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Geophysical Records of Anthropogenic Sinkhole Formation in the Delaware Basin Region, Southeast New Mexico and West Texas, USA

Lewis Land, National Cave & Karst Research Institute, and New Mexico Bureau of Geology & Mineral Resources, New Mexico Institute of Mining & Technology, 1400 Commerce Dr., Carlsbad, NM, 88220, 575-887-5508, lland@gis.nmt.edu

Anthropogenic Sinkhole in New Mexico    Photo © 2011: nckri

Abstract

A significant minority of sinkholes formed in gypsum bedrock in the Delaware Basin 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. These events prompted the New Mexico Oil Conservation Division to review its regulations regarding brine well operations in the southeastern New Mexico oil fields. 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 have been conducted adjacent to the Eddy Co. sinkholes to assess the potential for additional subsidence or collapse events in the future.

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