SUBHASHIS SENGUPTA, ONGC, Mumbai, India, O P SRIVASTAVA, ONGC, Mumbai,
India, S CHANDRASHEKAR, ONGC, Mumbai, India.
[email protected], [email protected]
Hydrocarbon Potential of Deep Water Plays, Kutch Basin,
India
ABSTRACT:
The Kutch Basin located on the Western Continental margin covers an area over 80,000 skm
both on-land and off-shore. The on-land part of the basin has been extensively studied and the
basin is believed to be a part of a poly-history pericratonic rift fill basin which has evolved
since Early Mid-Jurassic (Aalenian) times and matured as a classic passive margin system.
ONGC began exploration activities in Kutch Mainland from 1956 with the initiation of field
mapping which was followed by seismic campaigns. Since then, five on-land wells and more
than forty off-shore wells have been drilled which have revealed a plethora of subsurface
geological information. Two deep water wells have been drilled North of Saurashtra Arch to
test Paleocene – Eocene carbonate mounds and deep-water channel splays. No significant
hydrocarbon shows were observed during drilling of these wells. Subsequently 3D seismic
data has been acquired over the Saurashtra Arch. The interpretation of this data has brought
out several new plays in the area. A major channel – levee – complex (CLC) was identified in
the area having a proximal source for sediment and thus suggesting possible presence of
coarser clastics in the channel load. Several other play types identified include carbonate
build up, mass transport complexes and an interesting unconformity play. The hydrocarbon
source potential has always remained a matter of conjecture. However the discovery of
hydrocarbon below the Deccan Trap basalt over the Saurashtra Arch in shelf area has
established the presence of a Mesozoic petroleum system. Collaborative studies undertaken
with different agencies have indicated possible source potential in the area. Recent success in
shallow water proving the presence of both oil and gas below the Deccan trap in Kutch basin
has clearly indicated the Mesozoic sediments as the source of commercial hydrocarbon finds.
Possible extension of Mesozoic basin in deep water area and presence of deep-seated faults
add value to the endeavour in exploring for hydrocarbon in Deep Water Kutch Basin.
KEY WORDS: Channel-levee-complex, Saurashtra Arch, Mesozoic petroleum system.
INTRODUCTION:
The Kutch Basin located on the Western Continental margin covers an area over
80,000 SKM both on-land and off-shore. The on-land part of the basin has been extensively
studied and the basin is believed to be a part of a poly-history rift fill basin which has evolved
since Early Mid-Jurassic (Aalenian) times and matured as a classic passive margin system.
The earliest geological field work in Kutch-Onshore area was undertaken by Wynne and
Fedden in 1872 and coincidentally ONGC spudded in its first wildcat well in Kutch onshore
(Banni-2) one hundred years later in the year 1972. A decade later hydrocarbon was
discovered in the well KD-1 (1983) with the finding of oil in Eocene calcareous-sandstone.
At the turn of the millennium deep water exploration of Kutch offshore basin was undertaken
(Fig 1). During 2004-2005 two deep water wells were drilled.
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BASIN EVOLUTION:
The offshore Kutch Basin is a poly –history rift and passive margin basin which has
evolved through multiple rift phases after the separation of the Indian and African plate. The
rifting episodes were initiated in late Triassic and continued in phases up to end of
Cretaceous.
During Permian period a significant rifting affected the Indian plate assemblage
(Hercynian orogeny). The Karoo rift system evolved during this time and was filled with
fluvial-lacustrine deposits. The main feature distinguishing Permo-Triassic orogeny is the
opening of Madagascar basins & other Gondwana basins of East Africa. Rifting led to
formation of network of deep rift basins (Majunga, Morondova and Ambilobe basins) in
West Madagascar which are considered as features formed due to initial break up East and
West Gondowana. Permian traps have been dated in well Marvi-1 in Pakistan. Deeper basalt
reported from well Lodhika-1 in Saurashtra is also believed to be of Permo-Triassic age. The
Panjal volcanics may have been caused due to this phase of rifting. Rift axis passes through
offshore Kutch and Saurashtra basin and may open up a new frontier in terms of an envisaged
Permo-Triassic basin in offshore Kutch and Saurashtra.
During Early to Middle Jurassic the North – South rift continued to further widen with
gradual marine incursion from North to South. All along the strike from Madagascar through
Seychelles, Kutch and Rajasthan dominant clastic facies was deposited in Fluvial to Marine
environment. The Datta Formation, Sinawari Formation and the Samarasuk Formation in
Upper Indus basin show a shallow marine environment of deposition. In Kutch basin the
Nirona Formation and the lower part of Kaladongar Formation show fluvial set up. During
Late Jurassic the Upper indus Basin went through a phase of non-deposition. The Baishaki
and Badesar Formations in Jaisalmer basin show marginal marine to near shore environmrnt
of deposition. In Kutch Basin the Juran Formation indicates a deltaic to marginal marine
environment.
The Early Part of Cretaceous is dominated by a sudden increase of fluvial sediments
due to the uplift of the leading Western Indian margin as a result of the separation of
Australia and Antarctica from the trailing edge at this time. The Himmatnagar Formation in
Cambay Basin and Bhuj Formation in Kutch Basin are deposited under fluovio-deltaic
condition with intermittent marine incursions. The Late Cretaceous period is marked by the
separation of Madagascar and northward drift of India and Seychelles. The Mascarene basin
opened up during this period. The sea floor spreading centre jumped north eastwards from
Mascarene Basin separating greater India from Seychelles. Around 65 ma India moved over
Reunion hotspot, which is marked by effusion of Deccan Basalts. Indian plate moved in a
northwards direction at a very fast rate of 20cm/year towards the Eurasian plate.
During Paleocene and Eocene the Indian plate continued its Northward journey. On
Kutch on-land basin the Matanomadh and Naredi formations were deposited under lagoonal
to near-shore environment whereas in the offshore basin the Nakhtrana and Jakhau
formations were deposited in near-shore environmental condition. During late Eocene the
Indian plate underwent a soft collision with the north-eastern margin of the Indian Plate
docking first into the Eurasian Plate. Subsequently subduction commenced below the Tibetan
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and Burmese plates. Rate of NE-ward movement of the Indian plate reduced from an initial
20 cm/year to 10 cm/year between 60 Ma to 45 Ma (Mid Eocene). During Oligo-Miocene
period the maximum rise of the Himalayas was noted. The uplift probably led to a shallowing
up and a high input of clastic in all the basins in the western part of Indian plate. The general
stratigraphy of Kutch Basin is shown in Fig 2.
CRUSTAL STRUCTURE OF DEEP WATER KUTCH BASIN:
The crust underlying the Kutch Basin from shallow water near coast area to deeper
basin area is likely to consist of continental crust, transitional crust and oceanic crust (Naini,
B.R. et al, 1983). To help in understanding the nature of the crust in Kutch Offshore basin an
estimate of the extent of continental crust was attempted using gravity anomaly data. Fig 3
shows a smoothened Bouguer gravity map of the area. On the basis of the maximum rate of
change of gravity anomaly an attempt was made to interpret the extent of continental crust
below the Kutch offshore basin. Fig 4 shows the interpreted gravity map with all major
structural elements overlain and the possible continental oceanic boundary. It is evident from
Figure 4 that all major structural elements can be constrained with the help of gravity data.
Laxmi Basin shows a high gravity value while the Laxmi Ridge and the Bombay high area
show relatively low gravity values. The presence of a major transform North of GKDW-1
PEL block is established (Miles, P.R et al, 1998). The transform towards south is interpreted
on the basis of gravity and crustal thickness data (Fig 5). The COB (continental-oceanic
boundary) is likely to extend beyond the deep water area of Kutch basin. The presence of
continental crust below the deep water part of Kutch Basin augments the chances of
hydrocarbon find in the area emanating out of a Mesozoic – Tertiary petroleum system.
DRILLED WELLS:
Well GKA-1 was drilled at a water depth of ~1800m to explore the hydrocarbon
potential of Paleocene and Eocene carbonate build up (Fig 6). The well was drilled down to
the Deccan traps up to a target depth of 6050m. The well encountered seven basalt flows with
inter-trappean sediments late Cretaceous age while drilling 500m of Deccan Trap thickness.
The well did not probe beyond the Deccan basalt flows. The Miocene sediments encountered
in the well were essentially clay-stone whereas the Eocene and Paleocene sediments were
primarily fossiliferous limestone (Fig 7). Oligocene sediments were absent indicating a
hiatus. Eocene limestone section encountered in the well showed development of vuggy
porosity and also showed effects of leaching leading to secondary porosity development. (Fig
8) However the well was devoid of any significant hydrocarbon shows.
Well GKC-1 was drilled at a water depth of 2035m to explore the hydrocarbon
potential of a channel splay feature which was mapped as a high amplitude reflection package
(HARP) on 3D seismic data. (Fig 9) This well was drilled down to early Eocene level upto a
target depth of 4624m. Deccan trap basalts were not encountered in this well. Pliocene and
Miocene sediments encountered in the well were essentially claystone. The well penetrated a
late Miocene-Pliocene deepwater channel section wherein it encountered sandy facies
corresponding to channel fill sediments. The porosity of these sand fill were as high as 20%
to 23%. The Miocene channel splay feature encountered consisted mainly of silty-claystone
facies without significant development of any good reservoir facies. Eocene limestones were
encountered in the well. Eocene sediments represented a condensed section with a thickness
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of only 32m. Two distinct high gamma greyish black shale layers were developed in early
Eocene section. These two shale layers showed good TOC value corresponding to type-III
kerogen content. However these sediments were immature. This well too was devoid of any
significant hydrocarbon shows.
POTENTIAL PLAYS IN KUTCH DEEP WATER:
(I) CHANNEL – LEVEE COMPLEX
The Miocene and Pliocene section of Kutch deep water basin are characterized by remarkable
channel – levee complexes (CLCs) often of huge dimensions (Fig 10 to 14). These CLCs
have been studied in details and their geometries deciphered using 2D and 3D seismic data.
Initially the CLC trends were picked up from 3D seismic data through time and horizon
slicing methods. These trends were picked in 2D seismic data and the extension of the
channels mapped in detail. (Fig 12 & 13) A total of 13 major channel levee systems can be
mapped from 2D (10 X 10 km grid) and 3D seismic data in the area and followed for more
than 200 km in the Deep water Kutch basin. The channel-levee complexes stand out as
relatively high relief features with the maximum widths across channel-levee complexes
varying from 8km to 35km. The typical relief of these CLCs varies between 150m to 800m.
(Fig 14) Isopach maps and seismic amplitude maps clearly depict the meandering pattern of
the channel axis whose width varies from 0.3km to 0.6km (Fig 15). Upper fan submarine
channel towards North of the area follow the topographic lows which are developed amongst
the channel-levee accretions. The development of fan complexes mostly follows the trend of
channel avulsions where splays are formed due to the breaching of pre-existing channel-levee
complexes. This breaching of channels is probably due to the high sediment influx from
higher reaches towards north and north-east. Once the channels are breached the excess
sediment load accumulates as over-bank deposit known as splays or inter-channel lobes
(Slatt, R.M., 1988). These splays have also been described as High Amplitude reflection
packages as they appear to be amplitude bursts on seismic profiles. These are interpreted to
be sand-prone facies on the upper fan. Some sand rich facies are also expected to be
developed where the channel axes sediments are vertically stacked. These are depicted in
seismic sections as high amplitude discontinuous facies. The well GKC-1 drilled in the area
penetrated such a channel stack system in Pliocene level. At the Miocene level the well
penetrated a mapped splay system; however the reservoir facies was poorly developed.
A channel-levee-complex has been mapped in the KS4 3D volume which can be
followed on 2D lines for more than 200km. up to the Kutch shelf margin. In comparison to
the other CLCs this has a proximal source and is expected to be sand rich. (Fig 16 & 17)
Amplitude attribute study of this channel-levee complex shows the channel fill sediments to
have distinctly higher amplitude values thereby indicating presence of coarser clastics as fill
material. The presence of east-west trending faults across the CLC feature may provide updip seal and thus help in the entrapment of hydrocarbon. (Fig 18)
(II) PALEOCENE – EOCENE CARBONATES
During Paleocene period huge isolated carbonate platforms have been deposited over
an extensive shelf area under shallow water conditions. During early Miocene the basin
experienced a remarkable drowning event marked by the collision of the Indian plate with the
Eurasian plate. A major part of the platform area lying west of the Miocene hinge line
witnessed a major transgression and went into the realm of deep water environmental setting.
This drowning event resulted in the sudden closure of the carbonate factories and
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development of a distinct drowning surface over the carbonates. (Fig 19) Subsequently a
huge thickness of Mio-Pliocene Indus derived sediments covered the entire area.
Some of these isolated carbonate platforms have under gone sub-aerial exposure
during late Eocene and have developed secondary porosity in terms of karst layers. (Fig 20 &
21) These karst structures can be seen on seismic attribute slices and time slices. There is
every possibility that these layers may act as excellent reservoir facies for accumulation of
hydrocarbons. The adjacent lows to these carbonate platforms could act as the possible areas
for generation and short distance of hydrocarbons.
(III) PALEOCENE UNCONFORMITY
A distinct unconformity surface is seen at the Cretaceous – Tertiary boundary towards
the east in the area of study over the Saurashtra Arch. In view of the hydrocarbon find below
the Deccan traps in the Saurashtra Arch area this unconformity play at the K-T boundary
becomes a potential area for the exploration of hydrocarbon. Hydrocarbon has been
discovered after drilling more than 2400 Deccan Trap Basalt. In the GK-28 area hydrocarbon
has been established both in Mesozoic and Tertiary sections.
Due to lack of proper long offset data set in the intervening area the exact correlation of
Deccan Trap bottom becomes difficult. A proper estimation of Deccan Trap thickness
becomes nebulous in this context. (Fig 22 & 23) The area covered by the angular
unconformity surface is of the order of 100 SKM. Paleocene and Eocene carbonates are
developed over the unconformity surface and may also act as reservoir facies for any
hydrocarbon generated in the area. On the basis of gravity data interpretation in the area it
appears that the presence of Mesozoic sediments in the area cannot be ruled out. This
scenario makes the unconformity play interesting form an exploratory point of view.
CONCLUSION
The Kutch deep offshore basin is a relatively under explored area and has a potential
for future oil and gas discoveries in the clastic reservoirs of the CLCs developed from the
proximal slope. Only two wells have been drilled in an area of over 20,000 SKm. No well has
been drilled west of the Dwarka fault and close to Kutch Low (Fig 6). The huge carbonate
isolated platforms developed towards west of the area have good potential for hydrocarbon
accumulation being placed proximal to the Kutch Low area. The Karst layers developed
within the carbonate mounds add value to the hydrocarbon potential in terms of good
reservoir facies. The unconformity play developed towards east of the area over the plunge of
the Saurashtra Arch appears to be interesting. However, better imaging techniques below the
Deccan traps basalt will help in firming up this interesting play.
ACKNOWLEDGEMENT:
The authors are indebted to Shri G C Kathiar, ED-Basin Manager WOB, for his constant
encouragement. The authors are grateful to Shri K R Nambiar for his critical review of the
paper.
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REFERENCES:
Biswas, S.K., 1982, Rift basins in Western margin of India and Their Hydrocarbon Prospects
with special Reference to Kutch Basin: The American Association of Petroleum Geologists
Bulletin, Vol. 66, No. 10, p. 1497-1513.
Miles, P.R., Munschy, M. and Ségoufin, J., 1998, Structure and early evolution of the
Arabian Sea and East Somali Basin, Geophys. J. Int. 134: 876–888.
Naini, B.R. and Talwani, M., 1983, Structural framework and the evolutionary history of the
continental margin of western India, Proc. of the Hollis Hedberg Symposium, Galveston, TX,
January 12–16, 1981, Studies in Continental Margin Geology, AAPG Memoir 34, pp. 167–
191.
Slatt, R.M., J.M. Boak, G.T. Goodrich, M. B. Lagoe, C. L. Vavra, J.M. Bishop and S.M.
Zucker, 1988, Depositional facies, paleoenvironments, reservoir quality, and well log
characteristics of Mio-Pliocene deep water sands, Long Beach Unit, Wilmington Field,
California, in A.L. Lomando and P.M. Harris, eds., Giant oil and gas fields: A core
workshop: SEPM Core Workshop No. 12, p. 31–88.
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FIGURES:
KUTCH
SHALLOW
WATER AREA
Study Area
KUTCH DEEP
WATER AREA
Fig 1: Areal extent of Kutch Basin with offshore bathymetry definition
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Fig 2: Generalised stratigraphy of Kutch Basin
Fig 3: Bouguer gravity anomaly map with ONGC Deep water block GKDW-1.
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Fig 4: Bouguer gravity map with overlain structural elements; showing the interpreted
continental oceanic boundary (COB) with white line
Fig 5: Seismic line showing crustal configuration below Saurashtra Arch, Laxmi Basin and
Laxmi ridge.
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Fig 6: Map at Deccan Trap Basalt top showing well location and major features mapped
GKA-1
Fig 7: Well GKA-1 drilled in Carbonate Build Up
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Fig 8: Algal limestone from well GKDWA-1 showing development of good vuggy porosity
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Fig 9: Well GKC-1 penetrated a HARP feature (channel splay)
Fig 10: Miocene channel trends from 2D and 3D seismic data
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Fig 11: Miocene and Pliocene CLCs as seen in 2D seismic data
Fig 12: Miocene CLCs as picked up in 2D seismic lines
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Fig 13: Pliocene CLCs as picked up in 2D seismic lines
Fig 14: CLCs developed at different stratigraphic levels
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Fig 15: Isopach map of KS4 channel with maximum thickness of sediments up 400m
Fig 16: Indus channels and identified proximally sourced channel
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Fig 17: 3D visualization of proximal KS4 CLC
Fig 18: Amplitude attribute indicates presence of coarser clastics as channel fill
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Fig 19: Isolated carbonate platforms with drowning unconformity surface
Fig 20: Karst layer as seen in seismic data
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Fig 21: Karst layers identified on time slice
Fig 22: Shallow to deep water seismic line showing potential unconformity play
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Fig 23: Unconformity play
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