THE FLUID-ROCK SYSTEM NEAR AN ANCIENT SALT WELD

Salt welds are curviplanar structural discontinuities that form when salt bodies are evacuated by sedimentary loading or tectonic deformation. Although numerous welds have been interpreted in seismic data, very few of them have been directly accessed and we consequently know little about their detailed structural, compositional and hydrologic characteristics. Understanding these properties is important because salt welds in various locations have been observed to alternately serve as either hydrocarbon seals or migration pathways. Predicting which of these behaviors predominates is a significant obstacle in hydrocarbon exploration in salt basins.

We conducted a combined field and laboratory study of an exposed salt weld in the La Popa Basin of northeastern Mexico to better understand the variables that control the hydrologic behavior of salt welds. The ~25 km long weld involves a Jurassic through Eocene stratigraphic section that includes a lower carbonate-dominated interval and an upper clastic interval. The deformed zone around the weld is dominated by brittle fractures in three major sets: one early, bedding-parallel set, and two, later bed-perpendicular sets, many of which are mineralized with calcite and quartz. Stable isotopic analysis of vein and host rock calcite yielded δ¹⁸O values of 20-25 ‰ (vSMOW) and δ¹³C of +15 to -13‰ (vPDB). Fluid inclusion homogenization temperatures in veins range from 84.3˚ - 205.7˚C and salinities range from 2-25 wt% NaCl equivalent. Methane and degraded oil were observed in inclusions in some veins. Strontium isotopic analyses of vein calcite yielded ⁸⁷Sr/⁸⁶Sr values between 0.706792-0.708007.

Analysis of the structural and stratigraphic distribution of the results suggest that deeply sourced, exotic fluids, including hydrocarbons, migrated along the weld at more than one time and more than one place. The migration route was most significantly controlled by juxtaposition of sands, as well as by a major bend in the map trace of the weld.