General Geology and Major Oil Fields of Recôncavo Basin, Brazil<xref ref-type="fn" rid="ch18fn1"><sup>1</sup></xref>

Abstract

The Reconcavo basin, on the Atlantic Coast near the city of Salvador, has an area of about 10,000 km2 and is the principal petroleum province of Brazil. Since 1939, approximately 255 wildcats have been drilled and have discovered 43 accumulations which total 942 million bbl of producible oil and 992 billion ft3 of gas. API gravity of most oil ranges from 35 to 40°. The Bahia Supergroup, the main objective for petroleum exploration, has o maximum thickness of 6,500 m. This nonmarine unit ranges in age from Late Jurassic(?) to Early Cretaceous. The Upper Jurassic(?) typically consists of a redbed sequence (Alianca Formation) overlain by a blanket sandstone (Sergi Formation). The Sergi is the best reservoir rock of the basin. The Lower Cretaceous (Wealden) strata are mainly dark-gray and grayish-green shale of the Itaparica, Candeias, and Ilhas Formations and are considered to be the oil and gas source rocks. The A Sandstone, the lenticular sandstone bodies of the Condeias Formation, and the Sao Paulo and Santiago Sandstones of the Ilhas Formation are the best reservoir rocks of the Lower Cretaceous section. The Reconcavo basin is an intracratonic half graben. Intensive faulting occurred during the deposition of Candeias and lower Ilhas when the basin became a rapidly sinking trough. Accelerated growth of the Salvador and Mata-Catu uplifts, the most prominent structural features of the basin, produced the two principal (NE-and NW-trending) sets of normal faults. A late post-Sao Sebastiao tectonic phase reactivated ancient faults and caused new ones to form. Consequently, the basin is characterized by a complex system of faulted blocks. The six major fields, containing 96 percent of the total producible oil, are related to the structural evolution of the basin. It is believed that the early period of faulting, contemporaneous with the deposition of the Candeias and lower Ilhas, was a decisive factor in the control of petroleum migration to, and accumulation in, the Sergi and A sandstones. The horst blocks of Agua Grande, Buracica, and Dom Joao fields (the first two having been partly uplifted during this tectonic phase) trapped about 623 million bbl of recoverable oil in these two sandstone bodies. Accumulation in Ilhas reservoirs was controlled mainly by the later, post-Sao Sebastiao, phase of faulting. Folds developed in the downthrown blocks of normal faults, but the folding was not caused by compressional stress. These folds form traps for accumulations in the Sao Paulo, Santiago, and other Ilhas sandstones. Examples of such traps, in which about 187 million bbl of producible oil accumulated, are the Miranga and Taquipe fields. The genesis of the lenticular sandstone reservoirs in the Candeias field--a stratigraphic trap--is related to syntectonic deposition of the Condeias Formation; fractured shale and limestone also are reservoirs in this field. Condeias trapped about 93 million bbl of producible oil.