A large-scale magnetotelluric (MT) survey was designed to supply additional information to seismic studies for petroleum exploration in an area characterized by Cenozoïc normal faulting with graben-like structures filled up with clastic rocks . Seismic data is of high quality within the valleys and of very poor quality in between, due to outcropping hard limestones and/or igneous rocks. Initial drilling results show that even in high--quality seismic data areas , interpretation was erroneous because of unexpected thick interbedded igneous rocks .