Exploration of Hydrocarbon Reservoirs by Modelling of the Magnetotelluric Data

Abstract:

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. The Sehqanat oil field is located in sedimentary Zagros zone comprising ranges of the biggest oil fields in Iran. The most interesting target in this survey was geological contact between Gachsaran and Asmari formation. According to the electrical logs information, large resistivity contrast exists at the boundary of two mentioned formations. The Gachsaran formation is formed by tens to hundreds meter of evaporites which is highly conductive (ca. 1-10 ohm-m) and the Asmari formation, in contrary, comprising of dense carbonates with more electrical resistivity range (more than 100 ohm-m). With respect to the petroleum geology of the study area, the Sehqanat oil field has been formed by a gentle and moderate-size anticline called “Sehqanat” which acts as a structural oil trap. The role of cap rock is played by Gachsaran formation as an ideal impermeable seal covered the Asmari formation which is the biggest reservoir in the region containing huge amount of hydrocarbon. The broadband magnetotelluric data were collected in a 3-D pattern at more than 600 stations along five parallel southwest-northeast profiles crossing the main geological trend of the study area. Although dimensionality and strike analysis of the magnetotelluric data showed that the local 2-D assumption of the subsurface geological features is satisfactory but there were also remarkable 3-D effects in some of the sites and periods. Therefore, in order to image a comprehensive subsurface resistivity map throughout the Sehqanat oil field, three-dimensional inversion was performed on the magnetotelluric data as well as two-dimensional routine. As one would expect, the 2-D inversion results depicted the main resistivity structures while the 3-D inverted model shows significantly more details and consistency. 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.