Investigating subsurface structures of Gachsaran oil field in Iran using 2D inversion of magnetotelluric data

The magnetotelluric method is most often considered in hydrocarbon exploration in cases which are difficult for seismic imaging. In 2012, the National Iranian Oil Co. (NIOC) conducted an electromagnetic survey which included magnetotelluric and time domain electromagnetic (TEM) methods in the Gachsaran oil field in the south-west of Iran to delineate the reservoir formation of the region. Magnetotelluric data were collected at 5215 sites and a regular site spacing of 200mwas utilised. We chose a 16.5 km profile perpendicular to the main geological strike direction in the study area. The Bahr's skew and Mohr diagrams were used to perform the dimensionality analysis of the magnetotelluric (MT) data and indicated that the subsurface structures are one dimensional or two dimensional at shallow depths, whereas they are mainly three dimensional at lower depths. The phase tensor showed that the dominant geoelectrical strike in the survey area is in the NW-SE direction. Two dimensional inversion was utilised to acquire a realistic resistivity model that was compromise between the spatial smoothness of the inversion model and theMTdata fit. Apparent resistivity and phase data were modelled using the smoothness-constrained least-squares method. Models obtained of the TM, TE and TM+TE mode data were examined to have the best possible interpretation. The resulting 2D model revealed the main anticline and overthrust zone in the region. The near surface layer in the model which has a low resistivity, was identified as the cover rock of the region. The formation of the top of the reservoir in the region is estimated to be located at the depth of 1400-1900m below sea level. The resistivity model is in good correlation with the geological features and the results of well drilling.