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A. Nallapa Reddy ¹, B. C. Jaiprakash ¹, K. V. Bhaktavatsala ¹

Affiliations
1 Oil and Natural Gas Corporation Ltd., 3, First Lane, N.H. Road, Chennai - 600 034, IN

Abstract

Quantitative foraminiferal data in a temporal succession identifies a type 1 third order Miocene depositional sequence (Ravva Formation/Sequence) of sechron ∼6 Ma in Godavari shelf area. This sequence has developed as a shelf delta build-up. This sequence is divided into a transgressive systems tract (TST) and a highstand systems tract (HST) separated by a maximum flooding surface (MFS), based on faunal frequency trends. The TST plays host to all reservoir facies, suggesting that the sand depocentres have migrated landward, concurrent with progress of the transgressive phase. The Ravva Sequence is bounded below by a type 1 unconformity spanning ∼7 Ma across the Oligocene-Miocene boundary. This unconformity surface hosts reservoir facies in well GS-AE-D. The sequence is bounded above by a type 1 unconformity of ∼10 Ma across the Miocene-Pliocene boundary. This unconformity is a diachronous surface produced by a progradational highstand systems tract. This surface represents possibly the only period during the Neogene, where large quantities of middle to late Miocene sediments may have by- passed the shelf. These by-passed sediments, according to the proposed sequence depositional model, should occur as Iowstand systems tract deposits (shelf edge deltas and fan deposits).

Keywords

Sequence Stratigraphy, Sequence Boundary, Systems Tracts, Maximun Flooding Surface, Sechron, Miocene, Godavari Offshore.

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A. Nallapa Reddy ¹, K. Satyanarayana ¹, K. V. Bhaktavatsala ¹, K. Narasimha ¹, M. Nagaraj ¹

Affiliations
1 Regional Laboratory, ONGC Ltd., N.H. Road, Chennai - 600 034, IN

Abstract

Detailed chronostratigraphic biodatums and lithofacies in combination with correlative seismic horizons recognized two 3^rd order depositional sequences designated as Sequence 1 and Sequence 2 in stratigraphic order within Miocene in KD field. The biostratigraphic correlation has brought out that the Sequence 1 in shelf well GS-AD is sandrich and bounded by unconformable sequence boundaries, whereas coeval sequence in slope wells KDA and KDB, bounded by correlative conformities are dominated by clay rich system deposited during highstand sea-levels in earlymiddle Miocene. The clays are planar bedded with glauconite and pyrite nuggets with abundant deepwater benthics in association with shelf derived foraminifera, inferring that these clays were derived into intraslope basins mainly as debris flow/slump process. The Sequence 2 in wells KDA and KDB is represented by multiple sands with mudstone alternations, deposited during lowstand Sea-Levels. Biofacies are mixed with dominantly shelf derived benthics and the tests are commonly broken, abraded and occasionally ferruginised, suggesting reworking into intraslope basins as a result of relative Sea-Level fall during middle and late Miocene. The occurrence of abundant Paleocene to early Miocene dinoflageIlate cyst assemblage are also suggesting reworking process into upper slope by network of active channel system and were redeposited by Gravity-Flow processes into intraslope basins. The present study indicates each intraslope basin acting as an independent petroleum system and matured organic rich Paleogene beds lying at the bottom are contributing hydrocarbons to middle Miocene reservoirs through active growth fault system.

Keywords

KD-Structure, Sequence Stratigraphy, Systems Tracts, Depositional Setting, Krishna-Godavari Basin.

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