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Halokinetic Rotating Faults, Salt Intrusions and Seismic Pitfalls in the Petroleum Exploration of Divergent Margins

Varela, Carlos L.1 and Mohriak, Webster
1[email protected]

"In the eastern Brazilian basins, the well to seismic data correlation indicates that some thin evaporite layers inserted into sediments are associated with peculiar flat seismic events with anomalous amplitudes that crosscut the surrounding layering. This work discusses the presence of salt associated with weakness zones, such as faults and fractures. Seismic profiles and time slices are used to illustrate this possible association. We suggest that the formation of salt intrusions basically starts with the creation of extensional faults, commonly related to crestal collapse graben. At early stages 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. 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. As domation progresses, these faults are then rotated and migrated to structural flanks, creating interesting seismic features here referred as halokinetic rotating faults, some of them associated with thin salt apophyses.

The implications of these features for petroleum exploration may have been overlooked. When fault planes present sub-horizontal dips and high reflectivity, due to 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. "

 

AAPG Search and Discovery Article #90166©2013 AAPG International Conference & Exhibition, Cartagena, Colombia, 8-11 September 2013