Exploratory efforts made in Eastern
Region
The Tripura region is situated in the
north-eastern sector of India, and is surrounded by the territories of
Bangladesh and Burma, except in the north-eastern part, which is bordered
by the Indian states of Assam and Manipur. Geographically, it is bounded
by the latitudes 22 N and 24 30' N and the longitudes 91 10' E and 93 30'
E.
Geomorphologically, this region
is characterised by an alternating succession of ridges and valleys of
roughly north-south trend. The general elevation of the region rises eastward
from a few tens of metres in the areas adjoining Bangladesh plains in western
Tripura to about 1800 metres in eastern Mizoram bordering the Chin Hills
of Burma.
The Tripura region had attention
of geologists since long on account of its location in the petroliferous
Assam-Arakan basin and the existence of a series of large folded structures
with gas shows, but the progress of exploratory efforts in this region
has been slow on account of its geographic remoteness, poor accessibility
and industrially underdeveloped condition. Following some geological reconnaissances
carried out by the Geological Survey of India in the early part of the
twentieth century, the Burmah Oil Company prepared a photogeological map
of the region and carried out large scale mapping in parts of Tripura and
traverses in Mizoram between 1911 and 1959.
Status of Exploration
ONGC took up systematic geological
mapping of Tripura in 1962, along with shallow drilling at selected locations
and have so far carried out detailed geological survey covering almost
the entire Tripura state. These surveys have resulted in delineation of
fourteen large closed anticlinal structures, comprising thick dominantly
deltaic sedimentary successions of Neogene age with favourable geological
prospects, viz., Rokhia, Tichna, Gojalia, Baramura, Bulamura, Atharamura,
Batchia, Langtarai, Harargaj, Machlithum, Khubal, Sakhan, Langai and Jampai
anticlines. In addition, a buried domal structure was suspected on the
basis of geomorphological features in the wide synclinal trough between
Rokhia and Baramura anticlines and this has later been confirmed by seismic
surveys and named as Agartala Dome. Photogeological studies of air photos
and LANDSAT imagery have also indicated the occurrence of sixteen geomorphological
anomalies possibly related to buried structures under the cover of Quaternary
sediments in the Agartala, Khowai, Dolai, Manu and Dharamnagar synclines.
In order to delineate the subsurface stratigraphy and structures, seismic
survey have been carried out by ONGC since 1977, mainly in the western
part of Tripura, till date. Two seismic parties (combining together) are
as presently working in south of Tichna and Rajnagar structures. Aeromagnetic
survey has been carried out over the entire Tripura - Mizoram region by
the Geological Survey of India in collaboration with ONGC in order to delineate
the basement configuration and to estimate the thickness of the sedimentary
column.
Exploratory drilling was initiated
in Tripura in 1972 on Baramura anticline. As on 1st March 1998, 77 wells
have been drilled in the western part of Tripura and 5 wells have been
drilled in the eastern Tripura. Out of the 87 wells drilled so far,43 wells
proved to be gas bearing. The exploration delineation efforts is limited
to Middle Bhuban sequence. Additionally, the shallow prospect of Bokabil
sequence proved to be commercial producer in Agartala Dome. The exploration
for deeper Barail sequence (equivalent oil bearing sequence of Upper Assam)
yet to be probed by drilling.
As on 01. 04. 1997, 30.11 BCM of
GIIP and 17.24 BCM of Ultimate reserves of gas have been established in
Tripura.
As on 01.05.1997,45 exploratory
and 4 development locations are available.
High exploration cost coupled with
logistics, insurgency, law and order situation, land acquisition problem
associated with reserve forest and rubber plantation area severely affected
exploration in Tripura. Moreover, low offtake and subsidised price of gas
hinders further investment.
The exploratory efforts in Tripura
till 01.05.1997 and reserve established till 01.04 1997 is annexed. In
1998-99 ONGC has produced 306.712 Mnsocn gas.
Ongoing Exploration Programme
Environmental situation compelled
ONGC to limit the exploratory activities in two structures of Western Tripura
(Rokhia and Agartala Dome.) All the three rigs, at present, deployed at
Rokhia and Agartala Dome.
Acquisition of 2D seismic data in
new structures, like Rajnagar. Efforts to explore the shallow prospect
within Bokabil/Upper Bhuban sequence prioritised. Major areas (East
of Baramura) awarded on Production Sharing Contract.
MOU for Joint Study between
ONGC and M/S UNOCAL, signed-proposed study planned to be carried out from
March 1997 onwards.
Thrust Areas During IXth
Plan
-
Three tier exploratory strategy to be
implemented
-
Exploration for shallow prospects (1800/2000m):
Bokabil/Upper Bhuban prospect (Agartala Dome, Tichna and Rokhia structures)
-
Exploration for Middle Bhuban prospects
(3500m) in and around existing gas fields and available virgin structures
-
Exploration for deeper Lower Bhuban/Barail
prospects (4500+m) in the existing gas fields and new structure like Tulamura
-
Initial exploration to be limited to
the logistically and environmentally favourable areas
-
Exploration of virgin structures, like
Rajnagar and Tulamura during the early years of plan period
-
Further exploration in the southern
part, like Tichna and Gojalia, depends on the results of Rajnagar coupled
with the improvement of law and order situation
INTRODUCTION
Bengal Basin which has a
total effective basinal area of about 40,000 sq. km. is a pericratonic
basin, its geometry being governed by step faults.
The eastern part of the basin is
a mobile flank. Four tectonic zones have been recognised in the basin :
a basin margin fault zone, b) stable shelf, c) hinge zone and d) deep basin.
Exploration for hydrocarbons in
Bengal Basin has continued for about four decades without commercial success.
The main constraints for occurrence of any commercial accumulation appear
to be lack of well-defined structures in the platform area where sands
are present, and the lack of reservoir faces in the deep basin area where
only small amplitude structures could be deciphered. Some features interpreted
by seismogeological analysis (e.g, Eocene reef, flat spot or bright spot
anomalies) were not confirmed by drilling data.
Geochemical prospecting has been
carried out in West Bengal and this has brought out certain geochemical
anomalies in Ranaghat-Krishnanagar depression.
The Soviets were offered as per
their request an area in south Bengal onshore to be explored under Intensive
Integrated Exploration Project (IIEP). Their exploration targets are strati-structural
prospects in Miocene and Oligocene sequences and Eocene carbonate build
ups along hinge zone. 3D seismic surveys have been carried out by them
in selected areas on hinge zone. One well, Golf Green -l drilled which
was proposed by Soviets as well as by ONGC experts.
Exploration History
Exploration for hydrocarbon in Bengal
Basin was initiated by ISPP through geophysical, geological and photogeomorphological
mapping. Mainly low amplitude time structural anomalies within Tertiary
section were tested by ten wells. However, ISPP called off its exploration
activities apparently because of non-discovery of commercial volume of
hydrocarbons and drilling difficulties in the deep basinal wells on account
of overpressured Neogene section.
ONGC (1962) followed up and pursued
the exploration activities in onland part by introducing more sophisticated
data acquisition tools and techniques for unravelling the subtle features,
e.g., stratistructural, stratigraphic as well as low order time structural
anomalies. Powerful computers aided by sophisticated software, did improve
the quality of seismic sections.
Several drilling locations in onland
part were identified and 30 of them have already been tested by drilling
on time structural anomalies, subtle Eocene reef/carbonate build ups, lateral
seismofacies variations within carbonates, wedge outs, clastic build ups,
rollover fault closure within clastic sections, but again without commercial
success.
Initially two wells drilled in offshore
Bay of Bengal to probe the hydrocarbon potentiality of Neogene sediments,
located on the giant time structural features identified by seismic survey
by Carlsberg-Natomas during 1974-76. Subsequently five offshore wells planned
by ONGC have already been drilled without a commercial strike.
Thus 47 wells have been drilled
so far in West Bengal (excluding one well or CBM exploration) without commercial
success.
REVIEW OF EXPLORATORY EFFORTS
IN DIFFERENT PLAYS
Fractured Basement
Three wells (Galsi-1, Palasi-1 and
Mainagar-1) were drilled through the Gondwanas in the western shelf. None
could find hydrocarbon accumulations. Instead of crystalline metamorphic
basement, rocks of gabbroic and diabasic affinity were found at Palasi-1
and Mainagar-1 respectively. A structural closure at basement level has
been tested for fracture porosity at Mainagar-1 well, which proved to be
dry.
Gondwana Prospect of North
Bengal and Western Shelf
To date 7 wells have been drilled
in West Bengal which penetrated Gondwana sequence. Of these, 4 wells (Palasi-1,
Galsi-3, Mainagar-1 and Chandkuri-1) were in South Bengal and 3 wells (Rajganj-1,
Karandighi-1 and Lahil-1) were in North Bengal. The later two wells fall
in the eastern flank of Purnea depression.
Drilling of these wells revealed
the presence of considerable thickness of Lower Gondwana coal-shale and
sandstone units which show good source potential but very early generation
and subsequent exposure to erosion. Upper Gondwana is relatively thin and
dominantly arenaceous. There is lack of caprocks as also source rocks.
Palaeocene-Cretaceous plays
of Ranaghat depression and Western shelf
The shales of Jalangi Formation
have very good potential for generating hydrocarbon in the basinal area
as well as in the shelf area. So far 7 wells have been drilled for Cretaceous
prospect and 4 wells have been drilled for Palaeocene prospect. However,
none of these wells could establish the presence of hydrocarbons (except
the well Ichapur-1, where indications of hydrocarbons were found).
Eocene Limestone Prospect
The thick Eocene Limestone sequence
remained the most elusive target for hydrocarbon prospecting in Bengal
Basin. Eocene carbonate build-ups indicated by seismic responses were probed
by 12 wells. No positive results were obtained except indications of hydrocarbons
in well Golf Green-1 and some dissolved gas. The limestone is light and
practically devoid of porosity. Some sand bands within limestone in SME-1
indicated high pressure with presence of dissolved gas. These clastic facies
are discontinuous within short distances. The limestone drilled at SME-5
was massive at base and without any indication of hydrocarbons in the clastic
Palaeocene section below it. The algal-foraminiferal limestone has not
indicated any growth of reef facies in any well. Selective 3D seismic surveys
have been carried out to delineate porosity pods, if any, in suspected
reefs in Amtala area and in Golf Green area for integration of 3 D as well
as well data.
Oligocene Prospect
So far, 3 wells have been drilled
for Oligocene prospect in the western shelf and hinge zone. One well (Ichapur-1)
although targeted for Paleocenc also tested for Oligocence where indications
of hydrocarbons were obtained. In view of drilling results of SME -4 which
were targeted mainly for Oligocene wedge prospect, it may be inferred that
the Oligocene sequence (except Ranaghat-Krishnanagar depression) is very
tight and composed mostly of shale with interbed of argillaceous limestone
in the basinal side lying south of Damodar graben alignment.
In Ranaghat-Krishnanagar de-pression,
the Oligocene sand bodies are found to be isolated in nature. 3D seismic
survey has been carried out to establish the geometry of the sands. Integration
of seismic and well data is being carried out.
Mio-pliocene Prospect
18 wells have probed different structural,
stratigraphic and strati-structural features without commercial success.
While well defined mappable structures are rare, several features like
bright spot and flat spots as seen in the seismies were not proved by drilling.
The absence of reservoir facies therein restricts further exploratory efforts
in this direction.
Coalbed Methane
An well has been drilled for coalbed
methane, an unconventional source of energy, in Durgapur depression without
any commercial success.
Summing Up
The exploratory efforts made in
the onland part of Bengal can be categorised 4 distinct phases:
-
Geological, aeromagnetic, gravity and
seismic surveys leading to 10 exploratory wells by ISPP, mainly in the
fifties, for basin evaluation and prospect identification.
-
Exploration of structural traps, particularly
in Miocene and Eocene, by analog seismic surveys with single fold technique
by ONGC between 1963-70 and drilling of ONGC's first exploratory well Bodra-1.
-
Deliberate efforts by ONGC for identification
of stratigraphic pros-pects in Eocene and Miocene by advanced digital seismic
technique with higher foldage between 1970 till date and drilling of 29
exploratory wells in this basin.
-
For exploration of coalbed methane,
an unconventional source of energy.
Exploration in the offshore part
of Bengal Basin can be divided into 2 phases :
-
24-fold CDP seismic surveys in an offshore
contract area of the Bay of Bengal, followed by drilling of two offshore
wells, BB-A-IR and BB-B-I during 1975-76 by the Carlsberg-Natomas Group
of USA.
-
Acquisition of CDP seismic data in the
shallow marine part of Bay of Bengal and in the tidal rivers in the Sundarbans
by ONGC during 1979-81 followed by drilling of five offshore wells from
1986 to 1990.
In the current phase of exploration
seismic surveys are being carried out in Bongaon-Jaguli (2D seismic survey)
and in the south-west of Ichapur (31) seismic survey) areas.
presently no drilling is planned
in West Bengal, except for coalbed methane exploration, due to non-availability
of drillable locations. For coalbed methane exploration, two locations
are awaiting drilling in Raniganj and Birbhum areas after completion of
the second well in Durgapur depression.
Introduction
The state of Bihar, having
an area of 1,73,880 sq.km is situated in the north central part of Indian
Peninsula. Two-third of the area in the state is made of igneous and metamorphic
rocks. The sedimentary rocks, which are considered to be of interest from
hydrocarbon point of view,occur towards north and north-east part of the
state, with an area of 74,000 sq.km and are buried below alluvium. The
area falls in the eastern part of Ganga basin and is divided into three
depressions, viz Gandak, Madhubani and Purnea, caused by the intense block
faulting in the basement. The Vindhyan and Lower Siwalik developed in Gandak
and Madhubani depressions and Gondwanas developed in the Purnea depression
are the primary targets for hydrocarbon exploration.
Status of exploration
The exploration activity in the
eastern part of Ganga valley was initiated with Aeromagnetic survey in
1956 conducted by Canadian Aeroservices Limited Which established the broad
tectonics and geology of the basin.
The results of the Gravity-Magnetic
surveys during 1959-68 and 1978-80, corroborate the data of Aeromagnetic
surveys and provided better understanding of the sub-surface configuration
of the basin.
The earliest conventional seismic
survey (analog) was carried out by Prakla-Seismos of Germany and ONGC during
1960-65. This has revealed broad structural and stratigraphic information
and a prominent regional unconformity at the base of Siwaliks. Later CDP
seismic surveys (12/24/48 fold) were conducted in Gandak, Madhubani and
Purnea depressions.
Geological surveys were carried
out by ONGC during 1964-67 in the north-west part of Gandak depression,
close to Indo-Nepal border, where Siwaliks are exposed. The mapping indicated
presence of three plunging assymetric anticlines like Ganauli, Roghia and
Someswar. Geological surveys also carried out in the coalfield area of
Damodar graben, lying south of Ganga valley.
In the eastern part of Ganga valley
six wells have been drilled, out of which four wells are in Gandak depression,
viz., Raxaul-1, Gandak-1, Ganauli-1, and Kadmaha-1. One well in Madhubani
depression, viz., Madhubani-1 and one well in Purnea depression, viz.,
Purnea-1. Additionally, two more wells, viz., Lahil-1 and Karandighi-1,
drilled by West Bengal Project, also falls in Purnea depression. However,
all the wells proved to be dry.
In the current phase of exploration,
there in no plan for exploratory drilling in Bihar due to non-availability
of drillable locations. However, one 2D seismic party is engaged in Purnea
depression for delineation of structural configuration of Gondwana. Additionally,
for coalbed methane exploration, one location has recently been released
in Jharia which is awaiting drilling.
Environment activities in Tripura
Project, ONGC, Central Region
ONGC in Tripura Project works for
exploration, drilling and processing of hydrocarbons in various gas fields.
The activities related to drilling and production processing cause adverse
impact on environment. In order to minimise/mitigate the impact of various
pollutants, preventive measures are adopted for protection and conservation
of environment. Assessment of the quality of air, water, noise and land
pollution components are being done through actual data acquisition on
the sites and monitored periodically to check the environment quality.
Quality of air is kept at standardised
levels by using eco-efficient and environment friendly vehicles and equipments
with Internal combustion engines. These are being maintained for efficient
combustion to reduce vehicular emissions upto the required standard. Ambient
air quality for SPM,Nox, SO2 and HC are also monitored once in every year.
Flaring of gas is minimised and complete combustion is allowed. Care is
being taken for early detection and control of toxic gases like, H2S, CO,
SO2 etc. Green belts are developed in and around the production installations.
In addition, regular monitoring
of surface and ground water quality in and around the sites are carried
out. No waste is discharged outside the area of operation. Appropriate
sanitation facilities are provided for employees to reduce impact on quality
of water resources. All affluents are stored in impermeable pits and are
treated to meet the required standard and subsequently disposed off.
To control noise pollution, vehicles
and mechanical equipments are regularly serviced which also minimise the
exhaust emissions. Ear plugs/mufts are provided to the personnel working
in noisy areas.
To make the operation area free
from pollution, drilled solids are collected remixed with soils and spread
over the drillsite to neutralise the impact of the pollutants. Eco-friendly
chemicals and bio-degradable additive are used in drilling fluids. Leakage
of effluents discharged from the site is avoided by constructing waste
pits with impermeable lining and raising height of bunds around the site.
In and around the production installations,
green belts are developed to mitigate fugitive type of emissions, noise
abatement, cooling of atmosphere, prevention of soil erosion, restoration
of ecology and improve the quality of environment.
In case of disaster, efforts are
made to monitor the quality of air, water and land by generating adequate
data as per industrial norms. |