Well Log Analysis of Volcanic Rocks: Examples from the Nevada Test Site, Nye County, Nevada

Lance Prothro and Sigmund Drellack
Bechtel Nevada Corporation, Las Vegas, NV

As the exploration for hydrocarbon resources continues to move into non-traditional areas, and as the search for additional energy resources such as geothermal continues, subsurface geologists will encounter rocks not usually found in typical sedimentary-basin settings. One such group of rocks rarely encountered by most subsurface geologists is volcanic rocks. Volcanic rocks, however, are common at and near the surface in many parts of the world.

Volcanic rocks have a variety of litho-facies with significantly different fluid-flow properties. The accurate characterization of volcanic litho-facies is critical in evaluating the potential of volcanic deposits for energy resource exploitation. As with sedimentary rocks, well logs provide valuable data for analyzing the various volcanic litho-facies encountered in drill holes. However, unique aspects of the composition, deposition, and alteration of volcanic rocks require a certain amount of special knowledge about volcanic rocks and their responses to various well logs in order to accurately evaluate and characterize volcanic litho-facies.

At the Nevada Test Site (NTS) in southern Nye County, Nevada, more than 4,000 holes have been drilled as part of the underground nuclear weapons testing program and subsequent environmental restoration and groundwater modeling activities. Because the NTS lies within the southwestern Nevada volcanic field, many of the holes have encountered volcanic rocks, resulting in a large amount of well log data from volcanic rocks, and extensive experience in the subsurface characterization of volcanic litho-facies. Presented here are examples of well logs from a variety of volcanic litho-facies.