--> --> ABSTRACT: Oil and Igneous Intrusions in Nevada, by M. Lee Allison, Jeffrey B. Hulen; #91002 (1990).

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ABSTRACT: Oil and Igneous Intrusions in Nevada

M. Lee Allison, Jeffrey B. Hulen

Oil production from Paleozoic rocks in Nevada is almost entirely from wells drilled on or adjacent to igneous intrusions. These intrusions range in composition from granite to granodiorite, and Cretaceous to Tertiary age. Six, and possibly seven, of the eight producing oil fields and one noncommercial gas field are all associated with such intrusions. Of the 12 wells in eastern Nevada that penetrated igneous rocks, at least ten (83%) are either productive or lie within 1 mi of production.

Igneous intrusions may be critical to hydrocarbon accumulations by enhancing: (1) trap creation (uplift, doming, and fracturing or hydrothermal fracturing); (2) oil maturation (heating of submature source rocks); and (3) migration (flushing of oil by circulating hydrothermal waters). In the Blackburn field, fracture permeability in Devonian carbonates appears to be the result of high-temperature hydrothermal brecciation. Grant Canyon permeability may have been enhanced similarly or by mechanical fracturing during the intrusion process. A strong water drive circulating around the intrusions may increase production.

Tertiary volcanics produce in three of the fields (Eagle Springs, Trap Springs, and Tomera Ranch) while Paleozoic rocks produce in six (Eagle Springs, Grant Canyon, Bacon Flat, Blackburn, Kate Spring, and North Willow Creek). The Jiggs gas field and the Eagle Springs field produce primarily from the pre-volcanic Eocene Elko Formation and Sheeps Pass Formation, respectively.

Current exploration models include Paleozoic highs beneath a sealing Miocene unconformity, Mesozoic thrust belt, or Paleozoic patch reefs. If the oil is instead controlled by intrusions, then gravity surveys may be the best exploration method.

AAPG Search and Discovery Article #91002©1990 AAPG Rocky Mountain Section Meeting, Denver, Colorado, September 16-19, 1990