1 ONGC, Western Onshore Basin, Vadodara,

2Schlumberger Information Solutions,Vadodara

Email: jbratha@rediffmail.com

10th Biennial International Conference & Exposition

P 317

Stochastic modeling: A qualitative approach for generating 3D depositional

model, Jambusar Field, South Cambay Basin, India.

J.B. Rath1, R.E. Kadam1, A. Moharana2, M.S. Rawat1, M.C. Kandpal1

Summary

The Jambusar Field located in the North eastern part of the Gandhar Field is an established hydrocarbon bearing area in

South Cambay Basin where Middle Eocene Hazad sand is main reservoir.Oil dynamics in Jambusar field is controlled strati-

structurally and sand distribution pattern is guided by deltaic depositional process in lower delta plain environment.Though

sediment dispersion pattern is primarily controlled by river dominated process, role of tidal influence cannot be ruled out in

shaping the final stacking of sand bodies.

The key objective of the paper is to analyse the prospective area of hydrocarbon accumulation which is controlled by deltaic

depositional architecture. Facies variation both laterally as well as vertically by discrete nature of sand bodies and reservoir

heterogeneity play critical role in oil dynamics.To map the facies variation and delineate the reservoir boundary based on

petrophysical characters, seismic attribute study combined with facies and petrophysical modelling is very important aspect

of reservoir characterisation.

The spatial distribution of physical properties ofdepositional processes is difficult to predict deterministically.Scarcity related

to sampling data patternis another factor that complicates the prediction of the subsurface properties. Hence, prediction of

the spatial occurrence of rock properties via stochastic modeling process has been proved to be a better approach in designing

conceptual facies and petrophysical model by extrapolating trend and distribution pattern. Subsequently, correlated seismic

attributes aids in making the fine tuning of model by bridging the gap in sparse data and finally validation of model with

drilled well data improves the probability of success in unknown prospective area.

Present paper critically analyses all the G & G data available in the area and brings out 3D geological model incorporating

basin architecture to sediment dispersion of Hazad sediments.Petrographic study of core and cutting sample aids in

understanding the deltaic depositional process.Facies and petrophysicalmodeling have been carried out using stochastic

modeling approach of applied geostatistics in Petrel software.Variogrammodel has been designed using well data in the area

and correlated seismic attributes have been incorporated into the model. Reservoir heterogeneity brought out through facies

and petrophysicalmodeling has been corroborated with seismic impedancemodel.Finally, the model has been validated with

well data for generating sand dispersion pattern in delineating future prospects in and around Jambusar area.

Keywords: Stochastic, Deterministic, Variogram, Inversion

Introduction

Cambay Basin is a pericratonic rift basin along western

continental passive margin of India and Jambusar Field is

located in South Cambay Basin adjacent to major oil field

of Gandhar(Fig.1).The basin experienced different stages

of extensional tectonic episodes, essentially confined to

Cenozoic era and related stratigraphic evolution.The

stratigraphic setupprincipally facilitates the maturation of

Lower Eocene source faciesi.e. Cambay Shale and

distribution of various play style within prime reservoir

clasticfacies of Ankleshwar formation of middle Eocene

age. Hazad Member lies in bottom part of Ankleshwar

formation having development of better reservoir

faciesand is hydrocarbon producer in Jambusar Area.

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Fig:1 Location map showing Jambusarstudy area.

Rift Basin architecture and sedimentation.

As a part of Cambay basin, Jambusar area also experienced

different tectonic episodes of rifting throughsyn-rift,post

rift and structural inversion stages and bears structural

elements of Pre-Cambriantectonic trends.(Fig.2) The

Deccan Trap constitute the technical basement which is

unconformably overlain by non-marine Trap derived

materials of Olpad Formation during Synrift

phase.Trangressive Cambay Shale overlying Olpad

formation occupies most part of basin and lower Eocene

tectonic upliftment of the Cambay basin separates

Younger Cambay Shale from Older Cambay Shale by a

regional unconformity.Post rift phase of the Basin is

characterized by onset of distinct fluvio-deltaic system

depositing Middle Eocene clasticfacies which constitutes

the regional reservoir rock represented as Hazad member

in Narmada and Broach-Jambusarblock.It has been

established that the influx of sediment is largely from the

North, North-east provenance and oblique fault system of

the basin facilitated the deposition of progradational

deltaic sequence.The late phase of the Cambay basin

experienced structural inversion resulting in favourable

conditions for hydrocarbon entrapment.TheNeogene

sequence is characterized by post-Kand regional

unconformity and the depositional environment of the

sediment is principally continental-fluvial in the north and

lagoonal-intertidal in the South.

Fig:2 Pre-rift, Syn-rift and post rift sequences of Jambusar area

Structural style and depositional pattern of

Hazad Member, Jambusar Area

Time structure map on top of Hazad member shows gentle

monoclonal structure with few structural trends of highs

and lows in Jambusar area(Fig.3). Deeper faults seem to be

dying out in shallower level of post rift sequence. Hence,

Jambusar area at Hazad level is devoid of significant faults.

Hydrocarbon bearing wells are situated in minor high trend

axis aligned with the structural nose of Gandhar formed

during compressive episode.

Fig:3Time structure map on top of Hazad Top, Jambusar area.

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Time Structure Map on Top of Hazad,Jambusar

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Time Structure Map on Top of Hazad,Jambusar

3

Minor sands were shedfrom the basin margins, with larger

influxes occurring during deposition of late Cambay Shale

(Middle Eocene) as a result of progradation of deltaic

systems from the northeast. In the late Middle Eocene,

southwest tilting of the Indian continent, possibly as a

result of the onset of collision with Asia, resulted in a

major influx of sandrich deltaic sediments (Ankleshwar

Formation) which are the main hydrocarbon-bearing

reservoirs in the basin.

Reservoir Architecture and Hydrocarbon

Accumulation pattern

The hydrocarbons of the Jambusar Field arecontained in

the lower part of the Middle-UpperEocene Ankleshwar

Formation, which was deposited on a

fluvial/tidedominated delta and comprises a sequence of

fineto coarse-grained, argillaceous sandstones

andsiltstones, interbedded with grey to greenish-

greyshales that are locally carbonaceous. Hydrocarbons in

Jambusar field are mainly controlled by stratigraphic

entrapment where structure played minor role in re-

distribution of fluids with in reservoir.Facies variation both

laterally as well as vertically by discrete nature of sand

bodies and reservoir heterogeneity play critical role in oil

dynamics. The field contains 3 stacked reservoir sandstone

units (GS-6 to GS-9) within the Hazad Member, each

containing a separate hydrocarbon pool sealed by

intraformationalshales and ultimately sealed by the

overlying 8-12mthick Kanwa Shale member. The

reservoirs tend to shale-out towards the west parallel to the

strike of the palaeoslope, and towards the edge of the

subsiding basin in the north and NE.

Fig:4 Electrolog correlation along wells showing Hazad sand

distribution pattern.Jambusar area.

The electro-log correlations have brought out vertico-

lateral continuity/discontinuity of pay sands and

intervening shale distribution (Fig.4)

Sand geometry and analysis of Depositional

sequence

The main purpose of generating a depositional model is to

understand the oil dynamics which is controlled by sand

dispersion pattern and petrophysical characteristics. Facies

change has played a critical role in entrapment and

accumulation of Hydrocarbon in this area.

Depositional pattern of sediments depends on various

factors starting from sediment supply to depositional

setup.Change in any process is reflected in final pattern of

sediments deposition. Various controls like

accommodation space, rate of subsidence, upliftment,

eustacy, climate and lastly basin architecture come into

play in deciding depositional pattern.

Depositional pattern of sand can be inferred using all G&G

data like paleo-thickness distribution derived from well

data, attribute study,inversion of seismic data etc. which

makes the model more realistic. Isopach map of total

Hazad sand indicating paleo-depositional pattern and

orientation of feeder drainage system during deposition of

Hazad members (Fig.5).

Fig:5 Isopach map of Hazad sands, Jambusar area.

Fig.6 shows isopach map of GS-6 sand which is part of

Hazad member. This also indicates the same channel trend

indicating depositional pattern.Both the isopach maps have

been prepared on a regional scale showing regional

channel orientation from NE to SW direction.

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Fig:6 Isopach map of GS-6 sand, One of the reservoir with in

Hazad member, Jambusar area.

The analysis of logs of the drilled wells show

predominantly channel influences as the logs are

characterised by fining up sequence. The log data and

seismic data indicate that the pay sands with in Hazad

members are isolated sand bodies with limited lateral

extent and having complicated fluid distribution which

may be due to local structural influence and stratigraphic

trapping mechanism.Thus most rapid deposition of the

coarser material took place at the deltas where distributary

mouth bars are formed. The margin of the channel system

has fine grained levees having low porosity. The

distributary mouth bars are formed due to coalescing of the

drainage pattern and contain good quality of reservoir

sands.The progressively finer sediments get deposited in

seaward direction with the distributary mouth bar grading

into silts and fine sands which in turn progrades into

prodelta clays.The geological model thusconceptualised

on the basis of geoscientific data analysis is depicted in

Fig.7 showing distributary channel system and distributary

mouth bars in lower Deltaic depositional environment.

Based on Petrographic study of litho-association and

sedimentary structure with occasional presence of limonite

and glauconite suggest that Hazad GS-6 sand is deposited

as distributary channel/mouth bar deposit. Palynological

study also indicate tidal swampy environment of

deposition.

The drainage is mainly through proto-Mahiand Proto

Dadharrivers , which are flowing from North East and east

direction.Since, there was no appreciable changes in

structural disposition of Basin during post rift

sedimentation, hence the direction of present Mahi and

Dadhar rivers are assumed to be the same during paleo

deposition of Hazad sands.

Fig:7 Conceptual Deltaic depositional environment of Hazad

members, Jambusar area.

Distributary channel and mouth bar sands are excellent

producer of hydrocarbon.It has also been proved from the

reservoir and production performance data of Jambusar

wells.

Taking analogy from deltaic depositional pattern of

various modern delta of world, progradation as well as

lateral shifting/switching of distributary channel in lower

delta plain is a common phenomenon of delta building

process.In addition to that depositional environment shows

better continuity of facies parallel to depositional strike

and similar facies and depositional characteristics can be

expected in adjacent area.

Fig:8 Sand Isolith map of GS-6 sand showing sand dispersion

pattern, Jambusar-dabka-kural-Gajera area.

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Based on all the G&G study and Electrolog analysis,

regional sand isolith map of GS-6 sand was prepared which

has been validated through drilled well data, reservoir data

and pressure production data. The Sand isolith map shows

parallel channel orientation in NE-SW direction separated

by interdistributary shale.However, facies model indicates

same distributary channel orientation towards western part

of Jambusar area. This is the conceptual model prepared

by taking analogy from sand dispersal pattern in known

area.

Seismic Attribute study

Seismic inversion study was carried out to validate the

conceptual sand model. Various cross plots have been

generated to derive relationship between petrophysical

properties and seismic impedance. It was observed that

there was a better correlationbetween log derived

impedance with seismic impedance. Log derived

impedancealong with seismic impedance were plotted in

same track of log section which shows very good match

(Fig.9).

It has been observed that impedancemap might not be able

to differentiate strictly between sand and shale as plotted

in cross plot.(Fig.10).However, density has better

correlation with impedance and based on this correlation,

one may differentiate between reservoir and non-reservoir

facies.

Fig.8 Correlation between Log derived impedance (Blue) and

seismic impedance (pink)

Fig:10 Cross plot of Gamma ray/P-impedance and Density/P-

impedance of Hazad sand (Well-A)

Inversion attribute map of GS-6 sand(Fig.11) shows a

correlative trend of sand dispersion pattern with the sand

isolith map and hydrocarbon bearing wells that are falling

in area of moderate impedance value.All oil bearing wells

of GS-6 sand have been validated and are falling in area of

moderate value of impedance. However, gas sands show

low impedance. Non-reservoir/tight facies are indicated by

high impedance values.

Fig:11 Sand isolith Map of GS-6 superimposed on impedance

slice at GS-6 with a window of 8ms below,Jambusar Area.

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Facies and Petrophysical modelling

Facies models are built to understand geological processes

to capture facies architecture, such as reservoir

connectivity and high level heterogeneity by honoring

descriptive facies information such as shape, size,

orientation, proportion and distribution.

Well log upscaling has been used in this study to generate

facies and petrophysical models using deterministic as

well as stochastic methods as quite a good density of well

data and 3D seismic data is available for such purpose.

The spatial distribution of physical properties of

depositional processes is difficult to predict

deterministically. Scarcity related to sampling data pattern

is another factor that complicates the prediction of the

subsurface properties. Hence, prediction of the spatial

occurrence of rock properties via stochastic modeling

process has been proved to be a better approach in facies

as well as petrophysical modelling. Since, key objective of

the study is to decipher sand distribution pattern in un-

explored area, hence stochastic method has been applied.

Facies and Petrophysical modelling were carried out using

Petrel software through Geostatistical approach. Proper

Variogram designing is very critical in facies modelling

process as aVariogram is a quantitative description of the

variation in a property as a function of separation distance

between pointsand direction. It brings out spatial

relationship of data and pattern by which the facies

proportion changes laterally and vertically. All G & G data

has been used anda sample variogram was designed for the

modelling purpose.(Fig.12).

Fig. 12 Variogram model for facies and petrophysical modelling.

Based on variogram modeling, Net-sand map of GS-6 sand

has been prepared using stochastic approach and it shows

very good channel pattern validating the well data of study

area.(Fig.13)

Fig. 13 Net sand map of GS-6 sand from stochastic modelling

using variogram.

Effective porosity map of GS-6 sand from stochastic

modelling using variogram was prepared which also shows

very good correlation with facies map.(Fig.14).

Fig. 14 Effective porosity map of GS-6 sand from stochastic

modelling using variogram.

Seismic acoustic impedance has strong correlation with

density (Fig. 14) hence, it is assumed that porosity derived

from density log can be correlated with impedance.This

has been used as secondary property in petrophysical

modelling process to bridge the gap in sparse data

area.Subsequently, effective porosity map of GS-6 sand

has been prepared from stochastic modelling using

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secondary attribute of seismic impedance (Fig.16) clearly

validating the well data in known area.

Finally, model has been validated by making synthetic log

from facies and petrophysical model in well locations A &

B which were predictional for modelling study

(Fig.17).The correlation of prediction with that from actual

data at well locations A & B show very good match.

Fig:15 Cross plot of Density/P-impedance of Hazad sand derived

from well data of Jambusar Field.

Fig. 16 Effective porosity map of GS-6 sand from stochastic

modelling using variogram, secondary attribute of seismic

impedance.

Fig. 17 Validation of model from synthetic log derived from

facies and petrophysical modelling.

Conclusion

Hydrocarbon accumulation in Hazad sand of Jambusar

area is controlled by reservoir heterogeneity. Facies

variation both lateral and vertical due to discrete nature of

sand bodies play critical role in oil dynamics.

Integrated study analysing all the G & G data available in

the area has brought out depositional model for Hazad

sands showing reservoir connectivity and heterogeneity

.Both facies and porosity maps show channel orientation

and sand dispersion pattern which are validated at well

location.

Finally, validation of model through synthetic log data

derived from facies and petrophysical model indicate the

effectiveness of the modelling process.

Acknowledgement

The authors sincerely acknowledge the ONGC authority

for permitting to present the data and the findings of the

study. The views expressed in the paper are solely of the

authors and not necessarily of the organization in which

they are working.

Reference

Caers, J., 2001, Geostatistical reservoir modelling using

statistical pattern recognition: Journal of Petroleum

Science and Engineering,v. 29, p. 177–188.

Madan Mohan 1995,Cambay Basin –A promise of oil &

Gas potential ,Journal of the paleontological society of

India,Vol.40,pp.41-42

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