Mapping reservoirs based on resistivity and induced polarization derived from continuous 3D magnetotelluric profiling: Case study from Qaidam basin, China

In Sanfu, Qaidam basin, China, traditional geophysical methods have failed to find subtle hydrocarbon reservoirs. In an attempt to predict and delineate gas reservoirs, we used a type of magnetotelluric (MT) profiling called 3D continuous electromagnetic profiling (CEMP). Electric logs indicate that gas-bearing formations have high resistivity relative to nongas-bearing formations. Obvious resistivity anomalies derived from MT sounding curves are interpreted to come from gas-bearing formations; we observed no such anomalous resistivity away from gas-bearing reservoirs. For CEMP, five electric components were recorded at each station; the inline electric components of all stations were measured using dipoles placed end to end. Becausethe survey area was quite wide, we divided it into three rectangular blocks for data processing and inversion. After noise removal and static corrections, the data from each block were inverted with a 3D nonlinear conjugate-gradient inversion method to obtain the spatial distribution of resistivity. Using this resistivity, we created a 2D model, which we inverted to determine the induced polarization (IP) parameters. We found that a high-resistivity anomaly and high IP anomaly are two key indicators when predicting and delineating the location of gas-bearing reservoirs. In our case study, a known gas-bearing formation had a high-resistivity anomaly and a high IP anomaly. We identified two similar anomalous regions outside the known gas-bearing formations. As a result, two new prospects were determined as targets worth drilling.