Identification of the structural trap of the hydrocarbon reservoir below volcanic rocks in Majalengka region, Indonesia, using the audio-magnetotelluric method

The exploration of hydrocarbons below volcanic rock is a significant challenge in increasing oil and gas reserves in Indonesia. Because mapping using seismic waves does not produce data of good quality in areas of volcanic rock, in this study, the audio-magnetotelluric (AMT) method was used to map the structural trap of a hydrocarbon reservoir. Geophysical surveys were also used to determine the subsurface resistivity values and phase values. The raw data were in the form of time series measurement obtained using equipment from Phoenix Geophysics. The data were processed further into apparent resistivity and phase curves toward frequency. Data processing involved various filtering methods and corrections. The result was a two-dimensional cross-section of the AMT measurement line. The data from the AMT model were interpreted in an integrated manner using geological data. The results showed a relatively good relationship between the AMT data and the geological data. The results indicate a fault zone in the area of measurement and bedding formations that have produced a petroleum system. The lower part of the Cinambo Formation, which consists of shale-dark, acts as a source rock. While the upper part of the Cinambo Formation which consists of massive sandstone with very fine sand to silty above the shale-dark is thought to be a reservoir. The hydrocarbon trap could be of the structural type because there is a fault zone in the area of measurement.