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S. K. Acharyya ¹, K. K. Ray ¹, D. K. Roy ¹

Affiliations
1 Geological Survey of India, Calcutta, IN

Abstract

The oceanic pelagic sediments from the ophiolite assemblages of Naga Hills (NHO) and Andaman islands (ANO) broadly correspond to Late Cretaneous to Paleocene age. Late Cretaceous ocean floor represented in ANO was dominantly below the carbonate-compensation-depth (CCD), whereas it was uneven but mainly above CCD during Paleocene-Early Eocene. Maestrichtian to Paleocene ocean floor appears to be dominantly above the CCD in NHO. The NHO and ANO have closely similar postemplacement ophiolite-derived mid- and mid-late Eocene cover sediments respectively. Emplacement of the Naga Hills ophiolites during mid-Eocene, was possibly caused due to initial collision of an ocean-island chain with the subduction zone beneath the Central Burmese continent. Emplacement of the Andaman ophiolites occurred during early-mid-Eocene and may have been caused by a similar situation. The Chin Hills ophiolites were also mainly emplaced during early Eocene. The accreted ophiolites of the Indo-Burman range and Andaman islands were thrust westward over the Paleogene turbidite pile, deposited on the down going Indian plate during late Oligocene, due to terminal India-Burma continent-continent collision. In the southern Manipur sector of Naga Hills, the olistostromal upper Disang Formation (mid to late mid-Eocene) tectonically flooring the NHO, stratigraphically underlies the flyschoid to molassic Barail (Oligocene) sediments. In Andaman islands, the Lipa Formation with early Eocene and older olistoliths, forms the dominant constituent of the lower tectonic sedimentary melange flooring the ophiolites. Towards west, the sedimentary melange overrides the Archipelago Group. (Neogene) which unconformably overlies the Andarnan Flysch of Eocene-Oligocene age. The ischolar_main-zone of the collisional suture appears to be concealed below younger sediments in the Central Burma basin and in the Andaman Sea, parts of the latter being created as a back-arc marginal basin. There is thus no straightforward relation between the Late Oligocene collision suture and the present subduction zone along the trench to the west of Andaman-Nicobar islands.

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D. K. Roy ¹, G. K. Ray ¹, A. K. Biswas ¹

Affiliations
1 MBA Basin, ONGC, Kolkata - 700 088, IN

Abstract

Abnormally high formation pressures are encountered worldwide, ranging in geological age from Cenozoic to Paleozoic, within a depth range of few hundred meters to as deep as six thousand meters while carrying out exploratory drilling by E and P companies. Several causes can increase formation fluid pressure i.e. rapid loading of sediments results compaction disequilibrium, thermal expansion of fluids, compression and/or upliftment of strata by tectonic forces, generation of oil and gas from organic matter and its volume expansion due to high thermal stress within the restricted pore volume in subsurface condition. Few global examples on overpressure occurrences have been compiled in the paper with special reference to Bengal Basin. Emphasis has been given on methodology and interpretation on abnormal pressure detection in Bengal Basin with a compiled data package on generated curves (Geologs), charts, tables in a systematic way to understand the depth/stratigraphic horizons proved/interpreted as proved or likely to be within transition and overpressure regime. The integrated analysis indicates that the wells drilled in the east of Eocene hinge zone in the onshore and offshore parts of Bengal Basin have penetrated overpressure formation within Miocene in the depth range of 2800 m to 5340 m and the mud weight used to control this overpressure zone was more than 2.0 sp gr mud. The generated Geologs can be used as reference to understand the regime of transition and overpressure, as a valuable document for exploration drilling planning and monitoring. The generated model curve (modified using available data after Hottman and Johnson,1956 curve) using sonic departure (i.e. Δt_ob(sh) - Δt_n(sh)) from drilled wells may be used as an additional tool to find out the expected formation pressure gradient and equivalent mud weight in all future wells. The correlation of wells based on the trend of dcs and σ logs will be useful for predicting transition and overpressure top provided all the parameters required for calculating dcs and σ log recorded smoothly during drilling phase. The study has brought out the detail procedure to generate the pressure profile in the future wells. The generation of pressure profile of a well prior to drilling has got immense importance in oil industry. The drilling of the well should be done by maintaining the optimum mud weight generated from the pressure profile. In case, during drilling, formation pressure is more than the mud pressure, resulted gas kicks or worse, blowouts of the well. Excessively high mud pressure can fracture the formation and cause lost circulation. The oil and gas companies, worldwide, attributed 15% losses due to various problems associated with drilling complications, mostly related to improper pressure prediction of a well. The losses include loss of material as well as drilling process continuity, called non-productive time (NPT). The generation of accurate pressure profile reduces drilling problems, cuts exploration and development costs and allows billions of dollars now spent on losses to be better spent-building and replacing reserves.

Keywords

Overpressure, Geologs, Dc Exponent, σ Log, Electrologs, Bengal Basin.

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D. K. Roy ¹

Affiliations
1 Indian Oxygen Ltd, Calcutta, IN

Abstract

The need for classification of wire, flux and the combination for sub-merged arc (S.A.) welding has been felt by all concerned and some steps have been taken at national and international levels for standardisation.

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S. Mallik ¹, D. K. Roy ¹

Affiliations
1 Indian Oxygen Ltd., Calcutta, IN

Abstract

The techno-economic possibility of rebuilding Steel Mill Rolls by submerged arc welding process is well-known. It has been adopted as a regular practice in U.K., U.S.A., U.S.S.R. and continental countries. Initiative has also been taken by some Steel Plants in India in this respect and welding procedures have been successfully established for a few types of rolls. It is felt, however, that there is considerable scope to enlarge the reclamation activities to cover a wider range of rolls and also in developing the technique for manufacture of "Weld Surfaced" duplex rolls, a relatively new concept in the manufacture of rolls.

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