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AAPG Bulletin

Abstract


Volume: 74 (1990)

Issue: 8. (August)

First Page: 1262

Last Page: 1272

Title: High-Temperature Hydrothermal Origin for Fractured Carbonate Reservoirs in the Blackburn Oil Field, Nevada: Geologic Note: (1)

Author(s): JEFFREY B. HULEN (2), S. ROBERT BERESKIN (3), and LOUIS C. BORTZ (4)

Abstract:

Preliminary petrographic and fluid-inclusion studies of drill cuttings and core from the Blackburn oil field reveal that many of the oil-saturated fractures, veinlets, and breccias in the dolomite reservoir rock may be of high-temperature hydrothermal origin. Textures of the earliest formed fracture/veinlet networks and breccias suggest development by explosive hydrothermal fracturing; the veinlets themselves preserve evidence that high-temperature fluids, favorable for such natural hydrothermal rock rupture, once circulated in these rocks. Primary fluid-inclusion homogenization temperatures indicate that early vein dolomite precipitated from hydrothermal brines at temperatures exceeding 350 degrees C (662 degrees F). Younger(?) quartz-sphalerite-galena veinlets developed from dilute aqueous solutions at temperature of more than 225 degrees C (437 degrees F).

The high paleotemperatures recorded by early vein dolomite in the now much cooler (about 120 degrees C or 248 degrees F) Blackburn reservoir rock imply an ancient magmatic heat source, in this case almost certainly a small granodiorite stock (Tertiary?) adjoining and possibly underlying the field. Other carbonate-hosted petroleum reservoirs in eastern Nevada are also intimately associated with felsic intrusives. We propose that igneous-related, high-temperature hydrothermal fracturing could have created reservoir porosity and permeability at some or all of these fields, and that future petroleum exploration in the Basin and Range might profitably incorporate the search for concealed plutons.

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