AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Mapping of Fluvial Meandering Channels in Lower Eocene in Mumbai Offshore Basin, India: A New Play Through Integration of Geology and 3-D Seismic Attributes
(1) GEOPIC, ONGC, Dehradun, India.
(2) GEOPIC, ONGC, Dehradun, India.
(3) GEOPIC, ONGC, Dehradun, India.
(4) GEOPIC, ONGC, Dehradun, India.
(5) GEOPIC, ONGC, Dehradun, India.
In quest of finding reservoir facies and entrapment model in the northern part of Central Graben in Mumbai Offshore Basin, India, 1700 km2 3-D seismic and regional/local geologic data was interpreted. Standard structural and stratigraphic interpretation workflows, consisting of log correlation, well-to-seismic calibration, correlation of horizons and faults, 3-D seismic attribute extraction, spectral decomposition, post-stack inversion, 3-D visualization and geobody detection, were adopted for the evaluation.
Regional log interpretation showed sandy facies with fining-upward and occasional coarsening-upward motifs indicating sand deposition in channels/crevasse splays in southwest and south of the area. In upper part of Panna Formation very high amplitude reflections (bright spots) were observed on vertical sections. High amplitudes are organised in meandering channel-like geometries on attribute maps. Paleo-structural reconstructions show a northeast-southwest oriented depositional low with relatively flat topography in upper part of Lower Eocene favouring flow of meandering channels from south to northeast. Integrating all the studies, high amplitude channel-like geometries were interpreted as fluvial meandering channel and point bar complexes filled with sands.
Possibilities of coals and carbonates within channel fills are negated through characteristic depositional geometry and impedance. The paleo-highs with granitic/metamorphic exposed basements towards southwest (Mumbai High) and south (Panna/Heera High) probably acted as provenance for supply of coarser sediments. The lateral/vertical aggradations are depicted by successive horizon slices and 3-D visualization. Channels progressively migrated towards north and northeast in the envisaged upper delta plain environment.
Sands are dispersed in form of scroll bars, point bars and crevasse splays and are spread over 40 km2 area with 6 to 20 m thickness. Sands have higher impedance than coals and lower impedance than enclosing shales manifesting known reservoir channel sands in nearby areas. High amplitudes associated with lower impedance, supported by presence of Eocene (Panna Formation) shales as source and seal rocks and stratigraphic entrapment conditions, favour gas accumulations in the newly mapped channel sands.
This paper deals with the methodology that has helped in deciphering the sand prone meandering channel system, which hitherto could not be brought out from concealed seismic signatures.