The Megabreccia Reservoir at Ghost Ranch Oilfield, Railroad Valley, Nevada
The reservoir at Ghost Ranch is a large catastrophic Neogene rockfall deposit, known as a sturzstrom. The Ghost Ranch sturzstrom is a limestone and dolomite breccia, which predominantly originated from Paleozoic carbonates. At the 58-35 dry hole east of the field, Tertiary valley fill sediments were encountered below the breccia, which confirmed that the reservoir is a late Tertiary landslide. The brecciated nature of the reservoir is clearly expressed on an imaging log. The intense fracturing imparts huge permeabilities implied by production tests (4320 barrels of water per day at the 58-35). The top of this deposit is a prominent seismic reflection that extends southward to Kate Spring field, interpreted by French, 1991, to be a landslide deposit. Because of the continuity of the seismic reflection, the Kate Spring reservoir is interpreted to be the same lithostratgraphic unit as the megabreccia at Ghost Ranch. The deposit covers approximately 1500 acres and the volume of the deposit is estimated at 0.3 cubic kilometers (380 million cubic yards). According to relationships derived for modern sturzstroms of this size, it is estimated that the Ghost Ranch rockfall had a runout of 8 kilometers (26,250 feet) and a drop height of 1.2 kilometers (4000 feet). This drop height is very similar to the present day relief from the Grant Range to the valley floor. Foliations within the arched core complex of the Grant Range dip into Railroad Valley and may have increased the tendency for such a large landslide to occur.