difference RC + RC -

Thai Shell 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.