Ponta Grossa Arch Research Articles

Tholeiitic dyke swarm from the Ponta Grossa Arch and genetic relationships to the Paraná flood volcanics (Brazil)

Tholeiitic dyke swarm from the Ponta Grossa Arch and genetic relationships to the Paraná flood volcanics (Brazil)

Tholeiitic hypabyssal dykes: How many clinopyroxenes?

An augite-subcalcic augite trend is found in the coarse-grained tholeiitic dykes from the Ponta Grossa Arch (Brazil). Clinopyroxene is optically homogeneous but, for clinopyroxene approaching subcalcic augite, TEM/EDS shows heterogeneous exsolution products (lamellar exsolution and spinodal decomposition). The calcium content of this subcalcic augite estimated by single-crystal refinement [0.67 (6) atoms p.f.u.] agrees with the value obtained by TEM/EDS analysis of exsolved augite lamellae. Therefore, subcalcic augite does not exist as a single phase: it results from “contaminated” microprobe analyses arising from associated pigeonite and augite. Evolutive trends based on subcalcic augite compositions should therefore be considered with caution, unless detailed structural and microstructural data are available.

Regional Magnetotelluric Surveys in Hydrocarbon Exploration, Parana Basin, Brazil

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 km2 (463,000 mi2), 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.

Using Magnetotellurics in Regional Hydrocarbon Exploration of Parana Basin, Brazil: 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 natur l 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. End_of_Article - Last_Page 531------------