Cairn India Limited - Ravva | Geology and Stratigraphy

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RAVVA Celebrating 16 Years of Technical
Excellence Innovating Development Geology
and Stratigraphy
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Ravva Innovating Development
Ravva Geology and Stratigraphy
The Ravva PKGM-1 is located offshore of the
Godavari Delta on the northern side of the KG
Basin, in Andhra Pradesh on the east coast of
India. The northwestern margin of the block
extends to the shore. The Krishna-Godavari Basin
was formed as a result of the breaking up of the
Indian sub-plate from the other Gondwana plate
during the Jurassic to Cretaceous Period. The
rifting made a series of North East - South West
(NE-SW) trending en-echelon horsts and grabens in
this basin. These NE-SW structures overprinted
the NW-SE trending Permo-Triassic
Pranhita-Godavari Graben, which possibly extends
into the Ravva offshore area. These grabens were
filled with thick middle Jurassic to early
Cretaceous clastics. Rifting ceased and the
widespread late Cretaceous clastics have filled
and buried the horst and graben topography. The
passive margin progradation towards the
south-east commenced during late Cretaceous.
During the latest Cretaceous to earliest
Paleocene, the Indian sub-plate was tilted
towards the southeast. This event was caused by
uplift of north-western India as it drifted over
the Deccan hotspot. The tilting caused a major
transgression, and increased the depositional
energy of the proto-Krishna and Godavari rivers.
The resultant influx of coarse clastics caused
vigorous passive margin progradation to
the southeast. Sediment input has been dominated
by the Krishna and Godavari river systems since
the Cretaceous period.
The two present-day delta promontories became
established in their present positions in the
early Miocene era.
The locale near the Ravva has received mostly
finer clastics in the Paleocene period. The basin
was getting filled vigorously till the end of
early Miocene period. The very high sediment load
on the shelf coupled with the energy released
from the tectonic activity relating tothe
collision of the Indian plate induced a failure
in the prograding shelf. The failed block buckled
and moved down, creating a huge depression for
accommodation during the middle and late Miocene
period followed by a massive progradation. The
basin underwent many episodic movements along the
pre-existing faults during Pliocene to the recent
times. It has tilted and rolled over the faulted
blocks to form rolled over anticlines in Ravva
block area.
Tertiary sedimentation has been primarily
influenced by eustatic sea-level fluctuations
along with uplift and erosion of the hinterland
caused by the Deccan hotspot, the Himalayan
collision and local gravity induced tectonism.
Within the basin, major thin-skinned extensional
and compressional deformation occurred in the
late Miocene to early Pliocene. The offshore
portion of the Tertiary includes depositional
systems ranging from the shore face to the
deep-water submarine fan sandstones.
These middle Miocene sands of the Ravva field
were deposited in a NE-SW oriented
wave-influenced, loweto upper shore face setting,
where wave action and long shore currents moved
the sediment along the coast. Although
fluctuations in the position of the shoreline
occurred throughout the middle Miocene time, the
overall sediment and facies distribution at Ravva
remained remarkably constant as a result of the
dominantly aggradational stacking. The
exceptional multi-darcy reservoir quality was
caused by an upsection increase in depositional
energy resulting from two significant sequence
boundaries.
During the lower late Miocene period many good
quality sands were deposited in a fluvial to
sub-marine slope environment as fluvial channel,
channel levees and distributory channels.
Subsequently, during the upper late Miocene
period, thick and widespread sheetsands were
deposited in a prograding shelf. During the base
Pliocene period, a major erosional unconformity
formed due to tectonic activty resulted by
the hard collison of the Indian subcontinent with
the Eurasion plate. Due to this major
unconformity a very thick shale sequence
comprising thin sands deposited as a slope fan.
At the end of the Pleistocene period a forced
regression coupled with a massive progradation
took place in the Ravva block area.
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Ravva Innovating Development
Sequence Stratigraphy and Middle Miocene
Depositional Model
Oil and gas were discovered from the middle
Miocene reservoirs in well R-1 in 1987 and oil
production commenced in 1993. The depositional
model of the discovered reservoir sands plays a
vital role in developing the reservoir sands
optimally. During the discovery phase of the
field, depositional model inferred from the
middle Miocene reservoir was turbidites.
During the phase-I field development, 14
development wells were drilled and added to the
immense wealth of Litho-biostratigraphic, high
resolution bore hole image and strati-structural
dip data, which has provided an opportunity to
revise the depositional model and sand unit level
paleogeography maps. The revised depositional
model and the Paleogeography suggest that the
middle Miocene sands have been deposited in a
wave dominated deltaic setup. At the time of
deposition, these sands were influenced by waves
and redistributed along the coast in a NE-SW
orientation in a lower to upper shore face
setting. The further detailed geological work has
provided good insight to the chronosequence
stratigraphy and helped in dividing the main
middle Miocene reservoirs in different
chronological reservoir units like sub M20, M20,
M30, M33, M34, M32, etc. Since the sediments were
constantly winnowed and reworked, the fine clay
particles were removed from the sand grains and
have improved reservoir properties like
porosity and permeability.
The Ravva main producing reservoirs comprise
porosity in the ranges of 22 35. This multi
Darcy permeable sands with high oil saturation
have already produced more than 220 mmbls of
oil. Based on the refined geological
geophysical data and good chronostratigraphic
insights, different sequence stratigraphic
systems were developed.
Building depositional and geological models is a
dynamic process. More and more lithostratigraphic
well information helps to refine and review
paleogeographic maps. One such review is in
progress in Ravva Asset to analyse and build
improved paleogeography for the producing
reservoirs sands by integrating new infill well
information and stochastically inverted seismic
properties like effective porosity.