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The AAPG/Datapages Combined Publications Database

AAPG Bulletin

Abstract


Volume: 52 (1968)

Issue: 3. (March)

First Page: 528

Last Page: 529

Title: Oil and Gas Accumulation in Reconcavo Basin, Brazil: ABSTRACT

Author(s): Joao Italo Ghignone, Geraldo De Andrade

Article Type: Meeting abstract

Abstract:

The Reconcavo basin, on the Atlantic coast near the city of Salvador, includes about 3,850 sq mi and is the principal petroleum province of Brazil. Since

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1939, 255 wildcats have been drilled, resulting in the discovery of 43 areas of accumulation, accounting for 942 million bbl and 992 Bcf, respectively, of producible oil and gas. The API gravity of most of the oil ranges from 35° to 40°.

The Bahia Supergroup, the principal objective for petroleum exploration, has a maximum thickness of 20,000 ft. These sediments are nonmarine, and range in age from Late Jurassic to Early Cretaceous. The Late Jurassic deposits consist of a typical redbed association, which is overlain by the blanket Sergi Sandstone, the best reservoir rock of the basin. The Early Cretaceous (Neocomian) sediments are composed largely of dark-gray and grayish-green shale of the Itaparica, Candeias, and Ilhas Formations, which are considered to be the source rocks for the hydrocarbons. The "A" sandstone, the lenticular sandstone bodies of the Candeias Formation, and the Sao Paulo and Santiago Sandstones of the Ilhas Formation are the best reservoir rocks of the Neocomian section.

The present architecture of the Reconcavo basin is an intracratonic half graben, developed chiefly during the times of deposition of the Candeias and lower Ilhas, when the basin became a rapidly sinking trough. The accelerated growth of the Salvador and Mata-Catu uplifts, the most prominent structural features, were responsible for the two principal northeast- and northwest-trending normal fault sets. A late phase of tectonic movements occurred near the end of, or after, deposition of the Sao Sebastiao, reactivating ancient faults and causing new ones to form. As a consequence, the tectonic pattern of the basin is a complex system of faulted blocks.

The six major fields, in which 96 percent of the total producible oil is concentrated, are related to the structural evolution of the basin. It is believed that the early period of faulting, contemporaneous with the deposition of Candeias and lower Ilhas, was a decisive factor in the control of petroleum migration and accumulation in Sergi and "A" sandstones. The horst blocks of Agua Grande, D, Joao, and Buracica fields, uplifted during this tectonic phase, trapped about 622 million bbl of recoverable oil in the two sandstones. Accumulation of Ilhas reservoirs was controlled mainly by the later phase of faulting. Folds, developed in the downthrown blocks of normal faults, but not related to compressional stresses, were the traps for accumulations in the Sao Paulo and Santiago Sandston s. Examples of such traps, which accumulated about 186 million bbl of producible oil, are Miranga and Taquipe fields. The genesis of the reservoir sandstone lenses in Candeias field, a stratigraphic trap, is related to syntectonic Candeias deposition. Fractured shale and limestone also constitute reservoir rocks in this field, where 94 million bbl of recoverable oil were trapped.

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Copyright 1997 American Association of Petroleum Geologists