STABLE ISOTOPIC CONSTRAINTS ON THE PALEOHYDROLOGY OF A SALT WELD

Differential pressures in subsurface salt bodies cause the salt to flow vertically and laterally.  When the salt exits an area it leaves behind a fault-like discontinuity called a salt weld.  Such welds are common in places like the Gulf of Mexico, where they play an important role in the migration or trapping of subsurface fluids.  This study uses stable isotopes to characterize the paleohydrology around 25 km long salt weld exposed in the La Popa Basin of northeastern Mexico.  Information constraining the paleohydrologic system was obtained from samples of veins collected from differing stratigraphic and structural positions along the weld.  Stable isotopic analyses of the vein minerals allowed us to characterize the vein-forming fluids and fluid migration events.  Centimeter- to millimeter-scale isotopic sampling allowed us to recognize intra-vein variation and interactions between the vein-forming fluids and host rocks.  Veins collected within 200 m of the weld have a relatively narrow d ¹⁸O range of 20-25‰ (vSMOW), but a wide d¹³C range of +15 to -13‰ (PDB).  Isotopic values in veins close to the weld are not consistently correlated with the isotopic values of the host rocks.  This differs from the rest of the basin where veins collected at greater distances away from the weld have d¹⁸O and d ¹³C values related to the original carbonate units.  An influx of light carbon near the weld could be responsible for the large range of d¹³C along the weld.  Preliminary analysis suggests the presence of light carbon may correlate with more permeable rock units that most likely contained or transmitted CH₄ or CO₂ where they intersect the La Popa weld.  Recognizing these leaky stratigraphic units allows us to identify their hydrologic behavior and characterize them as seals or reservoirs.