ABSTRACT

The magnetotelluric (MT) geophysical method has been used effectively as a hydrocarbon exploration tool in the intracratonic Parana basin of South America. The Parana basin, an area of about 1,200,000 km2 (463,322 mi2), extends over portions of Brazil, Paraguay, Uruguay, Argentina, and Bolivia. It has been only sparsely explored by drilling, and only a few of the 100 or so complete wells have penetrated basement rocks. The basin is covered by the world’s most extensive flood basalt complex, making geophysical exploration of the underlying Paleozoic sediments extremely difficult, although modern seismic techniques have begun to achieve good results. The thick surface basalts and buried diabase sills pose no problem for the MT method, because the natural electromagnetic fields utilized as the energy source pass easily through the basalt and underlying sediments to provide scattered signals containing critical information on thicknesses and resistivities of the geologic units.

The MT method has been utilized in a multidisciplinary exploration program recently completed in the Parana basin by the State of Sao Paulo. In the deeper portions of the Parana basin, MT interpretations show that basalts have thicknesses of up to 2 km (6,500 ft) and that basement may be as much as 6 km (19,700 ft) below the surface. In most of the basin, the basalts are covered by thin units of Upper Cretaceous to Holocene continental sediments and are underlain by 2-4 km (6,500-13,000 ft) of prospective Paleozoic sediments. In addition, interpretation of the MT sounding data with layered and fault-dike models outlines a linear uplift known as the Ponta Grossa arch.

Permian Irati sediments are an important source unit classified as mostly in the “oil window.” Good electrical contrasts occur between the Permian sediments and older units, so that MT measurements can indicate the regional thickness of the Permian and younger sediments for use in interpretation of migration patterns and possible traps. In addition to providing this stratigraphic information, MT and aeromagnetic surveys have delineated the influence of the Sao Francisco craton in truncating uplift and tensional features of the Ponta Grossa arch.

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