Application of Spectral Decomposition for the Identification of Sand Fairways and Prediction of Net Sand Thickness - Cb/Os-2, Gulf of Khambat, Western Offshore India - A Case Study

Sivasankar Josyula, Sumit Bhardwaj, Nikhilesh Dwivedi, and Kondal Reddy
Reservoir Development, Cairn India, Gurgaon, India

The Lakshmi Field, in CB/OS-2 Block, Western off-shore, India encountered oil sands of Lower Miocene age. Confined within a 75 metre (m) interval, there are five sands (sand body thicknesses vary 1-18m) which are interpreted as hybrid facies of channel origin. Their seismic response is poor due to phase distortion and the masking effect of overlying gas sands. The time separation between the sands is inadequate for mapping and amplitude studies and Pre/Post stack Inversion studies were unsuccessful to predict lithology. However, production and DST data suggested the existence of potential oil accumulations. In this context, Spectral decomposition of seismic data, which under favorable conditions enables resolution of thin sands which are not visible on normal seismic, has been attempted.

The Spectral Decomposition method involved the identification of time window, seismic band width, tuning frequencies of sands and generation and analysis of Frequency volume. It is observed that 5-6m sand have tuning amplitudes (high) for 80Hz whereas 15m sand standout for 30-40Hz. Tuning Amplitudes when displayed in a map form defined the sand prone areas. Data at drilled wells suggested a linear relationship between tuning frequencies and net sand thicknesses which was used to generate net sand map for the area.

The Spectral decomposition method identified two sand fairways in the Lakshmi field. Apart from validating the predictions at drilled locations this tool revealed large untested areas for development. Two wells, which were drilled as part of the oil development in the last quarter of 2007, proved hydrocarbons in these fairways. Reservoir characteristics of these fairways are consistent with the predictions and further appraisal drilling is in progress. The Spectral Decomposition tool delineated the areas of Reservoir distribution for Development.

Presentation GEO India Expo XXI, Noida, New Delhi, India 2008©AAPG Search and Discovery