Abstract: Tertiary Volcanic Stratigraphy of Trap Spring Field, Nye County, Nevada

Don E. French, Kevin J. Freeman

The recently discovered Trap Spring oil field is producing from a Tertiary ash-flow tuff. On the basis of lithologic and zonation similarities and stratigraphic position, the reservoir is correlated with the ash-flow tuff of Pritchards Station, which has been mapped in the Pancake Range, 10 mi (16 km) northwest of the field. The Pritchards Station tuff is a compound cooling unit of Oligocene age. It overlies the Stone Cabin Formation and underlies unconsolidated valley fill. The Stone Cabin Formation is also an ash flow that unconformably overlies Paleozoic strata. The Pritchards Station tuff can be distinguished in well cuttings from the Stone Cabin Formation by the greater abundance and size of biotite phenocrysts. Interbedded between the Stone Cabin and Pritchards Stat on and between the Pritchards Station and younger valley fill are impermeable beds of altered-ash tuff. These beds probably represent the nonwelded and poorly welded ash at the top of the ash-flow tuffs which has been altered to clay by weathering. At outcrop in the Pancake Range the Pritchards Station tuff has a system of cooling joints similar to most other ash-flow tuffs of the region. The cooling joints form a rectilinear pattern and seem to be best developed in the densely welded zones. Apparently fracture porosity caused by cooling joints has been enhanced by Basin-Range faulting. Densely welded ash-flow tuffs may be expected to be suitable reservoirs for oil and gas accumulations when associated with post-emplacement faulting and interbedded altered-ash tuff.

AAPG Search and Discovery Article #90964©1978 AAPG Rocky Mountain Section Meeting, Salt Lake City, Utah