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Cairn India Limited - Ravva | Geology and Stratigraphy






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Cairn India Limited - Ravva | Geology and Stratigraphy Cairn India Limited - Ravva | Geology and Stratigraphy Document Transcript

  •               RAVVA Celebrating 16 Years of Technical Excellence Innovating Development | Geology and Stratigraphy
  • Ravva | Innovating DevelopmentRavva Geology and StratigraphyThe Ravva PKGM-1 is located offshore of the Godavari Delta on the northern side of the KG Basin, inAndhra 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 theother 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 structuresoverprinted the NW-SE trending Permo-Triassic Pranhita-Godavari Graben, which possibly extends intothe Ravva offshore area. These grabens were filled with thick middle Jurassic to early Cretaceousclastics. Rifting ceased and the widespread late Cretaceous clastics have filled and buried the horst andgraben topography. The passive margin progradation towards the south-east commenced during lateCretaceous.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 tiltingcaused a major transgression, and increased the depositional energy of the proto-Krishna and Godavaririvers. The resultant influx of coarse clastics caused vigorous passive margin progradation to thesoutheast. Sediment input has been dominated by the Krishna and Godavari river systems since theCretaceous period.The two present-day delta promontories became established in their present positions in the earlyMiocene era.The locale near the Ravva has received mostly finer clastics in the Paleocene period. The basin wasgetting filled vigorously till the end of early Miocene period. The very high sediment load on the shelfcoupled with the energy released from the tectonic activity relating tothe collision of the Indian plateinduced a failure in the prograding shelf. The failed block buckled and moved down, creating a hugedepression for accommodation during the middle and late Miocene period followed by a massiveprogradation. The basin underwent many episodic movements along the pre-existing faults duringPliocene to the recent times. It has tilted and rolled over the faulted blocks to form rolled over anticlines inRavva block area.Tertiary sedimentation has been primarily influenced by eustatic sea-level fluctuations along with upliftand erosion of the hinterland caused by the Deccan hotspot, the Himalayan collision and local gravityinduced tectonism.Within the basin, major thin-skinned extensional and compressional deformation occurred in the lateMiocene to early Pliocene. The offshore portion of the Tertiary includes depositional systems ranging fromthe 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 alongthe coast. Although fluctuations in the position of the shoreline occurred throughout the middle Miocenetime, the overall sediment and facies distribution at Ravva remained remarkably constant as a result ofthe dominantly aggradational stacking. The exceptional multi-darcy reservoir quality was caused by anupsection 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-marineslope environment as fluvial channel, channel levees and distributory channels. Subsequently, during theupper late Miocene period, thick and widespread sheetsands were deposited in a prograding shelf. Duringthe base Pliocene period, a major erosional unconformity formed due to tectonic activty resulted by thehard collison of the Indian subcontinent with the Eurasion plate. Due to this major unconformity a verythick shale sequence comprising thin sands deposited as a slope fan. At the end of the Pleistocene perioda forced regression coupled with a massive progradation took place in the Ravva block area.
  • Ravva | Innovating DevelopmentSequence Stratigraphy and Middle Miocene Depositional ModelOil and gas were discovered from the middle Miocene reservoirs in well R-1 in 1987 and oil productioncommenced in 1993. The depositional model of the discovered reservoir sands plays a vital role indeveloping the reservoir sands optimally. During the discovery phase of the field, depositional modelinferred from the middle Miocene reservoir was turbidites.During the phase-I field development, 14 development wells were drilled and added to the immensewealth of Litho-biostratigraphic, high resolution bore hole image and strati-structural dip data, which hasprovided 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 havebeen deposited in a wave dominated deltaic setup. At the time of deposition, these sands were influencedby 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 andhelped in dividing the main middle Miocene reservoirs in different chronological reservoir units like subM20, M20, M30, M33, M34, M32, etc. Since the sediments were constantly winnowed and reworked, thefine clay particles were removed from the sand grains and have improved reservoir properties like porosityand permeability.The Ravva main producing reservoirs comprise porosity in the ranges of 22% – 35%. This multi Darcypermeable 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, differentsequence stratigraphic systems were developed.Building depositional and geological models is a dynamic process. More and more lithostratigraphic wellinformation helps to refine and review paleogeographic maps. One such review is in progress in RavvaAsset to analyse and build improved paleogeography for the producing reservoirs sands by integratingnew infill well information and stochastically inverted seismic properties like effective porosity.