Regional magnetotelluric surveys in hydrocarbon exploration, Parana Basin, Brazil

Abstract

The magnetotelluric geophysical method has been used effectively as a hydrocarbon exploration tool in the intracratonic Parana basin of South America. The Parana basin has an area of about 1,200,000 km 2 (463,000 mi 2 ), extending over portions of Brazil, Paraguay, Uruguay, Argentina, and Bolivia. The Paleozoic marine sedimentary rocks in the Parana basin are covered by the world's most extensive flood basalt complex, making geophysical exploration extremely difficult, although modern seismic techniques recently applied are achieving good results in some areas. The 1-2 km (3,300-6,600 ft) thick surface basalts and buried diabase sills pose no problem for the magnetotelluric method because the natural electromagnetic fields used as the energy source pass easily through the basalt. Data for the regional study were taken on six profiles with soundings spaced 8 to 15 km (5 to 9 mi) apart. The magnetotelluric sounding data outline a linear uplift known as the Ponta Grossa arch. This major structural feature cuts across the northeast-trending intracratonic basin almost perpendicularly, and is injected with numerous diabase dikes. Although its character is reasonably well known in the shallow, eastern parts of the basin, it is poorly delineated in the deeper parts of the basin where promising natural gas zones have been tested in several wells. In the survey area, MT interpretations show that basalts have aggregate thickness of as much as 2 km (6,600 ft), and basement may be as much as 6 km (20,000 ft) below the surface. Over most of the basin, the basalts are covered by Upper Cretaceous to Holocene continental sediments of a few hundred meters thickness and are underlain by 2 to 4 km (6,600 to 13,100 ft) thick Paleozoic sediments with possible hydrocarbon potential. The Ponta Grossa arch is interpreted to be the failed arm of a triple rift system that formed during the separation of the African and South American continents. Shales of the Devonian Ponta Grossa Formation are important Parana basin source rocks for hydrocarbons, as are several Permian units. Significant electrical contrasts occur between the Permian sediments and older units, so that magnetotelluric measurements can give an indication of the regional thickness of the Permian and younger sediments to aid in interpreting hydrocarbon migration patterns and possible trap areas.