Interpretation of magnetotelluric data from the Gachsaran oil field using sharp boundary inversion

The magnetotelluric (MT) method is used in hydrocarbon exploration in situations where seismic exploration gives low quality data, or is logistically difficult. Static effects caused seismic data collected in the Gachsaran oil field to be of low quality, so a magnetotelluric survey was used to investigate the subsurface structures of this important reservoir. A total of 79 MT sites were collected on a 16.5 km long profile that was oriented perpendicular to the main geological strike direction. Dimensionality analysis indicated that the resistivity structure of the region is 1D or 2D mixed with some local 3D structures at shallow depths. It was found to be mainly 3D at greater depths. Dimensionality analysis, phase tensor and tipper all showed that the prevalent geoelectrical strike in the Gachsaran oil field is in a NW-SE direction. The time domain electromagnetic (TEM) method was utilized to remove the effects of static shifts. To obtain physically reasonable 2D resistivity models of the Gachsaran oil field, the nonlinear conjugate gradients (NLCG) and sharp boundary inversion algorithms were used. In the inversion of the Gachsaran MT data, the NLCG method recovered the general resistivity distribution of the subsurface structures, but the sharp boundary inversion recovered a layered resistivity model with substantially less internal variation within the layers. The model obtained by the sharp boundary inversion showed that the top of reservoir formation (Asmari) in the study area was located at a depth of 1400-1900 m below sea level. The inversion results were in reasonable agreement with mapped geological features and well logs. The results of this study show that the magnetotelluric method can be utilized effectively in hydrocarbon exploration in the Gachsaran oil field, and similar types of hydrocarbon reservoir elsewhere in the world.