The Mahanadi Basin, a product of rifting and break up of Gondwana Land, situated
on the East Coast of India (Figs.1 & 2) is a basin with significant unexplored hydrocarbon
potential. Like its immediate neighboring, Krishna-Godavari Basin, Mahanadi Basin
also has a geological extension into offshore. The basin covers a total area of
55,000 Sq. Km. out of which about 14,000 Sq. Km. lies in the shallow offshore area.
The shelf break occurs approximately along 150 m isobath. The deep-water part of
the basin covers a much larger area. The prognosticated resource of the onland part
of the basin is in the order of 45 MMt and that of the shallow offshore basin is
about 100 MMt. Deep water part of the basin holds huge significant additional hydrocarbon
The onland part of the basin is limited to north west and west by Pre-Cambrian outcrops
belonging to the Indian crystalline Shield (Fig. 3). Towards northeast, it merges
into North East Coast region (NEC) with Bengal Basin lying further northeast. Onshore
Mahanadi Basin is located in the State of Orissa. Geographically, the shallow offshore
part of the basin lies off the coast of Andhra Pradesh and Orissa. The 850 E Ridge
occurring to the south of Lake Chilka forms the approximate southwestern limit of
the basin. Subsequent to Late Oligocene / Miocene period, Mahanadi-NEC area is so
greatly influenced by Bengal deltaic sedimentation system that its northeastern
boundary with Bengal Basin becomes obscure.
To date, four wells in onshore part (MNO-1 to 4) and seven wells in Mahanadi shallow
offshore (MND-1 to 7) have been drilled, some of which indicated significant hydrocarbon
shows during drilling. In NEC area, two wells viz. BB-A-1R and BB-B-1 were drilled
by Carlsberg, four wells viz. NEC 1,2,3,and 4 were drilled by OIL and in more recent
times another Company drilled 6 wells. Some of the wells gave very encouraging results.
Geochemical Sniffer Surveys conducted by M/s Interocean of USA revealed a number
of geochemical sniffer anomalies in the shallow offshore area. An independent comprehensive
basin modeling study for the area also gave encouraging results. These positive
observations indicate the hydrocarbon potential of the basin. The recent promising
finds in Mahanadi-NEC shallow offshore gave further fillip to the envisaged potential
of the basin.
A. TECTONIC EVOLUTION OF EASTERN CONTINENTAL MARGIN
The Gondwana paleo-geographic reconstruction shows juxtaposition of Antarctica to
the east coast of Peninsular India with Australia lying further to the east. The
eastern continental margin of India is a rifted passive margin evolved in response
to continental rifting. Indian Craton is circumscribed by rifted grabens and marginal
sag basins. The east-west trending Narmada-Son Tectonic Lineament, an important
line of discontinuity across the Indian shield is a mid-continental rift system,
which divides the shield into two halves; a northern ‘Foreland block’ that now forms
the Himalayan Foreland Region and a southern’ Peninsular Block’. The process was
initiated along the Satpura weak zone during Late Carboniferous / Triassic time
due to crustal stretching that also caused co-genetic rifting of Mahanadi and Godavari
grabens along NW-SE tectonic trend. During the Early Rift Stage (Permo-Triassic),
down warping of the Eastern Margin formed a series of northeast southwest trending
faults following an older set of Achaean fault lineaments ‘The Eastern Ghat Trend’.
Initial break-up of the Indian Craton was caused by rift initiation possibly by
the formation of a series of triple junctions forming two sets of cross trending
Northwesterly paleo-current directions during Permo-Carboniferous and Triassic times,
marine intercalations and deltaic facies of Godavari and Mahanadi grabens suggest
the presence of a sea to the north of these grabens. This and the formation of depocenters
at the meeting point of these grabens with the Narmada-Son rift (Satpura and Son)
prompted to reasonably infer the presence of a seaway along this major rift. This
rift episode seems to have ended by Late Triassic when the seaway was probably filled
up and uplifted as the Indian Plate had started drifting away. The failed arms of
these rifts forming aulacogens have been exploited by the Mahanadi, Krishna, Godavari
and Cauvery river systems.
During Late Jurassic to Early Cretaceous period, India – Australia plates rotated
from Antarctica with continental extension between Australia and Antarctica and
a right lateral transform movement along the southern part of the Coromandal coast
(east coast) margin of India (to the south of Krishna-Godavari Basin) and Indian
plate got separated from Antarctica. Thus the initial break up of the Indian Plate
from Australia-Antarctica possibly occurred prior to Early Neocomian, creating the
proto Bay of Bengal Ocean with the onset of sea-floor spreading. Since Cretaceous,
the Indian Plate rotated northwestward. Development of a new spreading center (?)
initiated spreading in north-south direction. It was during Late Albian that the
paleoslope was reversed in these graben basins with the southeasterly tilt of the
peninsular block. This event coincided with first basin wide marine transgression
during Cenomanian and continued up to Maastrichtian. The post-rift thermal subsidence
continued throughout remainder of the Cretaceous and Tertiary. An extensive uplift
of that major rift and erosion followed the end of Cretaceous prior to the volcanic
episode of Deccan trap. The northward drift continued till it had initial (soft)
collision with Eurasia in Paleocene. Sea level fall in Upper Oligocene followed
this. The southeasterly/ easterly tilt of the Peninsula had resulted in the increase
of fluvial energies of various fluvial systems leading to pouring of substantial
sediment loads into the adjoining sea. In Early Miocene, crustal shortening continued
as the Indian Plate continued its northward drift with the resultant further increase
of fluvial activity. Himalayan upliftment and deposition of sediments by the Ganges
and Brahamaputra commenced in Miocene. Thus Miocene and younger times witnessed
very significant increase in the sediment outpouring into the offshore extending
the basins into the deeper waters.
The evolution of East Coast of India as a passive margin set up resulted in the
formation of a number of peri-cratonic basins- Bengal, Mahanadi, Krishna-Godavari,
Pennar and Palar, while Cauvery basin developed as an intra-cratonic basin.
B. BASIN EVOLUTION/ GEOLOGICAL HISTORY AND STRATIGRAPHY
The Eastern Continental margin of India represents a rifted passive margin evolved
in response to continental rifting.
Regional Tectonic Element of the East Coast of India.
India separated from
southern super-continent Gondwana Land during Late Jurassic- Early Cretaceous, although
rift initiation was during the Permo-Triassic prior to continental splitting. Rift
initiation was in the form of linked rift triple junctions. One of the failed arms
of these rifts forming aulacogen north of the Mahanadi Basin has been exploited
by Mahanadi river systems (Mahanadi Graben). These aulacogens, representing favorable
locations for the development of fluvial systems, provided great drainage basins
for the progressive erosion of the cratonic hinterland. Predominantly continental
sediments of Upper Paleozoic and Mesozoic age were deposited and preserved in this
aulacogen – Mahanadi Graben. Rifting and synrift infill of the Mahanadi Graben was
continuing in Jurassic and Early Cretaceous.
- Rift Stage
Mahanadi Basin, a peri-cratonic basin was initiated probably during Late Jurassic
by rifting and subsidence of the Pre- Cambrian Basement by a number of major faults
with a dominant ENE-WSW trend and subordinate NNE-SSW and NNW-SSE trends. These
faults, which are parallel/sub-parallel to the present day coast line divided the
basin into a number of linear depressions and uplifts Bhadrak Shallow Basement Block,
Cuttack- Chandbali Depression, Bhubaneswar- Kendrapara Uplift, Paradip Depression,
Nimapara-Balikuda Uplift, Puri Depression and Konark Uplift from north to south
in the onland part.
Tectonic Map of Mahanadi Onshore Basin. The Konark Uplift extends into shallow
offshore part of the basin. Further southeastward, the shallow offshore part up
to the Eocene Hinge Zone has also been differentiated into a number of ENE-WSW trending
uplifted blocks and depressions.
Tectonic features of the Mahanadi offshore Basin. Further to northeast,
a NNW-SSE trending fault is present probably, due to which the Eocene Hinge Zone
takes a sudden NE-SW swing and further northeastward, it assumes almost a N-S trend.
North of this part of Hinge Zone is designated as NEC area.
A Generalized Lithostratigraphic column of Mahanadi shallow offshore basin.
Available regional geoscientific information suggests that the Basement in the Mahanadi
onland and shallow offshore area is of continental type to the north of the Eocene
Hinge Zone . It may be mainly of oceanic type further to the south of it. A transition
zone from continental to oceanic crust of varying width may be present in different
parts of the basin. Continental Basement is made up of Eastern Ghat granulites and
gneisses of Pre-Cambrian age. The oceanic basement has not been drilled so far in
any of the Mahanadi wells, but it is likely to have an affinity with the Early Cretaceous
volcanic rocks encountered in some of the drilled wells. The upper age limit of
the oceanic basement may be Early Cretaceous. The Bouguer Anomaly Map indicates
that the lineament from which ocean spreading initiated was coast parallel implying
that rifting and spreading was controlled at least in part by the NE-SW trending
Achaean Eastern Ghat trend. NNW–SSE trending faults divide the basin into transverse
blocks. Significant lateral movement has taken place along these strike slip faults
during the course of basin development. As mentioned earlier, both the basin strike
and the alignment of the Eocene hinge takes a sharp turn to NE along one of these
faults suggesting a post-Eocene movement.
Oldest exposed sediments in onshore part of Mahanadi Basin belong to Athgarh Sandstone
Formation of Late Jurassic to Early cretaceous age. These sediments directly overlie
Pre-Cambrian Basement rocks along the western basin margin near Cuttack and are
concealed under a widespread cover of laterite and alluvium of Pleistocene to Recent
age at places.
Predominantly coarse grained sandstone with subordinate shales and thin coal streaks
of Late Jurassic to Early Cretaceous age have been encountered in the onland wells
drilled in Depressions. These sediments have been inferred to be deposited under
fluvial conditions with occasional marine incursions. Several flows of basic volcanic
rocks of different thickness are met within this sequence. However, the shallow
offshore part of the basin witnessed more intense volcanic activity albeit with
a few interruptions. This predominantly volcanic sequence made up of basalts, tuffs
and inter-trappeans is resting unconformably over Pre-Cambrian Basement. These volcanic
rocks have both sub-aerial and sub-aqueous characteristics. The thickness of volcanics
varies from around 25 m in MND -3 to 850m in MND -7 indicating the variation in
volcanic activity over the basin. Based on palynological evidence, these volcanics
are correlated with Rajmahal Trap and have been assigned Neocomian-Aptian age (133
to 144 Ma). The inter-trappeans are mainly argillaceous and their thickness vary
from 150 m in MND-1 to almost nil in MND-7.
Accumulation of Early Cretaceous syn-rift sediments in complexly dissected relief
caused by rift development explains their composite and heterogeneous lithology
as well as vertical and lateral variations within the basin.
- Early Thermal Subsidence
The initiation of early thermal subsidence during Late Albian is marked by a southerly
/ southeasterly tilt of the continental margin leading to marine transgression.
The structural configuration continued to be of horst- graben type but less differentiated
Unconformably overlying the volcano-clastic rocks of Early Cretaceous is an appreciably
thick sequence of sandstones and shales with minor limestone at places deposited
under shallow inner shelf conditions in the present day shallow offshore part of
the basin. The thickness of this stratigraphic sequence varies widely from almost
nil to 500m. However, the equivalent sediments in the onland part were deposited
under fresh water conditions suggesting restricted marine transgression.
- Late Thermal Subsidence
The first soft collision took place in Early Paleocene as continental India collided
with the Eurasia Continental margin. Rifting had ceased and northward drifting was
initiated. Eastward flowing rivers along the East Coast of India deposited Paleocene
sediments that filled Cretaceous topography. Marine transgression continued through
Paleocene. However occurrence of Paleocene sediments in only two wells drilled on
shore viz. MON-3 drilled on Konark Uplift located relatively basinward and in MON-4
in Chandbali Depression, seaward part of the Cuttack-Chandbali Depression, suggests
that the marine transgression during Paleocene was restricted to the basinward periphery
of the present day onland part of the basin. While the sequence is represented by
shales, sandstones and thin limestones deposited in the shallow marine environment,
in the offshore part it is represented by a dominant argillaceous limestone, shales,
siltstone and sandstone deposited under deltaic to shallow marine conditions. Based
on the well data, it can be surmised that there was a paucity of clastic supply
into the area during Early Paleocene resulting in the deposition of limestone in
the present day shallow offshore area (i.e. where the wells are located). However
the clastic supply got resumed (possibly from a provenance located in NE part of
the onland basin with a southeasterly drainage direction) during Late Paleocene
resulting in the deposition of sands and silts with minor limestones possibly under
deltaic conditions in the southwestern part of the offshore basin. However, northeastern
part of the offshore part of the basin (MND-2&7) continued to be deprived of clastic
supply leading to uninterrupted deposition of limestone.
Dominant presence of fossiliferous limestone with minor clastics in the Eocene sequence
in the shallow offshore wells and presence of seismic mounded facies corresponding
to this sequence possibly represent carbonate build-ups along the hinge. Occurrence
of medium to fine grained sandstone and shales deposited under inner shelf conditions
in a lone well proximal to present day shoreline and total absence of this sequence
in other wells strongly suggest that major part of the onland part possibly remained
uplifted inhibiting marine transgression into the northern part of the onland basin.
As mentioned earlier, Eocene Hinge can be clearly identified through seismic data.
Clear evidence of deep erosion into the upper part of the Eocene sequence is reflected
in the form of uneven topographic surface and erosional channels and total absence
of Oligocene section in both onland and shallow offshore part of the basin indicates
a major Late Eocene-Oligocene marine regression from the basinal area. In contrast,
there was widespread sedimentation during this period in the adjacent Bengal Basin.
In the NEC area, the well NEC-1 located 40 km away from Mahanadi offshore well MND-2,
encountered 567m of Oligocene. However, in deep offshore, Oligocene sediments seem
to wedge out against Eocene shelf edge.
Crustal shortening initiated in Late Eocene - Oligocene continued into Early Miocene
during the northward drift of the Indian plate and the consequent upliftment of
Himalayas that initiated Gangetic sedimentation. Further upliftment of the Himalayas
during Mid-Miocene caused Gangetic delta to dominate the sediment deposition in
a significant part of eastern offshore viz.- Bengal, NEC, Mahanadi and northeastern
part of Krishna-Godavari Basin. It also altered the course of the Brahamaputra River
to join Ganges with a resultant spectacular increase in the rate of sedimentation
into Bay of Bengal. The most prominent feature in Bay of Bengal i.e. Bengal Fan
can be considered as the largest deep sea fan complex in the world with a length
of 3000 Km. extending from the mouth of Ganges to Latitude 7 0 S in the Central
The significant increase in the sedimentation rate during Miocene and younger times
had its own positive impact in terms of hydrocarbon exploration in the area. It
initiated ‘sediment induced tectonics’ in these sequences.
Eocene Hinge Zone paralleling the coastline represents a prominent tectonic element
in the area, providing slope with 10 – 2 0 dip at shelf-edge and 60 – 12 0 dip on
the slope. Beyond the slope, it flattens out to 10-20 in the deep basinal area.
This provided an excellent geological setting to carry the huge sediment loads into
the deep offshore area. During sea level lowstands, sediments were delivered to
the shelf edge initiating the formation of shelf edge deltas, levee-channel and
basin floor fan complexes.
Initiated in Late Oligocene, a broad regressive cycle comprising successive periods
of delta building followed through Miocene and Younger times. Frequent sea level
fluctuations alternated the periods of transgression and progradation.
Seismic expression of both Upper Miocene and Pliocene sequences exhibits deltaic
facies in the lower part while the upper part of the sequences exhibit deep canyon
cut and fill features. The canyon cuts are steep sided, narrow to wide and at times
1-2 Km deep. These canyons have created locally deep-water conditions over the shelf.
Channels within these canyons acted as conduits for transporting coarser clastic
sediments into deep-water areas. As the canyons got filled up, they gradually came
under shallow water environment. It is observed that those canyons fills, in general,
are of fine clastics with some sands representing the channel facies within them.
The cyclic nature of occurrence of shelfal canyon complexes clearly suggest periodic
spurt in the fluvial energy coupled with sea level fluctuations.
Data from a few wells drilled in the above discussed sediment induced tectonic setting
have indicated some very exciting hydrocarbon opportunities in the shelf and deep-water
The Geochemical Sniffer Surveys carried out by M/s INTEROCEAN of USA in part of
the Mahanadi shallow water area in 1988 have revealed the presence of a number of
Geochemical Sniffer anomalies. Basin modeling studies carried out by M/s BEICIP
of France indicated presence of mature source rocks in some of the onshore depressions,
suggesting similar possibilities in deeper parts of various depressions in the basin
. Presence of organic mater has been reported in different sequences in some of
the onshore and offshore wells. Temperature gradient in onshore and offshore wells
is found to be >2.5 0 C/100 m . Presence of structural and stratigraphic traps and
the above observations clearly indicate the hydrocarbon potential of the basin.
Significant hydrocarbon shows have been encountered in most of the exploratory wells
drilled in the Mahanadi Offshore basin. Gas recovered during Formation Interval
Testing (FIT) of Miocene sandstones and Eocene limestones in the well MND-2 contained
significant proportions of higher hydrocarbons including iC4. In addition, specks
of tarry matter were noticed in few sidewall cores from the Paleocene carbonate
sequence of MND-2. Recently, six exploratory wells drilled in Block NEC-OSN-97/2
by RIL have proved to be gas bearing within Upper Miocene to Pliocene sequence.
- SOURCE ROCK:
The onland well MON-2 drilled in the Cuttack Graben has a high content of TOC (upto
9.5%) in the Early Cretaceous sequence and the well MON-1 drilled in Paradip Depression
has adequate TOC of more than 2% in the Miocene sequence. A total thickness of 960
m of source rocks have been encountered in the well MON-2 with an average potential
of 7 Kg HC/ton rock (coal beds included). Most of the organic matter is of type
II and III. Maturation in the Cuttack Graben was obtained around 2300 m depth. This
indicates similar possibilities in the deeper parts of other depressions in the
Source rock studies on drill cuttings and sidewall cores from the wells drilled
in shallow offshore indicate presence of adequate organic matter (TOC 1.5-2.5%).
TOC values of more than 3% also have been recorded in some samples. The organic
matter within Paleogene sequence of well MND-2 has reached adequate maturity (Vro
of 0.55 at about 2800 m and 0.65 at about 3300 m. In the well MND-7, organic matter
within Paleocene and older sediments appear to be mature. This observation suggests
that better source sediments can be present in the deeper parts of the basin. The
organic matter in the Paleogene and older sequences in MND-5 and MND-6 shows marginal
maturity. Additionally, Rock-Eval studies show that migratory hydrocarbons are present
in the Miocene and older sediments of Mahanadi offshore wells.
- RESERVOIR ROCKS:
Sandstone reservoir facies have been encountered in the wells drilled in Cuttack
Depression in the onshore part of the basin. Average porosity in these sands is
in the range of 15 to 25%. The inter-trappean sequences with associated fractured
and weathered volcanic flows are also potential reservoir rocks.
The subsurface data in the shelf part of the basin suggest availability of good
reservoir rocks within Early Miocene and older sediments. Porous and permeable sandstones
and carbonates within Late Cretaceous, Paleogene and Early Miocene are the potential
Reservoir sands within Channel/ Levee complexes of Pliocene and Pleistocene and
Late Oligocene Wedge-outs against the Eocene Hinge in deep offshore part of the
basin are also the potential reservoir targets.
- CAP ROCKS
Presence of claystones and shales at different stratigraphic levels reported from
the drilled wells are likely to provide cap rock in the basin and in particular
a relatively thick sequence of Miocene comprised of claystones provides a good regional
cap rock for hydrocarbon accumulation.
- ENTRAPMENT/ TRAP TYPES
Structural And Fault Related Traps In Syn-Rift Cretaceous Sediments:
Presence of coarse-grained sandstones in the synrift Cretaceous sequence coupled
with availability of source facies within this sequence can be a very potential
situation for hosting hydrocarbons. The potential traps may be sealed vertically
by Paleocene / Eocene shales. Complex development of this sequence has brought out
diverse potential situations- local faulted anticlines, fault related structures
and positive erosional features over the Cretaceous surface .
Paleocene-Eocene Carbonate Buildups And Sand Bodies:
Carbonate buildups can be envisaged along/over the Eocene hinge. It is observed
that the carbonate bodies show distinctly less internal reflections in comparison
to the surrounding sequences. Faults within the underlying Cretaceous and Paleocene
sequences can act as effective migration pathways.
Paleocene To Miocene Wedgeouts And Pinchouts:
The 85 deg East Ridge to the southwest acts as a barrier onto which Paleocene, Eocene,
Oligocene and Miocene sediments onlap and can have entrapment potential. These units
may consist of thin sands, carbonates and shales terminating against the Ridge.
The eastward basinal tilt in post Late Cretaceous could have facilitated hydrocarbon
migration northwest from the basinal depocenters. Oligocene Wedgeouts against the
Eocene hinge can also have entrapment potential.
Submarine Canyon And Turbidite Fan Systems:
Submarine canyon and turbidite fan Systems are more characteristic of Pliocene-Pleistocene
sequences with slope failures caused by significant increase of sediment input into
the basin. Canyon fills can be interpreted as the Ganges delta builds into the basin
or as sediment forced during a highstand as the sediment supply outpaces the accommodation
space. Channel sands within the Canyons also can have potential entrapment conditions.
Thus, Mahanadi Basin- onshore and most significantly the offshore part is endowed
with a good combination of source and reservoir facies with potential entrapment
situations. The deep offshore part of the basin with the characteristic sediment
induced tectonics coupled with the envisaged good reservoir and source facies holds
high promise for challenge loving explorationists. Similar geological situations
in the neighboring Krishna- Godavari Basin already started paying rich dividends
to the explorationists.