Controlled-Source Electromagnetic (Csem): Complementing AVO as Prospect Qualifier, Offshore Sabah, NW Borneo

Amplitude versus Offset (AVO) analysis has been utilised to evaluate potentially hydrocarboninduced seismic amplitude variation with offset. One major uncertainty is that reservoirs with 10% gas saturation will have similar AVO responses to commercially saturated reservoirs (>60% hydrocarbon saturation). In frontier deepwater areas that lack of well control, an independent, non-seismic method like marine controlled-source electromagnetic (CSEM) survey becomes an important technique to assess the risk<br>of low saturation gas reservoirs. The CSEM measurement is sensitive to resistivity contrasts, it can potentially differentiate hydrocarbon saturated reservoirs (highly resistive) and the surrounding conductive sediments. Furthermore, it may also be able to discriminate reservoirs with commercial saturation (tens-thousands m resistivity) from those with residual saturation. Stochastic AVO modelling performed on Prospect X in Offshore Sabah, NW Borneo, indicates the presence of hydrocarbons as well as a chance of having low saturation gas. The CSEM interpretation on the Prospect X, however, reveals a 20-60% electric magnitude increase of the target response over a chosen background, which indicates a hydrocarbon-related resistive body. Further interpretation suggests that significantly thick sand with resistivity of 100 m is the most likely cause for the CSEM anomaly; hence, it derisks the possibility of low saturation gas being present in the prospect. The combined AVO-CSEM interpretation is a compelling prospect qualifier in the Sabah deepwater setting, where (1) drilling an expensive deepwater well is not justified based on amplitude anomaly alone, particularly when gas-charged siltstone and “fizz” gas reservoirs are common, (2) the absence of nonhydrocarbon highly resistive lithologies such as salts, volcanics, and thick limestones avoids misleading resistivity interpretations, (3) the water depth is sufficient to suppress the air-wave effect that might otherwise mask any potential highly resistive anomalies, and (4) the reservoir depth below seabed is suitable for this combined interpretation to be successful in finding commercially saturated hydrocarbon reservoirs.