Solution for Imaging in Logistically Constraint Areas: A Case Study from Upper Assam
Koustav Dasgupta, Rahul Dasgupta, Mayank Banerjee, Anup Kumar, and Subhodeep Maji
Geophysics, Oil India LTD, Duliajan, District Dibrugarh, India
The study area falls within the operational areas of Oil India Limited at Upper Assam. A discovered oil and gas field is present in the area. The area is characterized by presence of a national park at its north where seismic activity is prohibited. Additionally, the area, at its west and south, is bounded by a swamp, where conventional seismic survey is not possible. The swamp was more than 10 kms long and upto 2 kms wide at places. These are the constraints of seismic survey in the area. Thus, innovative 2D/3D seismic data acquisition was necessary to cover the area meaningfully by seismic.
The area was covered by seismic without any extra resources in cost effective manner in the following phases:
i) Phase I: 2D seismic survey was initially carried out. The lines were short and hence, data was acquired keeping all the receivers live for ever shot. This generated high fold data required for obtaining interpretable data. Additionally, two 2D lines were shot simultaneously, generating extra coverage between the lines. This 2D survey resulted in the discovery of an oil & gas field.
ii) Phase II : The area was then covered with 3D seismic survey. At the north, towards the national park, receiver line spacing was reduced so that foldage develops within a short distance.
iii) Phase III: Next phase is aimed at coverage of the data gap due to the swamp and providing continuous imaging at prospect level. A Close-Grid 2D Survey of 400 m line was designed with high channel count split-spread geometry having adequate shots and receivers placed on both side of the swamp with innovative CDP coverage-Offset compensation technique.
The seismic data gap is thus covered which is essential for exploration of a remote oil field.
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