Presence of Hydrocarbon Related Diagenetic Zone (Hrdz) and Evidence of Seepages in Kerala-Konkan Offshore

Kalyan K. Saha¹ and Siddhartha Sengupta^2
1Forward Base, ONGC, Agatala, India
²Remote Sensing & Geomatics, ONGC, Dehradun, India

The knowledge of hydrocarbon seepage and its detection through Synthetic Aperture Radar (SAR) technology has immensely helped in reducing the risk factor in high cost exploration environment. The present study aimed at recognizing seepage related anomalies in the seismic, over an area already identified by SAR, as having hydrocarbon seepages. The seepage points when plotted over the seismic map revealed that the seepages are near to the shelf edge, a zone of prominent weakness. The time pull up anomaly observed in one of the section remained more or less unchanged even after reprocessing with repeated velocity picking in closed interval. These shallow level linear anomalous features were correlated to Hydrocarbon Related Diagenetic Zone (HRDZ). The three characteristic features of HRDZ’s namely, Amplitude anomaly, Time pull-up and Stack response degradation was effectively seen in the section as in case of Tahbilik gas field. The strong reflection zone caused by the deposition of carbonates is supposed to have been formed due to the leakage of deep-seated faults bringing the gas into the shallower zone. On the other seismic lines, though there is no conspicuous HRDZ present but the area seemed to be highly affected by numerous shallow level faults. These features are related to HRDZ, as HRDZs generate hardened and faster portion of the sediments, leading to scattering and distortion of signals. The seepages were also overlaid over the magnetic and gravity map of the offshore but no anomalous features were noticed. The presence of seeps with HRDZ related features and reported BSR-like features points to the presence of a petroleum system within the Kerala-Konkan basin thus reducing the exploration uncertainty. It is recommended that a geochemical survey and also a close girded 2D or 3D multichannel seismic survey to be carried out over the area for further studies.

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