Marine controlled source electromagnetic inversion data from a field at Campos basin SE-Brazil, post-salt reservoirs constrained by seismic and well log

This work presents results from a controlled source electromagnetic (CSEM) set of inversion runs of real data in the Campos basin, Southeast Brazil. The Campos basin is in the Brazilian east margin, with origin in the Neocomian stage of the Cretaceous period 145–130 million years ago during the breakup of the supercontinent Gondwana (South America and Africa split). The clastic reservoirs in this basin have been the largest oil producer in Brazil for the past three decades. The present challenge in exploring clastic reservoirs has moved to deeper waters, enforced by the complex geology posed by the tectonics associated with giant salt domes. Electromagnetic (EM) methods are sensitive to subsurface resistive variations and have been frequently used in exploration programs for fresh water, mining and for hydrocarbon (HC), mainly in joint approaches with seismic to minimize ambiguity in the interpretations. Reservoirs filled with HC are generally more resistive than the host rocks which is an advantage for the EM method. Combining resistive models with seismic results considerably improves the resolution of the subsurface structures and the geometry of the HC reservoirs. A set of 40 Controlled Source EM (CSEM) receivers were deployed at the seafloor (water thickness is around 1.7 km) in a grid-shape array with varying spacing between 5 km to 10 km. The electric field components have been recorded from an active electromagnetic source towed 50 m above the seafloor. The processed data consist of inline components of the horizontal electric field (Ex and Ey) for four main frequencies (0.125 Hz, 0.25 Hz, 0.5 Hz and 1.25Hz). The CSEM 3D inversion models have been obtained using the modified version of Modular System for EM inversion (ModEM code) which is under development within a research project at Observatório Nacional – Brazil. Preliminary 3D CSEM inversion results yielded a good agreement with resistivity values observed in the well logs. In the second step of this work, information from seismic and well logs are being introduced as a priori information to improve resolution of the CSEM inversion in different scenarios involving hydrocarbon accumulations in thin post-salt reservoirs in Campos basin.