Bombay Basin

8/4/2019 Bombay Basin

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Sedimentary Basins Of India

The total sedimentary cover in India up to 200m isobaths, spans 1.79 million

km2.This increases by an additional areal extent of 1.35 million km

2if one

considers the area beyond 200m isobaths in the offshore. Thus, the total area of

sedimentary basins accounts for 3.14 million km2. Twenty six sedimentary

basins have so far been identified based on geologic and tectonic considerations.

Out of the total sedimentary area only 0.626 million km2

(20%) area is

moderately to well explored, and around 1.13 million (36%) km2

remains poorly

explored to unexplored (source: DGH). Balance part is under different stages of

initial exploration. As per records by Directorate General of Hydrocarbons,

(2005-06) exploration & exploitation of oil & gas in India is being carried out

by more than 20 companies in public and private domains. ONGC tops these

with approx 54% of the total PEL area. In India, hydrocarbons occur across a

large temporal range of geological setup from Cambrian (Rajasthan basin) to

Cenozoic (Cambay, Assam, Mumbai offshore), however, a majority of oil and

gas occurrence, and their exploitation has been from Cenozoic sediments, both

on land and in offshore. The Mesozoic, in India, was either inadequately

deposited or found at such depths that are difficult for production. Therefore,

they have been lesser contributors to hydrocarbon, till date. The Gondwana

sequences are now being targetted in some of the basins like Krishna-Godawar

Assam area but most other basins are still to contribute significantly. Out of 26

identified basins, only seven petroliferous basins have been put on commercial

production. These are Mumbai offshore, Cambay, Assam-Arakan including the

Tripura-Cachar area, Cauvery, Krishna-Godavari, Mahanadi and Rajasthan

basins. The remaining basins are in different stages of exploration

Introduction

The Western Continental Margin is a passive margin having six sedimentary

basins namely Kutch, Saurashtra, Bombay Offshore, Konkan, Lakshadweep and

Kerala. Out of the six basins Bombay Offshore is the biggest basin having an

aerial extent of 1,20,000 Sq.Km. Bombay Offshore basin which is an

intracratonic basin, is the southern extension of Cambay basin and encompasses

the Gulf of Cambay and adjacent shelf part of the Arabiansea. It came into existence during Upper Cretaceous time as a result of faulting

in the Deccan trap basement. This basin is divided into several tectonic units

such as Cambay Gulf Shelf to the north followed to the south by Dahanudepression, Bombay Ratnagiri shelf occuyping


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central and southern part of the basin and Western clastic basin fringing

Bombay Ratnagiri shelf.

Geographic Location of the basinMumbai Offshore basin is located on the western continental shelf of India

between Saurashtra basin in NNW and Kerala Konkan in the south.

Category of the basinThe basin falls under the category I, which implies that the basin has proven

commercial productivity.


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AreaIt covers an area of about 116,000 km2 from coast to 200 m isobath.

Age of the Basin & Sediment-thicknessThe age of the basin ranges from late Cretaceous to Holocene with thick

sedimentary fill ranging from 1100-5000 m. Though possibility of occurrenceof Mesozoic synrift sequences in the deep-water basin have been indicated by


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the recently acquired seismic data by GXT, it needs to be further ascertained by

future studies.

Exploration historyThe first offshore seismic survey pedormed by ONGC between 1962 and 1965,

resulted in the discovery of the first offshore structure at Aliabet. Besides this,

the Russian - Seismic party carried out reconnaissance and semi-detailed

mapping covering many offshore sedimentary basins of India. A large number

of structures including Bombay High, Panna, Bassein, Diu, Dahanu and South

Tapti, etc. were delineated (fig.2). Subsequent detailing was carried out by

CGG, a French Company and GSI in the years 1972 and 1973, respectively. The

first oil discovery in this basin was made in the Miocene limestone reservoir of

Mumbai High field in February 1974. Subsequent intensification in exploration

and development activities in this basin have resulted in several significant

discoveries including oil and gas fields like Heera,Panna, Bassein,

Neelam,Mukta, Ratna,Soth tapti, Mid Tapti etc.In addition number of marginal

fields like B-55, B-173A, B-119/121, D-1 and D-18 have been put on

production in the last decade.

Recent Discoveries (2007-08)

Block/Prospect Discovery Formation Operator

B-55-5 Gas Mukta ONGCL

B-12-11 Gas Daman ONGCL

D-1-14 Oil Ratnagiri ONGCL

B-172-9 Gas Panna ONGCL

BNP-2 Gas S1 Pay ONGCL

Type of Basin

Mumbai offshore is a pericratonic rift basin situated on western continentalmargin of India. Towards NNE it continues into the on land Cambay basin. It is

bounded :-

1. The basin is bounded by the western coastline of India in the east


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2. In the northwest by Saurashtra peninsula,3. In north by Diu Arch.4. Its southern limit is marked by east west trending Vengurla Arch to the

South of Ratnagiri .

Geological Setting & History

Mumbai offshore basin, a divergent passive continental margin basin, is located

on the continental shelf off the west coast of India

The basin was formed due to extensional tectonics at the time of rifting of the

Indian plate from Madagascar during Late Jurassic-Early Cretaceous period.

Large-scale volcanic eruptions, which covered most of the basin, followed this

episode. Deccan trap basaltic lava flows were deposited in a pre-existing

pericratonic trough on the western continental shelf of India. The west coast and

other parallel faults, including the Bombay High east fault, were more active

during the Palaeogene period as compared to E-W faults north of Bombay High

As the rifting continued, the immature sediments deposited at the toe of faults as

alluvial fans, filled the initial morphotectonic depressions during Paleocene.This was followed by the first marine incursion towards the close of Paleocene

and beginning of early Eocene. Although, the crestal region of Bombay High,

remained an island for most of the Palaeogene period, a large part of its flanks

received considerable sedimentary cover during the same period. The geo-

thermal gradient in the Bombay Offshore

basin ranges from 4-6c/100 m. This gradient which In the Bombay Offshore

basin, like the Cambay a) basin, the end of the Palaeogene was characterised by

a regional unconformity Thus, early Eocene marks a widespread transgression.

Sediments were deposited in deltaic to restricted marine to shallow marineenvironments.


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Sedimentation during this period caused some adjustments in the basin. The

early Oligocene transgression covered most parts of the basinal area and

inundated parts of Mumbai high. Clastic section is present in the Cambay basin

in the Upper Oligocene-Lower Miocene, whereas this period in the BombayOffshore basin is represented by a shallow marine carbonate bank deposit,

interrupted by a few thin shale layers.

A major unconformity is noted at the top of lower Oligocene. Sea level rise

during early Miocene submerged large areas of the basin and terminated the .

Oligocene delta progradation. . In the upper part of the Lower Miocene,

carbonate bank deposition was terminated by the deposition of fine clastics,

with a few intervening short period of carbonate deposition. The Middle

Miocene other prominent period of carbonate bank deposition, represented by a

thick sequence which was terminated in the upper Miocene by a large influx of

muds and silts. The middle Miocene transgression marks the last phase of thewidespread carbonate sedimentation in the Mumbai highDCS area (Basu et al.,

1982; Zutshi et al., 1993). The Pliocene to. Holocene period witnesses a thick

deposition of shallow marine clay and claystone with a few layers of silt and

fossil shell fragments.

The basin has a NW-SE-trending horst-graben geometry. The grabens are

bounded by normal faults, and the horsts/ridges are dissected by NE-

SWtrending cross faults. On the basis of its structural configuration and itsnature, as well as the type of sediment fill, the basin is divided into six tectonic

blocks: Tapti-Daman, Diu, Heera-Panna-Bassein, Bombay high-DCS,

Ratnagiri, and Shelf Margin blocks.

Different Tectonic Zones with in the Basin

Five distinct structural provinces with different tectonic and stratigraphic events

can be identified within the basin viz.

1. Surat Depression (Tapti-Daman Block) in the north2. Panna-Bassein-Heera Block in the east central part3. Ratnagiri in the southern part,4. Mumbai High-/Platform-Deep Continental Shelf (DCS) in the mid

western side

5. Shelf Margin adjoing DCS and the Ratnagiri Shelf.

Documents

Bombay Basin