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Exploration Ramifications of Bowl Weld Variances in the Northern Gulf of Mexico

Abstract

Bowl welds are salt welds that are generally bowl-shaped. They constitute an important structural fabric in the deepwater northern Gulf of Mexico (GOM), juxtaposing secondary minibasins against older primary basins. In addition to varying in size and shape, bowl welds exhibit a range of salt root configurations. While many bowl welds are underpinned by a central feeder stem, others have offset stems, or lack a discrete feeder altogether, being directly rooted in the underlying salt layer (aka “bucket welds”), or in the lateral salt canopy. This root variability impacts hydrocarbon prospectivity. Secondary minibasins perched above the bowl welds pose substantial seismic imaging challenges for associated subsalt/subweld exploration targets. Sharp velocity contrasts across steep-sided bowl welds hamper seismic illumination of adjacent and underlying strata, while overlying secondary basins generate synclinal seismic artifacts that may overwhelm the reflectivity of sub-weld strata, resulting in erroneous interpretations. Bowl weld traps may also assume other elevated exploration risks: (1) upward-flaring bowl welds are a naturally dispersive geometry for hydrocarbons migrating up along the weld zone; (2) truncation traps along the outside of the bowl incur a generic risk of forming ribbon truncation closures; and (3) bowl weld truncation traps may also have enhanced seal risk, with seal capacity being impacted by the geometry and composition of juxtaposed minibasin fill strata, and by lateral variability in the weld-strata contact zone. Even with these perceived risks, bowl weld traps will likely comprise an ever-increasing percentage of future GOM subsalt play inventory as the more easily imaged subsalt traps are tested by industry. To help explorationists better assess the geologic uncertainties associated with bowl weld traps, we present a bowl weld classification which is based on two geometric criteria that exert a fundamental control on bounding trap configurations and risk assessments: (1) whether, or not, the bowl weld has been deformed into its present configuration; and (2) the deep root geometry. Together, these criteria are used to define five end-member bowl weld archetypes, each having its own associated generic trap configurations. While seismic ambiguity remains a substantial challenge for bowl weld traps, this classification serves to alert subsalt interpreters to the geologic variances inherent in these complex salt-stock systems.