Mapping of Reservoir Facies within Heterogeneously Deposited and Structurally Deformed Strata in Tripura Frontal Fold Belt of Assam and Assam Arakan Basin: Application of Sequence Stratigraphy, 3D Seismic Attributes and Inversion

Lal, Hari
GEOPIC, ONGC, Dehradun, India.

Identification and mapping of Late-Oligocene-Miocene-Pliocene reservoir sands in the Tripura frontal fold belt of Assam Ararkan Basin has been elusive due to relatively poor imaging caused by surface-logistics and subsurface complexities. The study area falls in the western part of a complex tectonic zone consisting of foreland basin setting initiated by collision and subduction of Indian Plate beneath the Burmese Plate towards east. The subsequent compresssional tectonics formed NNW-SSE to N-S Tripura-Chittagong Fold Belt consisting of series of anticlines and synclines arranged in en-echelon pattern. The sandstones of Bhuban Formation of Miocene age are gas bearing in both the anticlinal and synclinal parts.

The objective of present study was to map reservoir facies in intervening synclines by interpreting newly acquired 3-D seismic data. To meet the objective, robust interpretation workflows consisting of data enhancement, sequence stratigraphic analysis, seismic attribute analysis, spectral decomposition, impedance inversion and seismic guided multi-attribute log property mapping were applied to bring out the sand dispersal pattern. The available logs and geologic data were used for calibration and validation at the different stages of interpretation.

The enhancement in signal to noise ratio and continuity were achieved through spatial filters which minimised the acquisition footprints and random noise in 3D seismic. Sequence stratigraphic analysis consisted identification of basin setting (mostly from published data), paleodepostional environments (from published data, log motifs and seismic) and sequence stratigraphic framework building from identification of stratal terminations and chronostratigraphic surfaces in 3-D seismic.

Dip-azimuth, coherency and volumetric curvature attributes provided detailed geomorphology which was interpreted in terms of depositional environments. Attribute anomalies in linear to lobe-like geometries are aligned in the northeast-southwest direction. The stratal terminations, retrogradational and progradational stacking patterns indicate deposition by deltaic processes in transgressive to highstand conditions with possible provenance towards north to northeast. Integrating impedance and log properties, e.g., porosity, sand-volume etc., anomalies were interpreted as generated from relatively thin gas-sands embedded in shales. Both older "relict" features and younger channel fills are considered as prospective.

AAPG Search and Discovery Article #90155©2012 AAPG International Conference & Exhibition, Singapore, 16-19 September 2012