Cairn India Limited - Ravva | Geology and Stratigraphy

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Cairn India Limited - Ravva | Geology and Stratigraphy

  1. 1.                           RAVVA Celebrating 16 Years of Technical Excellence Innovating Development | Geology and Stratigraphy
  2. 2. 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.
  3. 3. 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.

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