2-D and 3-D inversion of the magnetotelluric data to explore hydrocarbon structures in the Sehqanat oil field, SW Iran

Among all the geophysical techniques, the magnetotelluric method has improved considerably in recent years and is widely being used in hydrocarbon exploration especially in regions where reflection seismic has difficulties. Areas which are covered with high velocity rocks in the near surface are most popular cases. A huge high resolution magnetotelluric investigation was conducted in the Sehqanat oil field, SW of Iran, in 2013 to map geoelectrical structure of the region from surface down to several kilometers. The Sehqanat oil field is located in sedimentary Zagros zone which is encompasses more than 95 percent of Iran’s oil fields. The main geological interface which is targeted to be imaged with magnetotelluric method, due to the large resistivity contrast (based on the well logs information), is the contact between the highly conductive evaporites of Gachsaran formation and the more resistive underlying carbonates of Asmari formation. Regarding the large thickness of the high-velocity (ca. 4500 m/s) and heterogeneous Gachsaran Formation outcropping in the Sehqanat oil field and several adjoining oil fields in the study area, imaging of the underlying layers is difficult with the reflection seismic technique. On the other hand, the big contrast of the electrical resistivity between the Gachsaran Formation and the underlying layers is favourable for MT exploration. The geoelectrical contrast is well documented from the full-set log measured along the explorative Sehqanat well. The high velocity and very heterogeneous Gachsaran formation is exposed on the surface and has a varying thickness from 500 meter to more than two kilometers in the region and also covers the Asmari formation which is the main reservoir in SW oil fields of Iran, as a cap rock. Geologically, the Sehqanat oil field has been formed by a gentle and moderate-size anticline called “Sehqanat” which its structural shape, due to the low quality of reflection seismic data, is not clearly known for geologists. The Sehqanat anticline acts as a structural oil trap from aspect of the petroleum geology. In order to collect more geophysical information about the subsurface morphology of the Gachsaran-Asmari formations boundary as well as Sehqanat anticline, broadband magnetotelluric data were acquired at more than 600 stations along five parallel southwest-northeast profiles crossing the main geological trend of the study area. Transient electromagnetic data were also acquired over 400 stations along the mentioned profiles to be used for static correction of magnetotelluric data. Dimensionality and strike analysis of the MT data show 3-D effects in a considerable amount of sites and periods. Therefore in order to get a comprehensive view through the subsurface resistivity distribution of the Sehqanat oil field, two- and three-dimensional inversions were performed on the magnetotelluric data. The 2-D and more precisely 3-D resistivity models, resolved the Gachsaran-Asmari formations boundary as a transition zone from high conductivity to more resistivity range. The Sehqanat anticline has also been delineated throughout the 2-D and 3-D resistivity models as a resistive dome-shaped body corresponded to the middle parts of MT acquisition profiles. Correlation of the magnetotelluric resistivity models with the adjacent 2-D reflection seismic sections is remarkable, letting us to accomplish more reliable interpretation of subsurface geology of the survey area.