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The Mumbai Basin contributes around 55% domestic crude oil and
70% of the gas production for India. IHS Markit forecasts the gas
demand to the domestic supply gap for India to increase from around
30% in Financial Year (FY) 2018 to around 60% by FY 2030 even with
a projected rise in gas production of around 700 Bcf/y by FY 2027.
How dependant is India on production from the basin? What plans are
in place to offset any foreseen declines in the future? This paper
gives a short geological overview of the Mumbai Basin and a look at
its future potential to support India's ongoing plans to reduce its
reliance on imports.
Geological History
The Mumbai Basin, also known as the Bombay Basin, is a large
divergent passive continental margin basin located mainly offshore
on the western continental margin of India. The basin covers an
area of over 170,000 km2 and is considered as the
offshore extension of the Cambay Basin, encompassing part of the
Gulf of Cambay towards the north. It's been described as having a
NW-SE trending horst-graben geometry and divided into several
tectonic blocks (the Surat Sub-basin, also known as the Tapti-Daman
block or Surat Depression, the Diu Arch, Heera-Bassein Block, the
Mumbai Platform-Deep Continental Shelf, the Ratnagiri Block, and
the Shelf Margin Sub-basin). The sedimentary fill ranges from
around 1,100 to 5,000 m, within which several large oil and gas
discoveries have been made. The presence of a working petroleum
system has been long established with multiple pay zones identified
throughout the stratigraphic column, from the recently discovered
clastic/fractured basement reservoirs to various established zones
such as the prolific Miocene carbonate reservoirs (for example in
the Mumbai High area), and the Mukta, Bassein, Panna, and Daman and
Mahuva formations. Principal mature source rocks in the basin are
present in the Upper Paleocene-Lower Eocene clastic sediments of
the Panna Formation reaching thicknesses more than 1,000 m.
Effective seals are provided by both widespread and interbedded
shale intervals along with cemented carbonates with the basin
displaying a wide array of both structural and stratigraphic
trapping styles.
The Mumbai Basin formed due to extensional tectonics during
rifting of the Indian plate from Madagascar in the Upper
Jurassic-Lower Cretaceous and is characterised by longitudinal
extensional faults giving rise to a series of horst and graben
features. Large scale volcanic eruptions and further rifting
followed resulting in the laterally extensive Deccan Trap
continental flood basalt accumulations covering a significant
extent of the basin. As rifting continued, the first marine
incursion occurred from the end of the Paleocene to the beginning
of the Lower Eocene during which sediments were deposited as
alluvial fans and filled the initial morphotectonic depressions.
The Lower Eocene marked the first transgression within the basin
with deltaic to shallow marine environments dominating and
subsequently from the Eocene through Middle Miocene,
carbonate-platform buildups occurred intermittently on the shelves
around much of the Indian plate. The Upper Paleocene-Lower Eocene
syn-rift period was terminated with a basin wide regression and
resultant development of an unconformity.
Following a period of peneplanation, the Mumbai Basin was almost
completely inundated during the Middle Eocene except for the Mumbai
High and the southeast Ratnagiri Shelf. Restricted clastic supply
followed during the Middle to Upper Eocene. The Surat Depression,
part of the Ratnagiri Block, the Deep Continental Shelf and the
Central Graben of the Heera-Bassein Block remained as classic
depocentres during this period.
At the start of the Oligocene a marine transgression took place
and resulted in the development of extensive carbonates which
continued during the Lower Oligocene in the Heera-Bassein and
Ratnagiri areas. Following this, global sea level fell and a
regressive phase began, associated with the initiation of a large
delta system in the northeast part of the basin. In response to a
sea level rise during the Lower Miocene, large areas of the basin
were inundated and the Oligocene delta progradation was
terminated.
The Middle Miocene marine transgression represents the last
phase of widespread carbonate sedimentation in the basin as
identified in the Mumbai High, the Deep Continental Shelf and the
Ratnagiri Block. Minor tectonic movements began by the end of the
Middle Miocene and some of the younger structures such as the
Saurashtra, Diu and Dahanu structures were formed and resulted in a
further westward tilting of the basin. The post Middle Miocene
witnessed flooding of fresh areas and an influx of clastics into
the basin.
Figure 1: Structural framework map of the Mumbai
Basin
Basin Insights
Traditional plays remain important exploration targets in the
basin and spatial extensions of these plays are being pursued.
These targets include the known plays in the Lower Miocene to Upper
Oligocene L-I to L-VI limestones with the main producer in the
Mumbai High field being the L-III interval, along with the Lower
Oligocene to Upper Eocene carbonate reservoirs of the Mukta and
Bassein formations, and the fluvio-marine clastic deposits of
Eocene-Paleocene Panna Formation.
In addition to these classic plays, new objectives such as
deeper plays have been identified in established areas and beyond
by recent discoveries in the basin. These play openers have
included the deeper pay zones of the fractured Basement and Basal
Clastic intervals within the syn-rift sequence. The basement play
is considered to have good potential and is understood to include
Archaen granite, basalt from the Deccan Traps and phyllite schist/
gneiss quartzite. Oil & Natural Gas Corporation Ltd (ONGC)
recently brought the fractured basement play on-stream within its
prolific Mumbai High field with basement production focused around
fault zones resulting in variable flow rates. The fracture network
continuity and density are heavily defined by the lithological
variation. If favourable, the network can be well interconnected
and composed of long network zones particularly within the granite.
Counterintuitively, disjointed systems found in the metamorphic
zone do not offer promising targets.
The Basal Clastic, described as either the basal layer within
the Panna Formation or a separate unit underlying the Panna
Formation, are understood to be heterogeneous in nature and
consisting of detrital clastics from the underlying basement and
localised limestone. Target trap types are thought to be on paleo
highs with further potential in the Panna Formation wedging out
against these highs. The potential of the Basal Clastic has been
proven in Mumbai High field where it directly overlies the
basement. Key to enhancing the extent of this play will be testing
along the margins of the Mumbai High and Heera fields.
Targeting the Pliocene in the frontier region of the Neogene
Shelf could be a promising future target. In 2015, ONGC flowed gas
within its MBS053NAA 1 discovery well opening up the play. Six
other prospects were identified within the block providing
potential for significant gas volumes. The potential utilisation of
a cluster development approach, combined with the relatively
shallow depth of these prospects, could lead to reduced time and
costs associated with commercialisation of these Pliocene
plays.
Figure 2: Stratigraphic Chart
of the Mumbai Basin
Recent Exploration Trends
Exploration within the basin is thought to have reached a mature
stage for the major hydrocarbon bearing zones. Production trend
forecasting by IHS Markit's Vantage product for asset evaluation
suggests that all the major on-stream discovered fields in the
basin (Mumbai High, Bassein, Heera & Neelam) are on a
production decline trend and are envisaged to be shut-in around
2040 in the event that no additional volumes are added, or other
recovery methods applied. However, significant potential does
remain adjacent to the main producing fields. ONGC has focused its
recent exploration drilling on marginal field areas to the west of
Mumbai High platform, with several successful wells discovering
hydrocarbons volumes from multiple intervals. One key example of
this is the WO-24 C discovery, made in June 2017, which is believed
to have found pay from six different reservoir zones, with one
interval flowing around 3,300 bo/d. This discovery is suggested to
add significant potential to the Cluster-7 (B-192, WO-24 and B-45)
fields and since ONGC has drilled at least five
exploration/appraisal wells in the same area. In February 2019 ONGC
tendered for the development of Cluster-7 where D-30 2 flowed 1,200
bo/d with gas from the secondary objective of the Panna Formation
in 2017. Future development plans include tying in any additional
new small pools from the current drilling activities into the
Cluster 7 development for fast track production.
India Dependency
Based on current development and known future plans, the
contribution from the Mumbai Basin to the overall domestic oil
production will reduce from around 55% to less than 10% by 2035.
IHS Markit envisages that by 2040, based on current estimated
ultimate recoverable volumes, it will have had production from
proved and probable oil recoverable maximised. To offset this, new
play targets previously discussed will need to be successfully
explored to add new volumes, as well as the application of recovery
techniques to established targets. Improved Oil Recovery (IOR) and
Enhanced Oil Recovery (EOR) techniques have the possibility to give
an increase of up to 10% recovery factor (RF) to offshore fields,
particularly in the western offshore area. Specifically, for the
Mumbai High field it is thought application of these techniques
would achieve a high case scenario of up to around 2 Bbbl increased
oil recovery. For the basin it is thought around a 5 to 7% RF
increase can be achieved and best-case scenario estimates for
Mumbai High field matches this trend, with around 1 Bbbl increase
of recovery. IOR and EOR are foreseen to have significant potential
for additional oil recovery in the basin but it is believed that
the cost to implement large scale methods for EOR, as of present,
makes them un-economic. These predictions are susceptible to change
and are highly dependent on both future oil and gas prices along
with the expected increased dependence on imported crude oil for
domestic demand in India.
Gas production forecast by IHS Markit's Vantage product expects
to see a domestic gas dependence transition within the next five
years to the Krishna-Godavari Basin, from the Mumbai Basin with the
basins contribution to reduce from ~70% to ~25% by 2030. Focus will
be on the offshore KG-DWN-98/2 ONGC blocks cluster developments,
with Cluster-II planned to commence gas production by 2020. In
addition, Cluster-I and III have considerable gas volumes for long
term development with the potential that Cluster-I will be jointly
developed with ONGC's recently acquired Deen Dayal field from
Gujarat State Petroleum Corp. Reliance and BP plan to bring
on-stream additional developments in neighbouring KG-DWN-98/3 block
by 2H FY 2021. The R-Series (Dhirubhai-34 field) is planned to
produce through existing facilities from D-1 and D-3 fields with
Reliance planning to prolong production life until D-34 is ready to
come on-stream. Dhirubhai-55 (MJ) field is projected for production
in 2022 (~1.1 Tcf of gas recoverable reserves) along with the
Satellite fields (Dhirubhai 2, 6 19 and 22), with Dhirubhai 29 and
Dhirubhai 30 adding ~650 Bcf gas recoverable reserves. However, it
is worth noting that during September 2018 Reliance recently shut
in the D6-MA field after it experienced a faster than expected
decline in production. Future production trends for both these
blocks will be watched with interest. Looking across projected gas
production for all basins in India, new projects will produce a gas
production ramp up to 1.85 Tcf/y by 2025 but then this is envisaged
to decline by 2040 to around 750 Bcf/y.
To conclude, the Mumbai Basin, over the past 40 years, has been
fundamental for India's domestic oil and gas production. With India
planning to transition away from oil to a more gas focused economy,
IHS Markit expects to see a transition to gas development and
production focus, looking to the eastern offshore area of the
Krishna-Godavari Basin. However, countrywide opportunities exist
with recent acreage awarded in basins across India under the new
revenue sharing contract model of the recent Open Acreage Licensing
Rounds. Vedanta Ltd (through its underarm Cairn Oil & Gas) has
acquired 41 blocks in OALP-I, across 10 basins, one block lies
within the shallow water zone of the Mumbai Basin. The company is
planning to drill around 150 exploration wells with work
commitments thought to be around USD 550 million with a projected
potential of 1.4 to 4.2 Bboe resources in these new blocks. In
addition, the government, under its hydrocarbon vision for 2030, is
pushing to increase production for gas from the Assam Shelf Basin
in the North East of India. Oil India Ltd (OIL) has recently
acquired blocks in the frontier zone along with Vedanta Ltd and it
is understood OIL's exploration plans are to focus on gas targets
to support this vision. ONGC recently announced two commercial
discoveries during Q3 FY 2018-19 in the Bengal and Vindhyan basins
with which India has the prospect of two additional basins moving
into its basin category-I classification. In February 2019, the
Indian Government approved the policy framework revision providing
gas marketing and pricing freedom for gas discoveries for Field
Development Plans yet to be approved. In addition, for existing
domestic fields, they offered fiscal incentives in the form of
reduced royalty rates by 10% for additional gas production to
projected normal production under a business as usual scenario. It
is understood that ONGC and OIL have not been able to monetize some
of its gas discoveries (onshore and shallow water) at the current
domestic gas price (USD 3.36/MMBtu) which is lower than the cost of
production. It is believed that of the total ~115 gas discoveries,
~90 gas discoveries might benefit from this re-form with
re-investment renewed in mature fields to increase the production
to gain from the fiscal incentives. If successful we might see a
significant change to the production forecasts of India for its
domestic production within the next 30 years. However, it is
apparent that significant gas volumes need to be found and bought
on-stream at a fast pace if dependency on imports is to be reduced,
otherwise more expenditure will be required to import oil and
improve the infrastructure to offset the gas demand through the
imports of LNG.
India's dynamic energy market, and the government's desire to be
domestically driven, along with the continued development of new
and proven plays, the application of new technology, and
utilization of an established infrastructure, will undoubtedly mean
that the Mumbai Basin will contribute significant volumes to
India's energy mix moving forward for oil but, based on the present
trend, gas dependence will begin to transition towards the eastern
coast.
Figure 2: Stratigraphic Chart
of the Mumbai Basin
