Seismic Artifacts – a case studyNiranjan Nanda*, Ram Singh** and Satinder Chopra****Petroleum Geophysicist (Consultant), Cuttack; **Reliance Industries, Mumbai; ***Arcis Corporation, Calgary,Alberta, Canada

Summary

Two structures G and H in an Indian offshore basin were testdrilled for Eocene prospects. The G-1 well turned out to bedry, but the H-1 well encountered a thick column of oil inEocene limestone reservoirs. Following the discovery, areview interpretation of seismic data for detailed reservoirmapping was undertaken, which on close examination, indi-cated specious seismic anomalies, that would have significantimpact on the economics of the field.

Earlier mapped ‘time lows’ to the north and west of the smallsized oil bearing structure were suspected to be artifacts dueto low velocities, associated with a ‘channel cut and fill’feature in the overlying thick Miocene carbonates. Properlycorrected for the velocities, the depth map indicated a largestructure, much disparate from the time map. A subsequentwell drilled based on the new map confirmed the seismicp i c t u re. Consequently, the re s e r v o i rgeometry and the estimated oil inplaceshowed a marked upward change infigures which upgraded the margin-ally economic prospect to a major highvalue field.

Introduction

‘Pitfalls in seismic interpre t a t i o n ’(Tucker 1973) and ‘Pitfalls revisited’(Tucker 1982) are two SEG publica-tions which have been very popularwith both geologists and geophysiciststhe world over. Tucker’s treatise onseismic artifacts, though fascinating,lacks, for whatever reasons, applica-tion in real case histories, withoutwhich, it becomes difficult to appre-ciate their value engineering aspect. The exploration industryhas since been highly commercialized and examples ofpitfalls discussed without relevance to their commerc i a lapplication and consequent benefits tend to become prosaic.

Of all the categories of pitfalls in seismic interpretation, themost elementary but important is the one related to veloci-ties. Seismic sections are still presented in travel time and forvalid depth sections, it is sine-qua-non to have a precise anddetailed knowledge of velocity. It is here, the geophysicistplays a very vital role in estimating an accurate velocity fieldfor transforming the seismic times to depth. Needless to saythat often a small discrepancy of the order of 10 m in depthestimation can make a tremendous pecuniary impact (of theorder of several hundred millions of rupees) on the commer-cial aspect of the prospect.

Not many cases of seismic artifacts have been reported in journals where their application had a direct and/or

immediate impact on field economics. We, however, presenthere a case study, where the identification and treatment of avelocity related seismic artifact have played a vital role in theturnaround of a successful development of a field.

The case study

Two offshore structures G and H, close to each other but sepa-rated by a small low as indicated by the seismic time map(Figure 1a) were drilled for Eocene hydrocarbon prospects.The first well G-1, though turned out to be dry, indicatedp resence of thick highly porous reservoir facies withinEocene carbonates. Pursuing the lead, the structure H wasdrilled, about 100 ms updip and encountered a thick columnof oil in the limestone reservoir.

The velocities at both the wells indicated the same value of2200 m/s and a depth map earlier made (not shown here) on

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Continued on Page 29This work was carried out when all three authors were employed by ONGC, India.

F i g u re 1. (a) Time map on pay top, (b) velocity field at pay top and (c) depthmap at pay top

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the single velocity function produced a map very similar to thetime map indicating limited oil reserves restricted to the smallarea of structural culmination only.

Subsequently, a review interpretation of the 2D seismic data fordetailed reservoir mapping was taken up for assessing thecommercial viability of the prospect for development. The studyexposed time anomalies related to severe lateral velocity varia-tions, which needed corrections for preparing an accurate depthmap at pay level.

On a typical seismic section O-83-41 (Figure 2a) across the ‘H’structure, a sag between 1.05 – 1.25s in the central part was inter-preted as channel feature. The geometry of the sag, the highamplitude flat reflection segments within it and the related edgediffractions led credence to the interpretation. However, the sagseen at the deeper pay level was doubted to be genuine as it wasseen right vertically below the channel and this necessitated

reprocessing of the seismic data forauthentication.

The re p rocessed seismic data( F i g u re 2b) clearly defined the‘channel cut and fill’ geometry atthe shallower level. The stackingvelocity analysis also indicatedsignificantly lower velocities asso-ciated with the channel fills.E rosional cuts in the youngerMiocene carbonate shelf-filled withlow velocity shale were known ageological entity in the nearby areaand validated the sag seen on theseismic section at the pay level asan artifact.

An isovelocity map at re s e r v o i rlevel (Figure 1b) pre p a red fro mre p rocessed 2D stack velocity dataafter calibration with the values atthe drilled wells, showed stro n gresemblance of trend with that ofthe time stru c t u re map. The stackingvelocity values at the sinks belowthe channel incision were furtherconfirmed quantitatively by compu-tation, taking into account the depthof the channel incision, the velocitiesof infill shale and the surro u n d i n gl i m e s t o n e .

The depth map (Figure 1c)p re p a red with the computedvelocity field indicated radicalchanges. The apparently innocuouslows to the west and south of thehigh seen on the time map weretransformed to features comprisingthe flank of a single large structurenow formed by coalescence of thetwo earlier orthogonal ‘time highs’indicated on the base map.

It was obvious that the new map indicated a large increase in thes t ructural area as well as in the thickness of oil column which culmi-nated in re-estimating the reserves many fold upward. An appraisalwell H-2 (Figure 2) later ventured in the ‘time low’ area, confirmedthe veracity of the seismic map and the increased reserves and wasdecisive in conversion of an economically lean prospect to a majorhigh value field.

Conclusions

Velocity anomalies play a very crucial role in defining the geom-etry and structure of a reservoir and consequently in the hydro-carbon estimates and its economics. Careful examination andanalysis of seismic data can expose apparently innocuous arti-facts which may have strong impact on the commercial value ofa prospect. The aberrations, however, can be obviated by theexperience and expertise of a geophysicist.

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F i g u re 2. (a) Segments of the earlier processed and (b) the re p rocessed seismic sections of profile O-83-41 passing thro u g h‘ H ’ s t r u c t u re .

In this case, a low velocity related seismic artifact at the target level caused by a channelin the overlying section, has been identified and its effect adequately corrected. This hasled to significant economic benefit in turning around a near marginal prospect to a highyielding economic field. R

Acknowledgements

The authors would like to thank the management, ONGC, for permission to publish thiswork.

ReferencesTucker, P.M., 1973, Pitfalls in seismic interpretation, SEG, Monograph 2.

Tucker, P.M., 1982, Pitfalls revisited, SEG, Monograph 3.

Niranjan C. Nandareceived a post-graduate degre ein Geophysicsf rom BanarasHindu Univer-sity, India in 1959and worked for 37

years with Oil and Natural gasCorporation Ltd (ONGC), India, in anumber of senior explorationmanagement positions. In addition tohis vast knowledge in hydrocarbonexploration and exploitation in Indianonland and offshore basins, he alsohas valuable experience in evaluationof a number of alien basins in foreigncountries, which he visited, as amember of ONGC exploration dataappraisal team.

In 1987, he received the NationalMineral Award of the Govt. of Indiafor his expertise in seismogeologicaldata evaluation especially in the fieldof Reservoir geophysics.

Niranjan has also been a very effectivefaculty member and has delivere dseveral lectures on various seismictopics like seismic stratigraphy, devel-opment geophysics, seismic interpre-tation techniques, 3D seismic, etc. togeoscientists in ONGC.

M o re re c e n t l y, as a Petro l e u mGeophysicist, he is a consultant forseveral petroleum E and P companiesin India. He also conducts trainingcourses and workshops on seismictopics for students and professionalsof geophysics and geology. He is avisiting faculty member to A n d h r aUniversity, Visakhapatnam and M.S.U n i v e r s i t y, Vadodra, India andteaches seismic interpretation tech-niques to postgraduate geology andgeophysics students. Niranjan was amember of SEG and currently holdsan honorary membership of SPG,Dehradun, India.

Satinder Chopra:Brief bio forSatinder Chopracan be found inthe 2006 SpecialIssue of the CSEGRECORDER onpage 121.

Dr. Ram J. Singh is Vice President (Exploration) in Reliance IndustriesLtd., India and is a leading geophysicist in the organization. He joinedReliance’s Exploration and Production Business group in the year 2000and has played major role in the world largest gas field discovery of theyear 2002 by Reliance in deep water KG Basin of India. Before joiningReliance, he worked for 18 years in ONGC as geophysicist and specializesin seismic interpretation and prospect identification. Dr. Singh has earned

his Master and Doctorate degrees in Physics from Banaras Hindu University, India in theyears 1978 and 1982 respectively and has published many papers both in physics and explo-ration geophysics. He is a member of SEG, SPG(India), AEG,APG.

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