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ABSTRACT: High-Temperature Natural Hydraulic Fracturing as a Reervoir Control in the Blackburn Oil Field, Nevada--Implications for Petroleum Exploration in the Basin and Range Province

Jeffrey B. Hulen, S. Robert Bereskin, Louis C. Bortz

Petrographic and fluid-inclusion studies of the Blackburn oil field reveal that fractures and breccias now hosting oil in the Devonian dolomite reservoir rock were induced in part by natural high-temperature hydraulic fracturing. Homogenization temperatures and ice-melting temperatures of fluid inclusions in vein dolomite suggest that this fracturing was effected by high-temperature (341-400°C) hydrothermal brines circulating above (and heated by) a small granodiorite intrusion. Oil entrapment followed cooling of this hydrothermal system after an unknown length of time; the system simply helped create the field's critical secondary porosity and permeability.

Four other (of a total six) commercial, carbonate-hosted eastern Nevada oil reservoirs (Eagle Springs, Kate Springs, Bacon Flat, and Grant Canyon) are intimately associated with small felsic plutons. Since such intrusives account for less than 10% of outcropping rocks in this region, the oil/pluton association seems more than coincidental. Our findings at Blackburn suggest that igneous-related hydraulic fractures and breccias could be key controls at some or all of these fields, and that buried plutons could become important exploration targets in the Basin and Range.

Although hydraulic fractures and breccias around plutons are previously unknown as petroleum reservoirs, they commonly host hydrothermal ore deposits. The geometries such ore-bearing fracture/breccia bodies (e.g., pipes, dikes, and inverted bowls) are well documented. We suggest that incorporating these features into Nevada petroleum exploration models could help with both development of existing fields and discovery of new ones.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990