Delineation of discontinuity using multi-channel seismic attributes: An implication for identifying fractures in gas hydrate sediments in offshore Mahanadi basin

Abstract

Seismic attributes derived from post stack seismic data are important tools to delineate and characterize the subsurface geological features and discontinuities such as fracture, fault and many more. In this paper we have used two seismic profiles (MH-38A and MH-38B) of 2D multi-channel data along north-south direction with seafloor depth ranging from 1450 m to 2175 m in the deep offshore Mahanadi basin. The gas hydrate indicator, bottom simulating reflector (BSR) is easily identified but the discontinuities such as fractures are not observed directly from the seismic data. Therefore, we have computed the total four most suitable attributes which are reflection strength, instantaneous frequency, instantaneous bandwidth and instantaneous quality factor. We have compared the relative changes of the attribute values along the layers in the four derived attributes and hence, have recognized total three and seven discontinuities for MH-38A and MH-38B respectively, at near and away from the well location. Most faults are passing in NNW-SSE and N-S direction either in the gas hydrate layer or below BSR. These discontinuities are called as natural fractures which are validated by the fractures observed from resistivity image log at well site NGHP-01–19 and NGHP-01–09 for CDP ranges 1561 to 1681 in line MH-38A and 1357 to 1458 in line MH-38B respectively. The discontinuities in the gas hydrate sediments are useful for anisotropy study and to map the chimney system through which the free gas can flow from below BSR to up direction.