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Structural Evolution of Encapsulated Oligocene Minibasins and Allochthonous Salt in the Northern Gulf of Mexico

Abstract

The deepwater northern Gulf of Mexico subsalt Paleogene play is an active exploration target for hydrocarbon resources. Primary minibasins, separated by salt massifs, are overlain by a complex salt canopy with secondary and tertiary minibasins. This framework creates an architecture with abundant subsurface complexities that elevate exploration risk. The refinement of trap geometries in primary minibasins is often limited even on advanced seismic data due to errors in velocity modeling caused by inaccurate interpretation of this complex system. The reintegration of well data and application of new concepts in salt tectonics combined with advancements in seismic acquisition and imaging leads us to describe additional stages of canopy evolution that can be used as inputs to refine primary minibasin geometries. Wells such as Cortez Bank (KC244), Sumatra (GB941) and Grand Cayman (GB561) intersected unexpected Oligocene-aged strata separated from primary and tertiary minibasins by salt. We interpret these strata to be encapsulated secondary minibasins that evolved during salt canopy emplacement and development. A revised subregional interpretation of 3D seismic tied to well data further supports the presence of numerous secondary minibasins within a highly evolved, multi-tiered salt canopy. We tested this concept using 2D structural restorations, which illustrate a plausible evolution of the current salt-minibasin architecture consistent with this interpretation. Primary minibasins separated by salt massifs developed through the Oligocene, which was contemporaneous with allochthonous salt breakout and downbuilding of suprasalt Oligocene-aged minibasins. These secondary minibasins became encapsulated during further coalescence of allochthonous salt and were subsequently rotated and deformed during later downbuilding of tertiary minibasins as the overlying canopy evolved. The improved understanding of these minibasins carries additional geological complexity compared to descriptions currently published. Incorporating these new concepts into petroleum system analyses has significant ramifications to prospect maturation in salt-dominated basins. The recognition of such minibasins can help improve interpretation and provide critically important imaging uplift in these structurally complex areas. Improved inputs to basin modeling, reservoir quality prediction, and trap timing can have a dramatic impact on reducing the overall exploration risk for complex subsalt plays.