Magnetotelluric signature of anticlines in Iran's Sehqanat oil field

Abstract

The magnetotelluric (MT) method has proved to be an effective tool in hydrocarbon exploration especially in areas with geological structures/formations where seismic reflection provides neither good quality data nor images. The Sehqanat oil field located in the sedimentary zone of Zagros in SW of Iran is a typical example. It is covered by the high velocity and heterogeneous formation of Gachsaran, which is exposed at the surface and has a thickness varying from 500m to more than 2km in the region. Gachsaran is composed mainly of salt and evaporites overlying, as a cap rock, the Asmari limestone formation which is the main reservoir in all oil fields of Iran along the Zagros range. The main geological interface which is targeted to be imaged with the MT method is the contact between the highly conductive evaporites of the Gachsaran formation and the underlying more resistive carbonates of the Asmari formation. MT data at more than 600 stations along five parallel SW–NE profiles crossing the main geological trend of the study area and transient electromagnetic data over 400 stations to be used for static shift corrections of the MT data were available. Dimensionality and strike analysis of the MT data show dominant two-dimensional (2-D) conditions in almost all sites and periods. The 2-D resistivity models resolved the boundary between Gachsaran and Asmari formations as a transition zone from highly conductive to resistive structures. The Sehqanat anticline has also been delineated throughout the 2-D resistivity sections as a resistive dome-shaped body located in the middle part of the MT profiles. There is a considerable correlation between the 2-D resistivity models and the adjacent 2-D reflection seismic sections so that a more reliable interpretation on the hydrocarbon trap of the Sehqanat anticline can be obtained.