New Mexico Geological Society Annual Spring Meeting — Abstracts

A significant minority of sinkholes formed in gypsum bedrock in the lower Pecos region are of human origin. These anthropogenic sinkholes are often associated with improperly cased abandoned oil wells, or with solution mining of salt beds in the shallow subsurface. In July, 2008 a sinkhole formed abruptly at the site of a brine well in northern Eddy Co., New Mexico. The well operator had been injecting fresh water into underlying salt beds and pumping out the resulting brine for use as oil field drilling fluid. Borehole problems had prevented the operator from conducting required downhole sonar surveys to assess the dimensions of subsurface void space. The resulting sinkhole formed in just a few hours by catastrophic collapse of overlying mudstone and gypsum, and in less than one month had reached a diameter of 111 m and a depth of ~45 m. Fortuitously, a seismograph had been deployed ~13 km southeast of the brine well a few months earlier, and precursor events were captured on the seismograph record a few hours before the subsurface cavity breached the surface. Four months later another sinkhole collapse occurred in northern Eddy Co., again associated with a brine well operation. A third brine well within the city limits of Carlsbad, NM has been shut down to forestall possible sinkhole development in this more densely populated area. Electrical resistivity surveys conducted adjacent to the Eddy County sinkholes indicate the presence of large, brine-filled cavities a few tens of meters deep in the subsurface beneath both sinks.