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Fracture Controlled Paleohydrology of a Salt Weld, La Popa Basin, Northeastern Mexico

Smith, Adam P.1; Fischer, Mark P.1
1 Northern Illinois University, Dekalb, IL.

Salt welds can be a substantial factor in the migration or trapping of fluids moving through the subsurface. The La Popa Basin, is home to a well-exposed, subvertical, secondary salt weld. This study examines the fracture-controlled paleohydrologic system in the vicinity of this weld. Fracture networks throughout the area formed during Laramide shortening and diapiric uplift from the Early Cretaceous to the Eocene. Veins that formed in these fracture networks contain information about the paleohydrologic system and can be grouped into three systematic sets of bed-parallel, cross-strike, and strike-oblique. We conducted stable isotopic analyses, fluid inclusion studies, and petrographic analyses on samples of each vein type collected from differing stratigraphic and structural positions along the weld. These analyses allow us to characterize the vein-forming fluids and the timing of fluid migration events. Petrographic analysis shows that veins are primarily comprised of calcite, with minor amounts of quartz and barite. Calcite predates quartz in nearly all veins, although there is some evidence for local co-precipitation. Microsamples of calcite were collected from multiple locations in each vein and its adjoining host rock. Stable isotopic analysis of these samples shows that the weld played a significant and complex role in the regional paleohydrologic system. Veins near the weld have a relatively narrow δ18O range of 20-25‰ (vSMOW), but a wide δ13C range of +15 to -13‰ (vPDB). The wide range of δ13C is restricted to certain formations, structural positions and vein types along the weld, and could be explained by an influx of externally derived, light hydrocarbon or CO2-bearing fluids. Away from the weld, a strong correlation between δ18O of the host rocks and veins suggests the background vein fluids were buffered by, and probably locally derived from the host rocks. Fluid inclusion data support the idea that the weld was a significant player in the regional paleohydrologic system, and that multiple fluid types moved along it or accumulated near it. These fluid types include low salinity (< 10 wt% NaCl equivalent) and moderate temperature (110-150 C), high salinity (> 20 wt% NaCl equivalent) and high temperature (190-210 C), and methane-, CO2- or liquid hydrocarbon-bearing aqueous solutions. Spatial variations in the inclusion data suggest that fluid flow may have been localized at a significant bend in the La Popa Basin weld.


AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009