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Abstract: Factors Affecting the Geometry of Fold-Thrust Belt Salients and Their Associated Petroleum Systems

Macedo, J.M. - Petrobras/Cenpes; Marshak, S. - University of Illinois

Salients are pronounced convex-to-the-foreland curves in the map-traces of orogens. Most salients involve fold-thrust belts that form along the foreland margin of orogens. It is common to observe a relative concentration of oil and gas fields in front of salient apexes. This study seeks to define factors that influence the geometry of salients, the structures within salients, and the relationship between salients, foreland basins and petroleum systems.

We begin by defining parameters that can be used to characterize the variability in shape, dimensions, degree of asymmetry, and trend-line patterns of salients. Then we describe the results of sandbox models that simulate formation of two main types of salients: indenter-controlled salients (whose geometry reflects interaction of a fold-thrust belt with a hinterland indenter) and basin-controlled salients (whose geometry reflects lateral variations in the thickness of predeformational basin fill). These models illustrate that variations in the mode of salient formation, convergence direction, basin shape and asymmetry, and indenter shape all affect salient geometry. For example, structural trend-lines within basin-controlled salients converge at the endpoints of the curve while in indenter-controlled salients they diverge at the endpoints of the curve. Further, the asymmetry of a salient reflects the asymmetry of the predeformational basin and/or the indenter as well as the convergence direction.

We constructed 2D finite-element models that portrays the map-view tendency of development of strain and stress fields within these two types of salient. These models shows the importance of the map-view shape of the pre-collision environment as well as the relative difference in rock stiffness between the wedge and the foreland. There is a good agreement between the strain field observed from the results of the numerical models and the strain field observed from natural examples of salients, as well as from our previous results of sandbox experiments. It was found good agreement between joint-array geometry of the basin-controlled model with a natural example: the Pennsylvania salient. The model suggests that joint-array geometry at a location may change with time as a consequence of changes in overburden in a salient.

We conclude by illustrating that study of salient geometry in map-view may help prioritize petroleum-exploration targets, for foreland-basin oil fields appear to be spatially related to basin-controlled salients. Examples include the Alberta, Taiwan, Santa Cruz, Wyoming, Northern and Eastern Carpathians, Verkhoyansk, and Sulaiman salients.

AAPG Search and Discovery Article #90933©1998 ABGP/AAPG International Conference and Exhibition, Rio de Janeiro, Brazil