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


Institute of Geophysics, Associate Professor


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.  


Main Subjects

Agard, P., Omrani, J., Jolivet, L. and Mouthereau, F., 2005, Convergence history across Zagros, Iran; constraints from collisional and earlier deformation, Int. J. Earth Sci., 94, 401-419.
Beamish, D. and Travassos, J. M., 1992, Magnetotellurtc imaging of basalt-covered sediments, First Break, 10(9), 345-357.
Cagniard, L., 1953, Basic theory of the magneto-telluric method of geophysical prospecting, Geophysics, 18, 605-635.
Constable, S. C., Parker, R. L. and Constable, C. G., 1987, Occam’s inversion: a practical algorithm for generating smooth models from electromagnetic sounding data, Geophysics, 52(3), 289-300.
Hoversten, G. M., 1996, Papua New Guinea MT: looking where seismic is blind, Geophysical Prospecting. 44, 935-961.
Jiracek, G., 1990, Near surface and topographic distortions in electromagnetic induction, Surv. Geophys. 11, 163-203.
Kalscheuer, T., Garcia, M., Meqbel, N. and Pedersen, L. B., 2010, Non-linear model error and resolution properties from two-dimensional single and joint inversions of direct current resistivity and radiomagnetotelluric data, Geophys. J. Int., 182(3), 1174-1188.
Kalscheuer, T., Hübert, J., Kuvshinov, A., Lochbuehler, T. and Pedersen, L. B., 2012, A hybrid regularization scheme for the inversion of magnetotelluric data from natural and controlled sources to layer and distortion parameters, Geophysics, 77(4), 301-315.
Mansoori, I., Oskooi, B. and Pedersen, L. B., 2015, Magnetotelluric signature of anticlines in Iran's Sehqanat oil field, Tectonophysics, 654, 101-112.
Mansoori, I., Oskooi, B., Pedersen, L. B. and Javaheri, R., 2016, Three-dimensional modelling of magnetotelluric data to image Sehqanat hydrocarbon reservoir in southwestern Iran, Geophysical Prospecting, 64, 753-766.
Martini, F., Hobbs, R. W., Bean, C. J. and Single, R., 2005, A complex 3D volume for sub-basalt imaging, First Break, 23, 41-51.
Motiei, H., 1995, Geology of Iran, petroleum geology of Zagros, Geological Survey of Iran, Tehran.
Orange, A. S., 1989, Magnetotelluric exploration for hydrocarbons, Proceedings of the IEEE 77, 287-317.
Oskooi, B., 2004, A Broad View on the Interpretation of Electromagnetic Data (VLF, RMT, MT, CSTMT). Acta Universitatis Upsaliensis, Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, 959, 68 pp. Uppsala. ISBN 91-554-5925-0.
Oskooi, B., Pedersen, L. B., Smirnov, M., Arnasson, K., Esteinsson, H. and Manzella, A., the DGP working group, 2005, The deep geothermal structure of the Mid-Atlantic ridge deduced from MT data in SW Iceland, Phys. Earth planet. Inter., 150, 183-195.
Oskooi, B. and Darijani, M., 2013, 2D inversion of the magnetotelluric data from Mahallat geothermal field in Iran using finite element approach, Arab. J. Geosci., doi: 10.1007/s12517-013-0893-6.
Oskooi, B., Pedersen, L. B. and Koyi, H. A., 2014, Magnetotelluric signature for the Zagros collision, Geophys. J. Int., 196, 1299-1310.
Oskooi, B. and Mansoori, I., 2014, Iodine-
bearing saline aquifer prospecting using
magnetotelluric method in Golestan plain, NE Iran, Arab J Geosci, doi:10.1007/s12517-014-1634-1.
Oskooi, B., Mansoori, I., Pedersen, L. B. and Koyi, H. A., 2015, A magnetotelluric survey of ophiolites in the Neyriz area of southwestern of Iran, Pure Appl. Geophys., 172(2), 491-502.
Pandey, D., MacGregor, L., Sinha, M. and Singh, S., 2008, Feasibility of using the magnetotelluric method for subbasalt imaging at Kachchh, India, Applied Geophysics., 5(1), 74-82.
Pedersen, L. B. and Engels, M., 2005, Routine 2-D inversion of magnetotelluric data using the determinant of the impedance tensor, Geophysics, 70, 33-41.
Siripunvaraporn, W. and Egbert, G., 2000, An efficient data-subspace inversion method for 2-D magnetotelluric data, Geophysics, 65, 791-803.
Swift, C. M., 1967, A magnetotelluric investigation of electrical conductivity anomaly in the southwestern United States, Ph.D. Thesis Massachusetts Institute of Technology, Cambridge, MA.
Takin, M., 1972, Iranian geology and continental

drift in the Middle East, Nature, 235, 147-150.
Travassos, J. M. and Menezes, P. T. L., 1999, Geoelectric structure beneath limestones of the Sao Francisco Basin, Brazil. Earth Planets Space, 51, 1047-1058.
Vozoff, K., 1991, The magnetotelluric method, in J. D, Corbett, ed., Electromagnetic method in applied geophysics-applications part A and part B, Society of Exploration Geophysicists, 641-711.
Warren, R. K. and Srnka, L. J., 1992, Exploration in the basalt-covered areas of the Columbia River Basin, Washington, using electromagnetic array profiling (EMAP), Geophysics, 57(8), 986-993.
Warren, R. K., 1996, A few case histories of subsurface imaging with EMAP as an aid to seismic processing and interpretation, Geophysical Prospecting, 44, 923-934.
Xiao, W. and Unsworth, M., 2006, Structural imaging in the Rocky Mountain Foothills (Alberta) using magnetotelluric exploration, AAPG Bulletin, 90(3), 321-333.
Zhang, P., Roberts, R. G. and Pedersen, L. B., 1987, Magnetotelluric strike rules, Geophysics, 52(3), 267-278.