6th International Symposium on Three-Dimensional Electromagnetics Berkeley, California, USA, March 28-30, 2017 3D magnetotelluric imaging of basalt-covered sediments and fragmented basement of central Paraná Basin, SE Brazil Ved P. Maurya 1 , Sergio L. Fontes 1 , Max A. Meju 2 , Emanuele F. La Terra 1 , Leonardo G. Miquelutti 1 David Taveira 1 and Irineu Figueiredo 1 1 MCTIC-Observatorio Nacional, Rio de Janeiro, Brazil 2 Petronas Upstream Exploration, Kuala Lumpur, Malaysia SUMMARY The Paraná basin is a major Phanerozoic feature in Brazil and adjacent countries. Data from 261 MT stations distributed along 3 lines (P1, P2 and P3) have been inverted in 3D to reveal the resistivity structure from surface to 50km depth. The average station spacing is 1.8 km and the frequency bandwidth is 1000 to 0.001 Hz. 2D inversion was also done for comparison with an earlier roughly parallel regional line P0 and found to be consistent with our new 3D model. The goodness of our 3D model was first evaluated by comparison with data from 3 deep exploration wells within our study area. The 3D model correctly identified resistive (>100 Ωm) basalt cover, underlying conductive (~10 Ωm) sediments (up to 5-6 km thick) and resistive (>1000 Ωm) crystalline basement. The MT results are consistent with other geophysical anomalies previously observed in the study area; the segmented midcrustal resistive layer found in a zone of depressed Moho is interpreted as evidence of magmatic underplating and has coincident gravity and magnetic anomalies. We interpret our model as presenting clear magnetotelluric evidence for a heavily fragmented basement and well-preserved half-grabens beneath flood-basalt in the central portion of the basin. We suggest that the basin is highly compartmentalised and hence has reduced hydrocarbon prospectivity in our study area. Keywords: 3D MT joint inversion, Pre-basalt exploration, well-based model validation, case study INTRODUCTION The intracratonic Paraná Basin comprises a thick and extensive sedimentary-magmatic sequence, covering an approximate area of 1,700,000 km 2 in central eastern South America (Brazil, Uruguay, Paraguay, and Argentina). It has an irregular oval shape with the major axis in the NNE direction (Figure 1a). The deep structure of the basin has been the subject of many geological (Figure 1b) and geophysical investigations but is still not well understood. Gravity and magnetic data have been interpreted to suggest a fragmented basement (e.g., Mantovani et al., 2005). Julia et al. (2008) also infer the presence of a fragmented cratonic root in the basin from receiver function analysis. Recently, 3D “whole-basin” inversion of GDS data integrated with 2D MT inversion of line P0 (see Figure 1a) by Padilha et al. (2015) reveal major crust-mantle conductivity anomalies associated with magmatism beneath the thick sedimentary cover. They focused on “continental flood basalt effects hidden beneath the thick intracratonic sedimentary basin” and the basinal features of potential significance in hydrocarbon exploration (Figure 1b) were not well resolved by this regional-scale imaging of GDS data with average station interval of ~100 km and MT data with ~10 km average station spacing. Following the early attempts using 1D MT methods to image below the basalt cover for hydrocarbon exploration (e.g., Beamish and Travassos, 1992), there is now a renewed interest in hydrocarbon prospectivity evaluation of the basalt covered sediments in the basin using advanced methodologies and the 3 new commercial MT lines (P1, P2 and P3) acquired by the Brazilian Agency of Oil, Natural Gas and Biofuels (ANP) with average station spacing of 1.8km, which are inverted in 3D here, are part of this effort. The issues we identified and hope to resolve with the aid of 3D MT joint full-impedance and VTF inversion include: (i) What is the basement structure and does it control surface deformation? (ii) What is the structural integrity of the basalt cover and/or diabase sills (potential seals); are they continuous or broken by faults? (iii) How accurately can MT map the sediments below basalts in the presence of diabase sills? Data from 3 wells (E, C & D in Fig 1c) will be used for ground- truthing of MT models. Interestingly, the isopach map shows 7km sedimentary thickness at the deepest part of the basin SW (just outside) of our MT study area (Fig 1a). Notice that the MT profiles did not cross the 7000m isopach, so we would not expect to find >6000m deep basin on any of the profiles. But can 3D MT imaging truly sense an off-line conductor corresponding to this deepest basinal section? (iv) Padilha et al (2015) image a 3D conductive feature at the position of Paraná River. Is the Paraná River conductivity anomaly real? This river is crossed by our