Some outcrops adjacent to the 25-km (15-mi)-long salt weld in the La Popa Basin, northeastern Mexico, are visibly stained and contain enrichment of organic carbon. Their presence, which is known only along the northwestern part of the weld and along the southwestern side of the weld, provides evidence for hydrocarbon migration along the weld. This distribution is presumed to be partially controlled by the restricted distribution of the source rock that generated the hydrocarbons within the salt-withdrawal basin southwest of the weld but may have been in part controlled by a previously existing salt wall, or the weld that formed as a result of salt evacuation, acting as a barrier to lateral migration and serving as a vertical conduit for fluid flow. Upturned halokinetic strata adjacent to the weld provided a conduit for migration along bedding planes. Although no flowing seeps were seen during this study, we documented discoloration and remineralization, as well as the presence of waxy-appearing soil that smelled like crude oil at some sites near the weld. Organic geochemical analyses confirm the presence of nonbiodegraded hydrocarbons along the salt-sediment interface and within adjacent, upturned lithologies up to 5 m (16 ft) southwest of the weld. The results document for the first time the definitive presence of actively migrating hydrocarbons associated with a subaerial salt weld. An oil-source rock correlation is inferred with the upper mudstone member of the Potrerillos Formation based on biomarker data. Thermal and subsidence modeling of the basin indicate that all of the strata in the basin are mature enough to have generated hydrocarbons. The lower part of the stratigraphic section entered the gas generation window, and much of the Cretaceous section and all of the Tertiary entered the oil window prior to uplift of the basin. Significant unroofing (as much as 7 km [4.3 mi] of sediment may have been removed) occurred sometime after the Eocene, as suggested by previous studies.

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