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GEOHORIZONS Vol . II

October 1997

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Integration of Well and 3D Seismic data for Prospect Evaluation

A case studyR J Singh* H L Kharoo*, P. Saha* S.N.Singh* & M. Chandra*

Introduction

Following the discovery of oil during

1958 in Cambay Basin, India the exploration

activities were intensified with major thrust to

tap structural prospects. Later, in the early 80s

the potential of subtle traps was realized in search

of finding more hydrocarbons in this basin. As a

consequence, a number of stratigraphic prospects

were discovered . The prospect discussed here is

one of them. Multiple pay reservoirs were found

in Kalol Formation ,but at the same time the geo-

logical complexities of facies development and

hydrocarbon entrapment mode ftrere also under-

stood. 3D survey was carried out to solve such a

problem of the X prospect in Cambay Basin over

which a number of wells were drilled but delin-

eation/development schemes to tap hydrocarbons

of the area are yet to be Finalised. The authors

evaluated 3D seismic data and found that the in-

tegration of geological data with 3D seismic may

lead to solve some of the problems of identify-ing facies and structural details.

Geology

The study area approximately seventy sq.

kms. is a prospect in north Cambay Basin. The

tectonics of Cambay Basin is very well docu-

mented dividing the basin into Mehsana,

Ahmedabad, Tarapur, Broach &Narmada blocks

separated by discrete cross trends (Fig. 1). Theclastic reservoirs within Middle Eocene account

for the major accumulation of hydrocarbons. The

entrapment established is structural, stratigraphic,

and of a combination type. The Eocene sequence

is characterised by three significant rapid trans-

gressive cycles during early Middle Eocene,

Middle Eocene, and Late Eocene with interven-

ing regressive cycles During regression, the ba-

sin fills are characterised by deltaic,backshore-

marshy lagoonal, and fluvial deposits.Lithostratigraphy of the area is shown in Fig.2.

Fig 1: Tectonic Map of Cambay Basin

Data Analysis

Methodology

Fortyeight fold 3D seismic data(bin size 15X75mts.) was acquired over the prospect in 1995 and

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unitwise brief discussion is given below.

K-X unit

K-X silt/sand reservoirs are developed with lim-

ited areal extent and smaller thickness below K-

X coal unit. A NW-SE channel is envisaged (Fig.

13b) based on the effective sand isolith and am-

plitude attribute map derived from seismic data.

K-IX unit

K-IX reservoir with sands or silty facies under-

lying the thick continuous coal unit is better de-

veloped in the area. These reservoirs are also oflenticular nature having limited areal extent and

preferred orientation. Seismic attributes were

analysed to identify the areas of facies develop

ment. For identification of facies both amplitude

and frequency attributes (Fig. 10 & II) were used

and the alignments brought out from these at-tributes were integrated with well data,especially

the effective sand isolith (Fig.l3a). A NW-SE

channel configuration is envisaged. The three

inlets of channels flowing from NW-SE are coa-

lescing around the well X-17 where the thick-

ness of sand is found to be more.

K-VI+VII unit

K-VI+VII reservoir facies is locally developed.

A channel(Fig.l4) is interpreted clubbing botheffective sand isolith map and seismic

data(Fig.l2)

Fig 4: Seismic calibration with Synthetic Seismograph (X-12)

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Fig 10. Sum of Negative amplitude within the time

window where K-IX reservoir is likely to be

present(Thich Horizon seiscrop)

Fig 11. Frequency attribute within a time window where

K-IX reservoir islikely to be present(Thick horizon

seiscrop)

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Fig 12: Sum of Negative amplitudes within atime window where K-VI+VII reservoir is

likely to be present(Thick horizon seiscorp)

Conclusions

Time and depth structure maps bring out

the structural details corresponding to the hydro-

carbon bearing stratigraphic units. K-X and K-

IX top units are represented by coals. K-IX coal

is thick and widely present whereas K-X coal is

Fig 14: Envisaged Geological model for K-VI

+ VII reservoir

Fig 13: Envisaged Geological model for (a) K-IX and (b) K-X reservoir

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relatively thin and pinches out. The pinch out boundary is identified from seismic data. Based on

seismic attributes and well data, different channels are mapped which are the likely areas for delin-

eation/development drilling in the area.

Acknowledgement

The authors are thankful to Mr.A.G Pramanik; Head, GEOPIC and Mr. S.N.Badola, HeadINTEG for their constant guidance and encouragement during the course of work and preparation

of this paper. The authors are also thankful to ONGG for giving permission to publish this paper.

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