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Shelf Delta to Deepwater Basin: A Depositional Model for Krishna-Godavari Basin*
By
Ravi Bastia1, Prasanta Nayak1, and Pankaj Singh1
Search and Discover Article #40231 (2007)
Posted February 21, 2007
*Adapted from extended abstract prepared for presentation at AAPG International Conference, Perth, West Australia, November 5-8, 2006
1Reliance Industries Ltd., Mumbai, India ([email protected])
Krishna-Godavari basin, located on east coast of India, is in a passive-margin
setting with a lateral (coastal) extent of 500 km, and it extends more than 200
km from the coast into the deep sea. This basin has been fed dominantly by
Krishna and Godavari river systems, along with numerous tributaries. The basin
represents a depositional setting of a 
well
defined shelf to shelf-edge delta to
deepwater. The area is adequately covered by several vintages of 2D and focused
3D seismic data which have been primarily used for subsurface imaging. Further,
the depositional units have been identified by interpreting seismic stratal
patterns and facies distribution in a sequence stratigraphic framework. This is
supplemented by information from wireline logs and cores. The gradational facies
pattern, sequence boundary, transgressive surface, maximum flooding surface,
channel architecture, etc. have been demonstrated in selected seismic sections
for developing the concept. The study is not only useful in understanding the
depositional processes in shelf/shelf-edge/deepwater and their linkages but also
as a good guide for the deepwater hydrocarbon exploration targets.
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Figures 1-4
The Krishna-Godavari basin (KG basin)
along the east coast of India (Figure 1)
covers the deltaic and interdeltaic areas of Krishna and Godavari
rivers and extends into the offshore. This stretch of sedimentary
tract contains a vast range of geologic settings, such as costal
basin, delta, shelf-slope apron, deep-sea channel, and deepwater fan
complex. The basin has emerged as one of the frontier areas for
future hydrocarbon exploration--after the multi-trillion cubic feet
supergiant gas discovery in the recent years. The basin has
significant hydrocarbon potential both in the Tertiary delta as
The basin was a major intracratonic rift within Gondwanaland until Early Jurassic. Exposures of Upper Cretaceous sedimentary rocks demarcate the basin margin toward the northwest (Rao, 2001). Onshore parts in the basin record a good account of Gondwana sediments. The basin evolved as a composite of rifted grabens, beginning in Late Jurassic, and formed a part of the development of the east coast divergent margin. The horsts and grabens were separated by vertical or steeply dipping faults. Since the Cretaceous, the basin has become a pericratonic rift basin. The initial rifting-drifting phase during this time generated fluvio-lacustrine sediments all over the basin. However, the transportation was over short distances, and therefore clastics of that time are devoid of sorting and are mainly argillaceous arkosic sandstones. The first marine incursion appears to have occurred during the Albian. The rift phase terminated by the end of Turonian in most parts of the basin, and subsequently, the post-rift sedimentary sequences prograded to the east with development of a continental shelf-slope system. The shelf areas received deposits of clastics and carbonate sediments while the slope registered deeper-water fan sediments. This setting, during which progradation was dominant, persisted throughout the Tertiary. The Paleocene and Eocene, in part, are considered to have been deposited during sea-level lowstands, thus forming fan complexes. From Oligocene onward, sea level began to rise, and more accommodation was available. In the Tertiary, the deepwater area became structurally deformed by numerous sets of growth faults and related features. In most areas, a major decollement surface is present near or at the top of the Eocene and marks a major tectonic event. A series of growth fault systems progressively developed, with increased sediment influx during the Oligocene through Miocene time. The onset of the Pliocene is marked by major sea-level fall and a prominent erosional surface. These lowstand conditions prevailed into the Pleistocene. The regional basement horsts (Bapatla, Tanuku, Kaza-Kaikalur, Kavali, Nellore and Nayudupeta) divide Krishna-Godavari basin into several sub-basins such as Pennar, Krishna, West Godavari, and East Godavari (Figure 2). These sub-basins contain thick Cretaceous and older sediments above the Archaean basement, with several intervening unconformities. The basin contains a 4-7-km-thick sediment column, ranging in age from Late Carboniferous to Holocene (Figure 3).
Deepwater sedimentation is likely to be most rapid during periods of sea-level lowstand because depocenters are most likely to be located at or near the shelf edge. However, deepwater sedimentation also occurs during highstands of sea level. Canyons that extend across the shelf and capture fluvial flow are active feeders as deepwater systems form during relative sea-level highstands (as in case of Congo River).
The facies and
Several dip-oriented seismic profiles (Figure
5) are selected to link the shelf-edge delta systems of Krishna
and Godavari deltas to the deepwater depositional systems.
Interpretation processes involve identification and sampling of
relevant seismic lines showing depositional features of different
geological age, identification and marking of reflection geometries
and patterns, establishing major bounding surfaces, identification
and marking of major bounding surfaces on deepwater depositional
features, and developing a
The seismic line is chosen in a way that
reflects the slope from shelf to deepwater and is oriented NW-SE. Since
the seismic reflectivity data was non-optimal in continuity, the phase
section has been used to identify seismic reflection geometries,
truncation patterns, and hierarchy of major bounding surfaces in a
seismic The interpretation (Figure-6) aims to bring out spatial and temporal relationship between highstand delta system, shelf-edge delta system, and slope/basin-floor fan systems. The
stratal pattern showing erosional cut defined by a truncation boundary
at the base and onlapping fill is indicative of incised valley. In
correlation, the base of incised valley was carried along the
truncations to define the
The downlap reflection terminations onto basin
floor fan and onlap to the Tracing the last downlap event on basin floor fan updip to the shelf will mark the maximum flooding surface (MFS). Sandwiched between the first transgression at the shelf edge and maximum flooding surface is the transgressive system tract (TST) indicated by backstepping – the termination of events. Incised valleys tend to be filled during early transgressive time and to be overlain by finer clastics during late transgressive time.
The highstand delta system represents the time
when the rate of relative sea-level rise slowed down and with increased
sediment supply shoreline started to regress, as indicated by prograding
clinoforms downlapping on the maximum flooding surface. Interpretation
of a highstand delta, in general, is difficult and tentative, owing to
cannibalization by the next lowstand depositional cycle. This framework
can further be used to analyse the facies distribution within the
This 3D seismic section is through a deepwater
This dip-oriented seismic line is located
north of the Krishna Delta connecting the shelf to the upper and lower
slope. Interpretation is made only for Late Miocene-Pliocene The
surface H1 can be interpreted as a
This seismic line is located near to mouth of the present-day Krishna River. Seismic section shows the shelf-edge delta system and lowstand fan complex. The lowstand fan complex consists of slope fan and basin-floor fan deposits. In the proximal part of the slope fan complex, subaqueous, high-sinuosity, sand-rich channel with associated levees may be present, although the whole system of slope fan complex is likely to be mud-rich.
The seismic line is located in the KG onland basin, near to the Krishna River. The line shows the shelf-edge-deltaic, prograding clinoforms of Cretaceous-Paleocene age. The downstepping clinoforms indicate subaerial erosion and sediment bypass to the slope/basin floor. The bright amplitude of foresets at the shelf edge indicates the presence of coarser clastics at the mouth of the delta.
This seismic section is from the Krishna delta and shows a lowstand shelf-edge delta system. The younger canyon system cannibalized the shelf and subsequently dumped these sediments on the basin floor as a basin-floor fan complex.
This seismic section shows the features from
shelf to deepwater. Typical reflection geometries defined either by
bimodal downlap on a Key to identify the shelf-edge delta system was to identify prograding and downstepping reflection characteristics at the shelf edge.
Conclusion
Burgess, P.M., and Hovius, N., 1998, Rates of delta progradation during highstands: consequences for timing of deposition in deepmarine systems: Journal of the Geological Society, London, v. 155, 1998, p. 217-222.
Posamentier, H.W., and Allen,
G.P., 1993a, Variability of the
Posamentier, H.W., and Vail,
P.R., 1988, Eustatic controls on clastic deposition II— Rao, G.N, 2001, Sedimentation, stratigraphy, and petroleum potential of Krishna-Godavari basin, East Coast of India: AAPG Bulletin, v.85, no.9, p. 1623-1643.
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