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Abstract

AAPG Bulletin, V. 97, No. 9 (September 2013), P. 14211446.

Copyright copy2013. The American Association of Petroleum Geologists. All rights reserved.

DOI:10.1306/02261312164

Halokinetic rotating faults, salt intrusions, and seismic pitfalls in the petroleum exploration of divergent margins

Carlos L. Varela,1 Webster U. Mohriak2

1Petroleo Brasileiro S.A.—Exploration and Production/Gerencia de Projetos Exploratorios/Interpretacao Regional de Bacias, Avenida Chile, 330/16th Floor, 20031-170, Rio de Janeiro, Brazil; [email protected]
2University of Rio de Janeiro State, Faculty of Geology—Departamento de Geologia Regional e Geotectonica Rua Sao Francisco Xavier, 524, Room 4024-A, Rio de Janeiro 20550-900, Brazil; [email protected]

ABSTRACT

This work discusses concepts related to the occurrence of salt along weakness planes, such as faults and fractures, which resemble igneous intrusions and may result in peculiar seismic features. We suggest that mechanisms for the formation of these structures basically involve the creation of extensional faults (commonly associated with crestal collapse grabens), which are rotated and migrated to structural flanks by domation, creating interesting seismic features here referred to as halokinetic rotating faults. At the time of their formation, some of these faults may be incipiently intruded by salt as a way of relieving sporadic intense internal overpressure episodes in the salt body, by regional compression, and/or by buoyancy effects compensating the density difference between salt and surrounding sediments. The relatively low overburden pressure at the crest of the diapir and the original high dip angles of these fault planes favor salt intrusions near the diapir apex. The process may occur in several cycles along the salt dome evolution, creating several generations of salt apophyses positioned in the diapir apex and flanks, resulting in different dips and areas of extension. These intrusions sometimes resemble the branches of Christmas tree structures, which are commonly formed by extrusive mechanisms.

Although well and seismic data point to the occurrence of salt along fault planes, we recognize that salt is not a low-viscosity fluid, and the mechanisms to allow its penetration along fault planes remain unknown. Some of the possible mechanisms, which are commonly associated with a later phase of regional compression, are discussed in this work.

The implications for petroleum exploration may have been overlooked in the recent exploration campaigns in the deep-water regions of the Brazilian margin. Halokinetic rotating faults, when partially filled with salt, are sometimes responsible for common pitfalls observed in seismic and well data interpretation. When fault planes present subhorizontal dips and high reflectivity, caused by the presence of salt, they have been mistakenly interpreted as flatspots, a well-known seismic hydrocarbon indicator. When drilled and proved to correspond to thin evaporite intervals in well data, these salt apophyses have also been misinterpreted as younger localized evaporitic events overlying the main salt body.

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