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In addition to the Brazilian part of the Parana basin this paper includes geologic data on the sedimentary basin extension into the neighboring countries of Argentina, Bolivia, Paraguay, and Uruguay, and also briefly discusses the geological development of the Bolivian Andean and the Argentinian Andean, Pampean, and Patagonian regions. The entire area thus studied lies between Lat. 16° and 40° S., and between Long. 46° and 70° W. The present area of the basin is approximately 1,200,000 sq. km. in Brazil alone; at various geologic ages the sedimentary basin embraced several times this area in south-central South America.
Only those stratigraphic units are described which are of interest as petroleum sources or reservoirs, or which had a bearing on the tectonism of the basin. These are the Devonian marine series, the Lower Pennsylvanian glacials and marine transgressions, the Upper Pennsylvanian coal-bearing beds and marine transgressional deposits, the Lower Permian group with the bituminous shales, the Upper Permian group with brackish to fresh-water formations, the Upper Triassic continental eolian deposits, and the Juro-Triassic extrusive lavas and intrusive diabase dikes and sills.
The section on paleogeography is accompanied by paleogeographic and environment maps for many of the discussed stratigraphic units.
Early geologic history of the Parana basin is traced, mainly for background reference, from the great geosynclinal troughs, and mountain-building periods of early pre-Cambrian, through the equivalents of the Laurentian, Penokean, and Taconic revolutions, to the Caledonian revolution at the end of the Silurian. Late Silurian outlined the general framework of the Parana basin and defined the limits of its subsequent oil prospective deposits of middle and upper Paleozoic and Mesozoic sediments.
The widespread deposits of black, marine, bituminous shales of lower Devonian are overlain unconformably by the successive cycles of glacial deposits of the lower Pennsylvanian age. These glacial sheets covered nearly the entire southern part of the continent. At least five distinct glacial advances from the south, with marine transgressions from the west and the northwest during the interglacial epochs, have been identified.
General epeirogenic fluctuations of the basin were responsible for the shallow-water type of deposition during upper Pennsylvanian and Permian time. Lower Mesozoic is represented by a period of eolian sand deposition, giving way in Rhaetic time to the great, deep fractured lava flows. These geoclases probably reached the salsima magmatic zone and caused the decompression of the basic magma chambers and the subsequent ascension of lava which, in successive flows, spread over the entire region of central South America in thicknesses up to 1,500 meters.
In the northern part of the basin supra-basaltic eolian sands were deposited during the Jurassic, followed by Upper Cretaceous fluviatile and limnic deposits. During Tertiary epeirogenic movements uplifted the Brazilian coastal shield, the elevation of the old mountain ranges being accompanied by faulting. Cenozoic deposits are represented by small and restricted flood-plain basins and by marine coastal deposits. The paleogeographic development of the neighboring countries is also briefly discussed.
The study then deals with the principal structural features and finishes with a discussion of the favorable areas and oil possibilities.
A combination of 1,200 meters of non-bedded glacial deposits and 1,500+ meters of extrusive lava deposits within a 4,500-meter section comprising also marine and eolian deposits at once renders normal methods of geological and geophysical exploration rather ineffective. Though the individual lava flows can be determined, they can not be effectively mapped by surface geologic methods over the vast area of outcrop. Neither is the seismic method of geophysical exploration effective since the energy of the charge is dissipated before reaching the base of the lava, even with a multi-hole shot pattern. Long reconnaissance gravity lines across the basin have given some indications of relative lava thicknesses but are not effective for detailed studies of deeper strata.
In resume the stratigraphic sequence demonstrated that, though the section can not be considered of prime favorability for oil generation and accumulation, it does offer sufficient possibilities to merit exploratory
drilling. The numerous oil seeps of southern Brazil, most of which originated from sediments of the same age as those producing in the north Argentinian and southern Bolivian oil fields, enhance the oil possibilities of south Brazil and do warrant continued exploratory investigations.
The factors considered as unfavorable to migration, accumulation, and possible production of oil may be summarized as: (a) lack of anticlines or folded structures, (b) very low regional dips, (c) no thrust of reverse faulting, and (d) the indication that the many epeirogenic subsidences which could have caused generation, migration, and accumulation of oil, also could have caused the oil to have been spilled from its trap, thereby allowing its updip escape to the outcrop.
The favorable factors are: (a) possibility that some hingeline oil still remains in fault traps along the present outcrop line, (b) a strong possibility that any Pennsylvanian or Permian generated oil did not migrate until, or after, the overburden of the basaltic lava, thus being accumulated in fault blocks against diabase dikes, (c) oil which may have been released for upward migration along dike lines after the lava flow may be accumulated in the eolian Botucatu sandstone underlying the lava cap (one outcrop occurrence known).
Regarding actual oil possibilities, an oil occurrence and specific gravity map was made to show the origin, movement and final accumulation of liquid hydrocarbons throughout the entire Parana basin and adjoining sub-Andean area. The conclusions drawn from that map are (1) the contact zone of lower Pennsylvanian and Devonian is responsible for most of the oil seeps in both the Bolivian sub-Andean area and in the Parana basin, and (2) almost all of the Parana basin oil seeps are on the supposed eastern hingeline of the basin. It is further significant that the eastern hingeline nearly approximates the assumed subcrop limit of present-day Devonian bituminous shale beds. This enhances the probability that any oil found in Parana basin will probably be Devonian generated and both Devonian nd Pennsylvanian reservoired oil.
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