Mapping Thin Sandstone Reservoirs: Application of 3-D Visualization And Spectral Decomposition Techniques
- _ Harilal (Oil and Natural Gas Corporation Limited) | C.G. Rao (Oil and Natural Gas Corporation Limited) | R.C.P. Saxena (Oil and Natural Gas Corporation Limited) | J.L. Nangi (Oil and Natural Gas Corporation Limited) | A. Sood (Oil and Natural Gas Corporation Limited) | S.K. Gupta (Oil and Natural Gas Corporation Limited)
- Document ID
- Society of Exploration Geophysicists
- 2006 SEG Annual Meeting, 1-6 October, New Orleans, Louisiana
- Publication Date
- Document Type
- Conference Paper
- 2006. Society of Exploration Geophysicists
- 2 in the last 30 days
- 77 since 2007
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The 3-D data of C-37 and adjoining prospects of Tapti-Daman sub-basin of Mumbai offshore Basin, India, have been evaluated for delineation and mapping of Mahuva pay sands. The pays were found over an anticlinal nosal feature in Upper Mahuva Unit of Lower Oligocene Mahuva Formation by one exploratory well in 1994. Conventional interpretation of 3-D data and subsequent drilling of wells towards up dip and down dip both could not map the areal extent of the pays. Post drill analysis of log and seismic data show that low impedance pay sands, embedded in high impedance shales, are separated in thin beds by limestone and/or shale streaks.
Delineation of these sands by conventional interpretation methods is difficult because of thin and discontinuous occurrences, high degree of vertical and lateral variability in net sand thickness, abundance of limestone streaks and limited bandwidth of seismic data. 3-D visualization of surfaces and volume attributes, neural network based seismic trace shape classification and spectral decomposition techniques have been applied with integration of well and log data. Amplitude attributes based on full bandwidth data were found more contaminated by thin limestone streaks. Spectral decomposition based iso-frequency sections and iso-frequency slices mapped areal extent and temporal thickness of pay zone. Voxel based 3-D visualization of selected frequencies from instantaneous frequency volumes and seismic trace shape classification maps provided comparable image of the reservoir sands. Marine sands near shore-zone areas during continued sea level fall are envisaged depositional system for the pay sands. The sandstones are spread over 90 km2 area in isolated sand bodies. The inferences are validated by drilled wells.
The C-37 prospect in Tapti-Daman sub-basin of Mumbai Offshore Basin, India was proved by one exploratory well in 1994. The discovery well produced hydrocarbons from sandstone reservoirs of Daman and Mahuva Formations of Late and Early Oligocene age, respectively. To test the up dip extension of pays one well was drilled at a distance of 3.7 km from discovery well but hydrocarbons could not be found because the particular sand, which produced hydrocarbon, shales out there. 3-D seismic data was acquired in 1996-97 and interpreted with application of instantaneous attributes and window based attribute extraction.
The stratigraphic and structural interpretation of 3-D data suggested two locations towards downdip, primarily for channel prospects in Daman Formation. Minor gas shows were observed but Mahuva pay sands were not encountered. Currently seven wells fall in 3-D survey area out of which one is hydrocarbon bearing in Daman and Mahuva Formations (C-37 prospect) and one is in Daman Formation (adjoining prospect). Post drill study has suggested lack of equivalent sands at other well locations. The 3-D data has been reevaluated for mapping the Mahuva pay sands by using 3-D visualization, neural network based seismic trace shape classification and spectral decomposition methods. The objective of this paper is to demonstrate delineation of thin sandstone reservoirs embedded in the predominantly shales, deposited in shallow marine environments by using 3D interpretation tools.
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