Cairn India Limited - Ravva | Geology and Stratigraphy
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Innovating Development | Geology and Stratigraphy
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
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
The two present-day delta promontories became established in their present positions in the early
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
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.
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
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.