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CENOZOIC SEQUENCE STRATIGRAPHY OF KRISHNA

MOUTH AREA AND BEYOND IN EASTERN OFFSHORE, IN-

DIA WITH SPECIAL REFERENCE TO DEPOSITIONAL SET-

TING AND HYDROCARBON HABITAT IN MIOCENE SEDI-

MENTSA. K. Das R.P.Gupta R.HussamS.N.MauryaBasin Studies Division, KDMIPE , ONGC, Dehradun - 248195

Abstract:

In the light of gas strike in Early Miocene clastic reservoir, the Krishna Mouth and its

adjoining Indian offshore area is considered as a possible thrust area for future exploration. The

area was mapped with the emphasis on understanding depositional setting and hydrocarbon habitat

in Rawa Formation of Miocene age using 1200 line tan of seismic data and information from ten

exploratory wells . A total of five major seismic sequences has been mapped in the Cenozoic

section , Paleoshelf edges at different stages of basin evolution have been outlined from seismic

data. Vadaparru -Rawa petroleum system is considered as the most effective one in the offshore

areas. Sands within Rawa Formation are the potential reservoirs and are expected to be sourced by

Vadaparru Shale ( Eocene- Oligocene ) and lower part of Rawa Formation. NE- SW trending arcu-

ate listric faults hading basinward played vital role in hydrocarbon migration and accumulation. A

deep water turbidite fan has been delineated within Late Miocene sequence, which may stand out

as an important target for hydrocarbon exploration.

Keywords : Turbidite fan complex, Rawa Formation, Krishna Mouth, Krishna-Godavari Basin.

Introduction:

The Eastern Offshore of India has a num-

ber of oil and gas bearing structures located in

shallow as well as in deep waters . In shallow

waters off Krishna Mouth, well G has produced

gas from Early Miocene elastics, affected by

Fig 1. Location map showing study area

growth, faults. The subsurface data of the area

covering this discovery and extending further

south in the offshore (Fig. 1) have been analysed

to work out the Cenozoic stratigraphy and to

evaluate the hydrocarbon prospects.

Review of Previous Work :

Ravikumar et. al. ( 1984) interpreted seis-mic data from Krishna Mouth to Kakinada Bay

and mapped a number of structural features in

Krishna Mouth area Rangaraju (1987) empha-

sized on understanding the role of turbidites in

petroleum exploration . He discussed about con-

cepts and methods of recognizing them with the

help of a few examples from Krishna- Godavari

Basin . The two approaches to infer a turbidite

basin or turbidite system in a basin are (1)

paleoshelf edge mapping and (ii) recognition ofcanyon on shelf and slope. Prabakaran and

Ramesh (1991) made an in- depth study of pa-

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leogeography of Krishna-Godavari Basin through

different geological ages. Das et. al. (1995) re-

viewed the seismic data of Krishna Mouth area

alongwith other geoscientific information and the

present paper is broadly the outcome of their

work .

According to Samanta et. al. (1994) lower

part of Rawa Formation and Vadaparru shales

are the potential source rocks in deep offshore

areas and the growth faults play a major role in

hydrocarbon migration and entrapment.

Cenzoic Stratigraphy:The available 2D seismic data have been

interpreted with the help of subsurface geologi-

cal input from ten exploratory wells drilled inonland and shallow waters (Table 1),

Lithostratigraphic nomenclature (Fig.2) proposed

by Venkatarengan et. al. (1993) has been adopted

with some modifications (Table H). In the land

part, some of the litho- units are time transgres-

sive (Nimmakurru Sandstone and Rajahmundry

Sandstone) and therefore, they do not exactly

correspond with their basmal colmter-parts

(Vadaparru Shale, Rawa Formatian and Godavary

Clay ).

A total of five seismic sequences have

been identified between Cretaceous top and Mio-

Pliocene unconformity (Figs. 3, 4, 5, 6). Paleo-

shelf edge positions at the end of Cretaceous, at

the end of Paleocene and at the beginning of

Miocene demarcated with the help of seismics

are shown in Figs .7, 8, 9. The Cretaceous top ,

marked by a regional unconformity, formed the

base for Cenozoic sedimentation and acted as

decollement surface for the growth faults devel

Fig 2: Generalised Stratigraphy, Krishna-Godavari Basin

oped in the overlying younger sediments. The

mapped sequences are described below .

Paleocene ( Sequence I ) :

Paleocene section is represented by

Razole Volcanics and Palakollu Formation in the

shelf part and by only Palakollu Shale in the ba-

sinal part. Razole Volcanics do not extend be-

yond Mantripalem, to the northwest of study area

, where it is only 25m thick . Palakollu Forma-

tion is composed of moderately thick limestone,

shale and sandstone in the shelf part. Beyond

shelf edge, the sequence is dominantly shaly and

increase in thickness in the basinal direction

(Fig.7). Deposition of sands attaining a thickness

of around 400m on the slope regime due to tur

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Fig 3: Seismo-Geological Section Along Seismic Line X-Y

Fig 4: Part of Interpreted Dip Seismic Profile AA

Fig 5: Part of Interpreted Dip Seismic profile BB

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bidity currents (?) has been inferred in the area

wells E to F (Fig. 10). However, presence of

Paleocene and Eocene sections has been estab-

lished in the well E- I by log and seismic corre-

lation (Das et. al, 1995), which were earlier in-

cluded in Miocene (Raju, 1982) without suffi-

cient paleontological control.

Eocene- Oligocene (? ) ( Sequence 2 ) :

This section is lithostratigraphically pre-

sented by lower part ofNimmakurru Sandstone (

Early Eocene-Early Miocene ) in the shelf area,

the upper age limit of which may well extend

upto Middle Miocene . Beyond shelf edge, the

sequence is dominantly shaly and named as

Vadaparru Shale for the Eocene section

(Venkatarengan et.al, 1993). Considering thesubjectivity of the Oligocene interval identified

by laboratory studies in well 1' and its reported

absence in well F, both Eocene and Oligocene

have been included in one seismic sequence (Seq.

2), the top and bottom of which have been fixed

with the help of log and seismic features (Das et.

al , 1995). Fig, 8 indicates a few depositional

lows trending NE- SW in the area beyond shelf

break. Development of small, subaqueous fan is

observed in the area of wells E- I and E- 2.

Fig 7. Isopach of Paleocene (Sequence I)

Fig 8: Isopach of Eocene (Seqence II)

Fig 6: Part of Interpreted Strike Seismic Profile CC

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Miocene (Sequences 3,4,5):

Lithostratigraphically , the Miocene se-

quence in the shelf area is represented by upper

part of Nimmakurru Sandstone and lower part

of Rajahmundry Sandstone (Late Miocene-

Pliocene), Narsapur Claystone being remarkablyabsent. Beyond shelf break, the sequence is

named as Rawa Formation. Isopach and com-

posite sand thickness map for total Miocene

(Figs. 9, 12) indicate a few fan lobes and reduc-

tion in thickness due to Post- Miocene erosional

cut. Considering the significance of this sequence

as potential reservoir and cap rock facies, it has

been mapped as combination of three different

seismic sequences (Seq. 3, 4 and 5). Sequence 3

Fig 9. Isopach of Total Miocene(SEQ. 3+4+5)

Fig 11:Composite Sand Thickness Map of Eocene(Seq 2)

is within Early Miocene and includes well G

paysand. The northern limit of this sequence is

marked by a NE- SW trending faults (Fig. 14).

The sequence is predominantly composed of

sandstone with minor shale.

Sequence 4, the next overlying unit in-cludes the remaining part of Early Miocene upto

Middle Miocene and persistently present over the

area with sizeable increase in thickness in the

basinal part. The sequence is made up of mainly

sandstone with shale. In the deeper parts, the top

of the sequence is marked by the Mio-Pliocene

unconformity (Figs.3,4,5,6).

The Late Miocene sequence has been

Fig 10.Composite Sand Thickness Map of Paleocene

(Seq -1)

Fig 12.: Composite Sand Thickness Map of Total

Miocene (Seq. 3+4+5)

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mapped as Sequence 5, top of which is marked

by a widespread erosional unconformity of Mio-

Pliocoie age. Within this sequence, an additional

sequence (Seq. 5A), sitting over the base of Late

Miocene, has been mapped in the area south of

wells G and F (Figs. 4,5,6,15). This sequence

(Seq.5A) is characterised by a reflection free zone(arenaceous fades) having oblique relationship

with the over-and underlying units and inter-

preted to have deposited as canyon related tur-

bidite fan complex. Time interval map of this

sequence indicates a number of depositional

lows with maximum thickness of 500 ms TWT

(Fig. 16). The sediment input for the fan is from

Fig 13. Time Structure Map at the base of Miocene

either northwest or northeast through canyon

mouths, close to Middle Miocene shelf edge ;

although canyons could not bemapped dueto seis-

mic data quality.

Pliocene- Recent :

The Pliocene sequence is represented by

upper part ofRajahmundry Sandstone and its

fades equivalent Godavari Clay. The sediments

are deposited over a maior erosional surface de-

veloped at the end of Miocene A number of cut

and fill features are quite apparent within this

Fig 14. Time Structure Map at the top of Seq 3(Level

close to well G pay zone within early Miocene)

Fig 15. Time Structure Map at the top of Seq 5 (Canyon-Related Turbidite Fan Comlex)

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sequence.

Depositional Model:

Towards the end of Cretaceous, the south-

easterly tilt of the Krishna-Godavari Basin and

development of a well defined basin margin -shelf-slope systemwerewell developed. During

that period, a major part of the study area was in

the slope regime resulting in deposition of thick

basinal shales. However during Paleocene, with

further fall in sea level, shelf edge steadily

prograded towards southeast exposing a substan-

tial part of the shelf area (Figs.7, 8). South-east-

erly flowing streams constituted a network of

delta system which prograded basinward. Mod-

erately thick limestones, sandstones and minorshales are the lithotypes in the shelf regime dur-

ing this period, which grade into thick monoto-

nous shales. Deposition of thick sand bodies in

the slope regime due to turbidity currents (?) is

envisaged in the area between wells E and F

(Fig. 10).

During Eocene, position of shelf edge had

shifted maiginally towards south-east (Figs.8 ,9).

The sandstone, limestone and claystone gradu-

ally grade into thick, monotonous shales in the

deeper part. Development of deepwater

sandbodies is observed in the well E-l (Fig. 11).

During Miocene, there is rapid

progradation of shelf to the south-east with fur-

ther fall in sea level (Figs 9, 17). Dumping of

elastics by river distributaries had resulted in

active channelling and submarine canyon forma-

tion. The canyon is generally associated with a

fan system on the downslope. One such feature

(Seq. 5A) is observed and mapped in the areabeyond wells G to F (Figs. 15, 16, 17).

The sedimentation pattern over the area

is governed by arcuate, listric, normal faults,

which have originated due to shale bulging in

the basinal part during Late Paleocene - Eocene

and onwards. The general trend of these faults is

NE-SW and they hade towards south to south-

east. Time structure maps (Figs. 13, 14) indicate

a number of small roll over structures associatedwith these growth faults.

Hydrocarbon Prospects:

Results of source rock studies carried out

in six wells are shown in Table III. In general,

source rock intervals capable of hydrocarbon gen-

eration are rare. However, the interval 2335-

2475m within lower part of Rawa Formation in

well G and interval 3675-3900m in well E-l

are considered to have marginally better source

potential. Also the Eocene and Paleocene shales

may develop into better source rock fades in the

downdip areas (in the regime of higher geother-

mal gradient) (Samanta et. al, 1994).

Fig 16. Time Interval Map of Seq 5A (Canyon Related Turbidite Fan Complex)

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Fig 17 A,B Depositional Model for Miocene Sediments

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Fig 18. Bathymetric Contours for the area of Canyon-Related Turbidite Fan Complex (Seq 5 A)

Fig 19. Standard Litho-Column and Exploration Plays in Study area

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In this area, the Rawa Formation is the

most prospective one, which has both reservoir

as well as cap rock facies. The NE-SW trending

listric normal faults hading basinwards are likely

conduits for hydrocarbon migration. Roll over

structures on the down thrown sides of these

growth faults are the favourable sites for accu-mulation of hydrocarbons (Figs. 13, 14),

An interesting stratigraphic feature is the

canyon related turbidite fan complex within early

part of Late Miocene mapped in the area south

of wells G and F ,close to the eroded Middle

Miocene shelf break which merits further explo-

ration (Figs. 15, 16). The fan body is oriented in

NE-SW direction covering an area of about 100

sq. km. and the water depth ranges between 200mto more than 500m (Fig. 18). Exploration plays

in the study area are summarised in Fig. 19.

Conclusions: A total of five seismic sequences (Seq. I to 5)

have been mapped in the study area within

the Cenozoic section. The seismic markers

correspond to Cretaceous top, Paleocenetop,

Eocenetop, a level (within Early Miocene)

close to well G pay zone, Middle Miocene

top and Mio-Pliocene unconformity. All

these markers have been integrated with geo-

logical and log data.

The role of paleo-shelf edge progradation in

sediment dispersal pattern through the geo-

logical times from Cretaceous end upto be-

ginning of Pliocene has been brought out.

A large part of the study area is affected by

shale tectonism of Late Paleocene-Eocene

age which had further continued during Neo-

gene, when the sediment supply was faster.Arcuate listric faults having NE-SW trend

and hading south-eastward have allowed

greater sediment influx during Miocene and

Pliocene in the basinal directions.

Structural features associated with growth

faults at the Miocene Base and at top of a

level close to well G pay zone (within Early

Miocene) are of interest for future explora-

tion.

A canyon related turbidite fan complex hasbeen delineated in the deepwater area , south

of wells G and F, which may be another

exploration target.

Acknowledgement:The authors are thankful to Director (Ex-

ploration), ONGC for his kind permission to

publish this paper. They wish to record their deep

sense of gratitude to Shri Kuldeep Chandra , ED(R&D), A. E. Ahmed, G.M. (Geol), S.Sahai,

G.M. (BRG) , B.K. Varma, Dy. G.M. and N.K.Lal

, Dy.G.M. for their keen interest in the work and

encouragement. They are highly indebted to Shn

R. Venkataroigan, Dy G M ., Dr. G . N. Rao CO

and Shn U. B. Samanta, SC for offering many

valuable technical comments.

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