43855-AC2
The Paleohydrologic System Near a Salt Diapir and Weld in the La Popa Basin, Northeastern Mexico
This project uses mesoscopic structural analysis, thin section petrography, isotope geochemistry, and fluid inclusion microthermometry to characterize the fluid-rock system near an exposed salt diapir and weld in the La Popa Basin of northeastern Mexico. Our research focuses on the fluids that moved through local and regional fracture networks. The geologic record of these fluids is today preserved in vein minerals such as calcite, quartz and barite that are found as fracture fillings throughout the area. Over the past year we conducted two additional weeks of fieldwork that focused on collecting vein and host rock samples from along the last remaining, fairly inaccessible stretch of the La Popa weld zone. We have completed carbon and oxygen stable isotope analyses on all of the samples we have collected thus far, and have also conducted fluid inclusion microthermometric and strontium isotopic analyses on a select suite of samples from each of the vein types that are common near the weld. Unfortunately few samples yielded quality fluid inclusion data, so our results in that area are somewhat limited. Nevertheless, having all these data in hand, we next conducted a geospatial analysis of the geochemical characteristics of the veins. This task was aimed at determining whether stratigraphy, timing or structural position exerted a controlling influence over vein geochemistry and by inference, fluid evolution and migration. The results suggest that stratigraphy plays the most significant role, and that structural position, namely proximity to the weld zone, plays a secondary role. The timing of vein formation and the type of vein do not correlate with vein geochemistry and are therefore interpreted to exert little influence on the paleohydrology of the area. In short, vein-forming fluids were present for a long period of time, they did not change significantly over that time, and vertical fluid migration was most efficient near the weld at selected “leaky spots” that correspond with presently unknown stratigraphic characteristics. These interpretations are based mostly on our new, abundant stable isotopic data that generally corroborates our preliminary data from last year. These data indicate that individual formations have unique isotopic signatures away from the weld, but that near the weld, may have distinctly different isotopic values. Among these different values is the interesting result that lighter d13C values in calcite tend to occur in veins close to the weld in some formations. This suggests that small amounts of light hydrocarbons may have migrated along leaky spots where formations such as the Viento are cut by it. The presence of methane in some fluid inclusions supports this idea, and inclusion microthermometry shows the vein-forming fluids were predominantly saline brines (20-25 wt% NaCl equivalent), at temperatures ranging from 100 to 200˚C. At standard geothermal gradients, these data suggest that veins were formed at depths between 3 and 6 km. Strontium isotopic ratios (87Sr/86Sr) from vein calcites are between 0.707163 and 0.708007. The wide spread in these values is inconsistent with vein fluids forming entirely as diagenetic brines that were captured during burial. Although this may be true for some veins, the ones with extremely lower strontium isotopic ratios almost certainly formed from fluids that interacted with or were derived from the Jurassic salt layer that formerly underlay most of the region. The impact of the project was best gauged by the response we received at the December, 2007 meeting of the La Popa Basin research consortium. Approximately 40 industry geologists attended this meeting and all of them were impressed by and interested in our work. Despite that we only had preliminary results to report at that meeting, we received high praise for the early interpretations and our presentation led to extensive discussion of fluid migration and the sealing behavior of salt welds. We will be presenting our research at the upcoming December meeting of the La Popa Basin research consortium, as well as at the annual AAPG meeting in 2009. I anticipate our first manuscript will also be submitted by the middle of 2009.
