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‘Evaluating Hydrocarbon Potential of Deccan Trap (Basaltic Reservoirs) in Padra Field of Cambay Basin for Its Effective Development Through Logging, Geological and Geophysical Techniques’

Dr. T. R.Varun1, B. K. Sainath2, Dr. N. B. Ishwar3

Abstract: The exploration activities in Deccan Trap (Basalt

reservoirs) in Padra field of Cambay Basin though started in

1960‟s, yet, the development stage could not be reached due

to uncertainties involved in discriminating reservoir facies

and its hydrocarbon potential, which required to be addressed.

Present attempt was to eliminate/ minimize such uncertainties

by having a relook on related aspects like; Development of

Porosity by Weathering / Leaching / Alteration by

Hydrothermal Waters, Log data of drilled wells (48 nos.)

drilled prior to the study in the light of Core data / Cutting

data / Testing details / Production history, Relevant Seismic

Data interpretation, Fault Pattern and Migration Pathways in

different Lava flows.

Deccan Trap consists of a number of basaltic lava flows. The

duration of each flow ranges from 0.5 Ma to 1.5 Ma. The time

duration of successive flows have a bearing on generation of

porosity in the upper part of exposed layer of each flow.

Schrock (1948) compiled the number of flows and surface

exposures. West (1999) has identified about 48 flows based

on petrographic studies of continuous cores taken in eastern

Saurashtra and Ahmedabad. When the duration between the

two lava flows is more, the top of the flow has undergone

weathering, till covered by the next flow. It is observed that

the upper layers of the lava flows in the drilled section holds

porous horizons, which are developed due to sustained

weathering effects.

Well logs have played a key role in identifying the porous

pods, the likely hydrocarbon seat, with neutron log leading

the way. Utility of neutron log in identifying porous pods as

compared to other logs has played a defining role in the

present work. Interestingly, the neutron porosity reading

ranging between 18-22% and resistivity values between 10-30

ohm-m indicate presence of oil in these weathered trap locales

in each lava flow. Development of porous and permeable

zones is also indicated by the SP deflection at places against

the same zones validated by production logging results in five

wells and build up study during conventional barefoot testing.

Integration of log data with seismic mapping and

interpretation as well as drawing the fault pattern in the whole

area could very well explain the depositional pattern, porosity

development and migration model. Migration of

hydrocarbons from Broach depression to the margins in the

Padra field, has been facilitated by long distance (100-125

kms) migration by the combination of E-W transfer faults and

their juxtaposition with Cambay Shale in the western side of

the area.

Based on this study, the development, of the area in the

middle host block as per our map at the Trap Top in closed

grid, have given the best lead and all development wells are

producing oil in large quantity more than the expected.

As per our map, other horst blocks are also required to be

delineated. Moreover, three exploratory wells, which were

kept on back burner due to lack of confidence, and one more

exploratory well P-ww based on the study were drilled and all

the four produced oil and gas from these reservoirs. The

testing of wells in basalt reservoirs need to be done by

lowering casing replacing the existing practice of testing

barefoot, as a aftermath of this study, which has spurred up

confidence to that level. This is also in line with the practice,

followed by Russians in developing such basement rocks

across the globe. This study is also useful for exploration and

exploitation in Karzen field, south of Padra field up to which

juxtaposition of Cambay Shale exists. .

Keywords : Padra field, Deccan trap, Lava flows,

Juxtaposition, Neutron log, Source rock, Migration

1. Dr. T. R. Varun, GM (W), ONGC, Palawasna,

Mehsana, Gujarat, India.

2. B.K. Sainath, CG(S), ONGC, CMDA Building,

Chennai, India.

3. Dr. N. B. Ishwar, CG (W), ONGC, WOB, Baroda,

Gujarat, India.

Introduction : Padra field located on the eastern raising

flank of Cambay basin within Broach-Jambusar Block is

surrounded by Dabka field in the west, Akholjuni and

Cambay fields in the north, Karjan fields in the south. The

Gravity- Magnetic survey was carried out in 1957- 58 to

understand the broad structural configuration of trap/

basement. The area is covered by seismic campaigns

SIG -351, 330, 294 and 320. The exploratory work in this

field was started in 1960 with hydrocarbon indications in the

well Padra-a. The known reservoirs are Trap and Olpad

Formations however, the hydrocarbons are also reported from

Ankleshwar Formation in a few wells. The thickness of

drilled section of Trap varies from 476m+ (Padra-d) in Padra

field to more than 3200m+ Ankleshwar Superdeep (ADP-a)

in deeper part of the basin. Barefoot testing of Trap section

was started from the well Padra-n onwards, Trap was drilled

40-200 mts, the well was cased and cemented with shoe

within the Trap top).

If the well, produced oil/ gas, it was completed in the Trap.

If the Trap section produced oil and water, then, the bottom

50m was sealed with sand/ cement plug and again trap section

was tested barefoot before completion of the well as a

producer. To confirm the hydrocarbon contributing zones,

production logging was carried out in Padra-q, which

indicated the contribution of oil from a 5m layer, 90mts below

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the Trap top. While in Padra-y‟, oil contribution was reported

from 25-30mts of trap section from the trap top. To confirm

this, three wells, padra-z, P-aa and P-cc were drilled upto

200m in the trap section. The wells which were drilled on

horst block, on testing produced oil. However, understanding

of hydrocarbon distribution pattern/entrapment remained

unresolved that from which level of Trap oil/ gas flows. The

understanding of hydrocarbon distribution pattern within Trap

in different flows has become a point of concern to delineate

the field/reserves and its extension, if any.

The Objective of the study was to understand the role of faults

in entrapment vis-à-vis the pattern of occurrence of

hydrocarbons and to understand the effective methods to

activate the trap section, to devise ways to have optimum

production, avoiding water contributing zones. Also, to

pinpoint the locales for further exploration/ exploitation of

Padra field, and to review the already released locations

PDBP, PDBL and PDAS in the light of the study which were

kept on back burner after a number of dry holes in this area.

Prelude about Deccan Trap :

A number of geoscientific studies have been carried out to

understand the phenomenon of the occurrence of oil in the

Traps of Padra field. Intertrappeans, if at all present are very

thin, less than 30 cm and indurated. The methodology evolved

for identification and evaluation of Deccan Trap reservoir of

Padra field by recomputing resistivity from porosity and then

evaluate water saturation from available Rt. PIVT studies

were also carried out at KDMIPE to predict fracture

distribution. TVN Rao etal, 1998 in their report, established

the presence of Olegophillic facies in Trap section of Padra

field (a feasibility report) using Electro Facies Visual Concept

and NGS logs. Painuly S. P. etal, 1995, in their report opined

that “Source of Oil accumulations in Padra area lie in the

organic rich Cambay Shales in the deeper parts of the basin”.

The authors explained that the reservoirs in Padra field are

charged due to this juxtaposition. P. H. Rao etal, 1999 while

proposing 3 locations in the south of Padra field reported a

network of fault system on the top of trap unconformity acted

as a conduit for migration that have charged the reservoir.

Pendkar N, Mathur A. K. and Anand S. R., 2000 in their

report “Fault Pattern Analysis, Identification of Flows and

Entrapment Conditions in the Deccan Trap”, reported that the

development of reservoirs appear to be controlled by a

combination of altered basalt with the fractures acting as

pathways for hydrocarbon migration.

Megascopically, the Deccan trap is characterized by basaltic

rock, which is divisible into four type of litho units‟ namely;

fresh, altered, weathered and amygdaloidal weathered basalt.

The rock mass is highly weathered and fractured, grain size

varies from fine to coarse. The fracture intensity is observed

moderate to good and filled with secondary mineral like:

feldspar, zeolite, calcite, siderite and dolomite. Out crop

samples collected from Rajpipla area have also been studied

and found corroborating with sub-surface samples of Padra

field. Prominent amounts of clay minerals are also found in

the form of Montomorillonite and Kaolinite, however Illite

has been observed in rare occurrences. The salinity in the

field varies from 13.0 gpl to 60 gpl as per data collected from

the testing of the wells.

Deccan Trap in Cambay basin consists of a number of flows.

The duration of each basalt flow ranges from 0.5 Ma to 1.5

Ma. The time duration of successive flows have a bearing on

generation of porosity in the upper exposed layer of each

flow. Schrock (1948) compiled the number of flows and

surface exposures. West (1999) has identified about 48 flows

based on petrographic studies on the continuous cores taken

in eastern Saurashtra and Ahmedabad. Authors reported in

their report that when the duration between the flows is more,

the top of the flow has undergone weathering, till covered by

the next flow.

In this field well nos.Padra-a to h (except well no. P-d) were

terminated in the first flow by drilling about 20m in the Trap.

As a result number of flows not clearly identifiable due to less

drilled depth and recording of logs by Russian tools, aa the

porosity logs were not available in those days. In the wells P-i

to m about 50 to 70m trap thickness was drilled covering

about 3 flows from the Trap top. The well nos., P-n to P-q

were drilled in the Trap (70 to120m approximately) covering

about 4 to 5 flows. Most of the wells from well no. P-r to P-dd

were drilled down in Trap upto 200m covering about 5 to7

flows. In the later phase of exploration of Padra Trap, wells

from P-ee to P-vv were used to drill down 100m

approximately with casing shoe about 5-10 mts down from

Trap top. After drilling of well no. P-vv there was a lull

period in drilling as 6 wells went dry, 2 wells in Padra (P-uu

& P-vv) and 4 wells in Karzon to better understanding of

entrapment and to devise ways for delineation. Well nos. P-

ww & P-xx were drilled based on the study proved prolific

producers. Already a released location PDBP, PDBL and

PDAS reconfirmed for drilling based on this study which

have been drilled now. Horst areas which have entrapped oil

has been identified and are under drilling from well no. P-bbb

onwards.

Thus, it is concluded that different lava flow surfaces (top

surfaces) which were exposed for a long time before the next

flow, developed porosity due to weathering, leaching by

surface discharge and alteration by hydrothermal waters. The

horst blocks have better development of porosity in the upper

portion of each flow as it was exposed for a longer duration

owing to its being located at structurally higher places.

Methodology and Techniques :

To meet the objectives for successful exploration and

exploitation of Deccan trap, conventional log data of all the

48 drilled wells were analyzed. Various flows in the trap

section were identified based on neutron, density, sonic,

resistivity, GR and SP logs by plotting them in log grids.

Based on lithology, average density was calculated as an

indication of weathered zones (2.35-2.90). The number of

lava flows are identified based on the study from logs and

numbered them from Trap top up to the drilled depth in each

well (Fig. 1& 2 are shown as representative well).

So far, in this field the maximum thickness of the trap has

been penetrated upto 476m in well P-d. About „20‟ flows of

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basalt are encountered. Most of the basalt flows are non-

vesicular. Generally, vesicles are concentrated on the top of

the trap (Weathered basalt), where best porosity development

is observed. On analysis, Neutron log was found

comparatively better log, which provided fair indication of

porosity variation within weathered, fresh and highly altered

trap zones. The neutron porosity in the range of 21-33%

(apparent) gives the indication of reservoir where neutron log

reading caliberated upto 21% is against the highly altered

basalt with negligible porosity. The sonic log porosity along

with density derived porosity also corroborates the same in

most of the wells. The other logs like resistivity, GR and SP

also support the above findings. However, SP is found to give

better information of porous and permeable reservoirs

occasionally.

The Late Cretaceous Deccan traps forms the economic

basement over most part of the basin. The structure and fault

pattern map was prepared at the Trap top integrating seismic

data of SIG-x94 and x30 with log data. Understanding of

different tectonic blocks vis-à-vis fluid anomaly, fluid

contacts, migration pathways, thickness of various formations

encountered and production testing details were made.

Attempt was also made to study the variation of geothermal

gradient on the occurrence of hydrocarbon within the Trap

section. Based on the study of maps, it is observed that the

total field is dissected into five blocks having roughly north-

south orientation forming horsts and grabens. All the five

blocks are cut by four transverse faults (F1, F2, F3 and F4) in

ENE-WSW direction. It is also observed that most of the

wells, which lie in the lows are water bearing except P-qq, P-

aa and P-vv. Most of the N-S faults terminate in Dadhar

Formation and some faults terminate in Ankleshwar

Formation.

The source rock is Cambay Shale, which pinches out to the

west of the Padra field. It is envisaged that Cambay Shale is

juxtaposed with the western faults trending roughly E-W

might have feeded Padra field through long distance

migration (Fertyl & Tissot Welte 1988, Studies on Cambay

basin). The long distance migration of 100-125 km. is

possible in a geological set up like that of Cambay Basin. The

hydrocarbons might have migrated along the four transfer

faults F1, F2, F3 and F4 oriented roughly east-west from

Dabka to Padra field. All these faults have down throw

towards south. The fault F5 has down throw towards Dabka

field. The fault F6, a margin fault in the east might have acted

as a sealing fault. The total area of Padra field resembles a

horst block from all the sides for holding of hydrocarbons in

the Trap flows, where porosity and permeabilities have

developed by the process as explained above. The total area is

dissected into five N-S blocks with intervening lows due to

tectonic activity. As a result, natural fractures are developed.

These fractures coupled with main faults acted as conduit for

migration of hydrocarbons (Fig. 4).

Well Log Data Analysis :

The Resistivity, GR, SP, Sonic, Neutron and Density logs of

more than thirty wells were plotted in their respective

grids/scales. In the first grid, Gamma Ray and SP logs, in the

second grid Resistivity logs and in the third grid Sonic,

Neutron and Density logs were plotted from top of the Trap

upto the drilled depth. The total drilled depth in the trap was

identified and divided in the flows as per log signature,

numbering from top to the bottom. The exposed layer in the

each flow was identified based on the character of porosity

logs. Resistivity, GR and SP logs were also considered in the

identification of porous layers. Neutron log is found to be

more susceptible to the porous layers developed in upper part

of the flows. On qualitative analysis, it is observed that the

neutron log showing more than 21% porosity and resistivity

varying from 10-30 ohms in the Trap flows seems to be oil

bearing and might have contributed in subsequent testing and

production . The same feature when drilled in the lows,

although have porosities, but produced water, owing to their

structural position. However, resistivity is found to be

affected by Trap matrix rather than formation fluid. The

results of the wells drilled in all the five blocks from east to

west are elaborated in (Fig. 3):

BLOCK-A (Eastern Most Block) :

In this Block, four wells i.e., P-s, P-cc, P-rr and P-uu are

drilled. The well P-rr drilled in extreme north-east of the

block is gas bearing in Olpad Formation and is still

producing. The porous layers developed in the upper part of

1st, 2nd 3rd, & 5th flow are charged with hydrocarbons. The

wells P-s and P-cc drilled south of this block are oil producers

from Trap. Porous layers in the flows seem to be the

contributors of hydrocarbons. The middle part of this block is

not explored and further exploration is warranted. In the

adjacent low towards the eastern margin, well P-uu was

drilled and was dry due to its structural position (Fig. 4).

BLOCK-B:

In this block, wells P-i, P-m, P-o, P-p, P-t, P-u, P-y, P-z, P-bb,

P-gg, P-ii, P-jj and P-ss were drilled. Wells P-z, P-ii and P-jj

are drilled in the adjacent low towards east and are water

bearing. The released location PDAS which could not be

drilled due to some problem. A large part of the area is

unexplored in the vicinity of „PDAS‟ which merits

exploration priority. A location towards north, south or east

may be drilled for delineation and exploration of this block.

Based on our recommendation PDAS was drilled as well no.

P-aaa, which produced oil (Fig. 3).

BLOCK- C, D, E, F:

This block lies in the middle of the field, where well numbers

P-hh, P-n, P-x, P-g, P-h, P-ll, P-pd, P-b, P-j, P-f, P-r, P-ee, P-

pp, P-oo, P-aa, P-uu, P-q and P-ff were drilled. All the wells

drilled in this block have produced oil from Trap except P-ll,

ff & vv. Well nos. P-a to P-j except P-d, require further

deepening or step out wells for exploration / exploitation of

Trap in this area. Based on our recommendation, area is under

delineation on a grid pattern and drilled wells numbering from

P-bbb to P-kkk as on date have produced oil (Fig. 3).

BLOCK-H, G:

In this Block, four wells P-l, P-v, P-w and P-mm were drilled.

Well no P-l has produced oil from Trap. Well P-v has

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produced oil from Trap and gas from Olpad Formation. The

rest of the area towards south of this block is not explored and

merit further exploration/ exploitation based on the model.

BLOCK-I:

This is the fifth block from the east and five wells P-dd, P-kk,

P-nn, P-qq and P-tt were drilled. Out of the five wells drilled,

P-qq and P-kk are dry. The study suggests that the location

PDBL need to be drilled on priority. In the southern side of

this Block, P-qq was drilled and produced water. The analysis

of 2D seismic interpretation suggests, the area forms a horst

block and likely to have hydrocarbons. The location PDBP

has opened up more area for exploration of Trap. To the west

of this fault, a prominent low exists towards Dabka field in

which Dabka-x5 is proved water bearing (Fig. 4).

An attempt was made to identify hydrocarbon/water zones to

know the oil water contact within the flows. However, the

same could not be established due to negligible changes in the

reservoir facies having hydrocarbon/water. It is also observed

that the flows are not correlatable with each other,

encountered in different wells due to differential degree of

weathering/alterations under different Palaeo conditions.

Based on the above study, several locations are proposed for

exploiting the remaining reserves in the trap. Thus all the four

locations, three recommended and one got released during the

study have produced oil, thus proved the efficacy of the

Model. The middle host block on delineation by close grid

drilling based on the study and prepared map, have produced

hydrocarbons from well nos. P-aaa to P-kkk (11 wells).

Recommendation :

It is recommend that future wells should be tested

conventionally by lowering the casing against the Trap, if

resistivity and neutron log character indicate good porosity

supported by other logs in the flows. During production

testing, choke size should be kept small i.e. from 3mm and

may be increased seeing the well behavour. Higher choke size

may result into water cut in a short span of production.

Neutron log along with sonic and density log should be used

as a good measure of porosity in each flow if it reads more

than 21% porosity on neutron log scale. Well completion in

each block should be done in the same flow itself and after

cessation of flow; well may be re-completed in the upper

flows. The wells in the Middle Block (3rd) have not penetrated

deeper in the Trap (well nos. P-a to P-j except , P-d). The

area shown by the brown colour need to be explored by

drilling, new wells or re-entering the old wells, if possible.

Wells which did not became active/proved dry and are located

in the horsts blocks need to be re-entered in the well by

lowering casing against the Trap section with conventional

perforation. The four wells namely; P-j, P-k, P-l and P-bb

could not be tested but appears to be interesting in the trap

section from hydrocarbon point of view. Wells P-y‟ and P-pp

seems to be hydrocarbon bearing in Olpad Formation,

whereas well no. P-ee seems to be hydrocarbon bearing in

Ankleshwar Formation and recommended for retesting, if

feasible (Fig. 3).

Acknowledgment :

The authors express their gratitude to Director (Exploration)

and ONGC Management for granting permission to published

this paper in SPWLA. The authors are also thankful to Shri

D.P. Sahsrabudhe, ED-Basin Manager, WOB, ONGC, Baroda

and Shri P. B. Pandey, GM (GP), Block-I, WOB, Basin,

ONGC, Baroda for encouragement from time to time.

Thanks are due to Shri S.P.Painuely, ex-GM-Block Manager-I

for overall guidance, facilities and review from time to time.

The authors are also thankful to Shri Asim Samanta, GM-

HOI-CEWELL, ONGC, Baroda for providing facilities for

preparation of the paper.

References :

Dhulia B.P. et. al, “Proposal for Release of 2D Exploratory

Locations in Padra-Umeta-Anklav area of Broach-Tarapur-

Cambay Blocks of Cambay Basin”, Unpublished Report,

1998, KDMIPE.

Painuly S.P. et. al, “Review of Deccan Trap Exploration in

Padra Area and Prioritization of locations”, Unpublished

Report, 1998, WRBC, Baroda.

Pandey A.K. et. al, “Reservoir Characterization of Olpad and

Ankleshwar sands in Padra Field, Broach Block, Cambay

Basin, Unpublished Report, 2000, RGL, Baroda.

Pendkar N. et. al, “Fault Pattern Analysis, Identification of

Flows in the Deccan Trap, Padra Field, Cambay Basin,

ONGC, KDMIPE, Unpublished Report, 2001.

Rao P.H. et. al, “ Proposal For Release of Three Exploratory

Locations in South of Padra-Karjan area in Broach Block,

Cambay Basin, Unpublished Report, 1999, WRBC, Baroda.

Rao T.V.N. et. al, “Establishment of Oleophallic facies in

Trap Section of Padra Field, ONGC, Baroda, Unpublished

Report, 1998.

Sinha N. et. al, “Hydrocarbon Distribution Pattern and Future

Exploration Strategy of Deccan Trap Reservoir in Padra

Area” Unpublished Report, 1998, WRBC, Baroda.

Varun T R et. al, “Integrated Study Of Deccan Trap In Padra

Field By Applying Suitable Geological, Geophysical And

Logging Techniques With Special Reference To Tectonics”

2003-04

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Fig 1 Fig 2

Fig 3

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