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Megaflaps Along the Edges of Steep Diapirs and Beneath Salt Sheets: Models and Examples

Abstract

Megaflaps are packages of deep minibasin strata that extend far up the sides of diapirs. They are near-vertical but may also be completely overturned in their upper reaches, at the transition between feeder diapirs and salt sheets. They occur adjacent to both primary and secondary diapirs and are distinguished from steep diapir-flanking strata of composite halokinetic sequences by their scale, with vertical relief up to 5 km. In some cases they are equivalent to shale sheath or one manifestation of carapace, but are defined purely geometrically with no connotation of lithology or genesis. Strata within a megaflap are usually subparallel but may be convergent. The lower boundary is either a concordant surface between the salt and the megaflap or a low-angle onlap surface. The upper boundary ranges between a prominent onlap surface and a more diffuse zone of gradual rotation and thinning. Megaflaps form through contraction, drape folding, or a combination. A contractional megaflap develops during shortening, as in the Atwater Foldbelt of the northern Gulf of Mexico (GoM) or at the Aulet diapir in the southern Pyrenees. In this case, it represents part of a fold or thrusted fold that extends along strike away from the diapir, but with squeezed diapiric salt between the two limbs. A drape megaflap forms in the absence of significant shortening, as in Walker Ridge (northern GoM), and represents the roof of inflated pre-diapiric salt that rotates into its present configuration during progressive minibasin-scale drape folding. The length of a drape megaflap is dependent on the width of the inflated salt body, the position along the roof at which diapiric breakthrough occurs, and the degree of bed-lengthening during folding. Finally, a composite megaflap, e.g. next to the Witchelina diapir in South Australia, is a drape fold that gets enhanced by subsequent shortening. If megaflaps are not recognized, wells drilled to test three-way truncation traps against salt will likely encounter significantly older and steeper strata than prognosed. However, an observed megaflap does not necessarily kill a prospect. If there is a zone of progressive rotation above the megaflap, reservoirs will pinch out within the thinning growth intervals. If, in contrast, the top of the megaflap is a prominent onlap surface, the megaflap itself may provide the lateral seal, in which case the degree of bed lengthening, and thus probable fracture development, may impact seal capability.