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Abstract

The main objective of this study was to investigate secondary migration in the A¸u Formation, including reconstruction of migration pathways, estimation of velocity of the migration fronts, and identification of key variables controlling the migration process. The Potiguar Basin in northeastern Brazil is an appropriate geologic setting for studying secondary migration. Almost all of the oil accumulated in the onshore A¸u sandstones was generated from offshore pods of active source rocks in the Alagamar Formation and has migrated laterally for long distances. The A¸u Formation, which is the main carrier bed and reservoir of this petroleum system, dips seaward as a regional monocline structure, with gentle folds, normal faults, and facies changes. In this study, we reconstructed the history of secondary migration using basin-scale modeling of 8272 grid cells. Rock and fluid properties used in this investigation are those typically found in most of the oil accumulations in the A¸u Formation.

Petroleum migration pathways and economic oil accumulations in the Alagamar–Acu petroleum system were controlled by northeast–southwest structural noses at the top of the A¸u Formation. Estimated ratios of oil displacement for the earlier oil migration fronts range from 3.0 to 7.0 cm/year. Oil in the main onshore accumulations of the Potiguar Basin may have been trapped between 1 and 5 m.y after the beginning of secondary migration. Significant loss of petroleum may have occurred along seepages from onshore outcrops of the A¸u Formation.

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