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Trap Types in Basin and Range Province

Donna Herring Flanigan

Seven oil fields have been discovered within Tertiary basins of the Basin and Range structural province in Nevada. These fields are similar in that (1) producing area is small, averaging less than 1,000 ac, (2) oil column is thick, commonly over 500 ft, (3) produced oil is low gravity, ranging from 10.5° to 28°, (4) associated water is fresh, (5) reservoir drive is water and/or gravity, (6) lateral seals are fault-bounded, and (7) vertical seals are semipermeable, allowing escape of associated gas and in some places creating an asphalt cap.

The fields occur as one of two structural types: secondary grabens or secondary buried horsts. Secondary grabens and horsts form on a secondary or minor scale within the major basin and range system of mountain-range horsts and whole-valley grabens. Analogy to individual anticlines within a large anticlinorium conveys the sense of scale involved. Both the primary and secondary horsts and grabens are superimposed over older fold and thrust systems.

N. H. Foster in 1979 described the Trap Spring and Eagle Springs fields as volcanic reservoirs preserved in downdropped smaller (secondary) grabens. He presented a geomorphic technique to explore for these graben traps in the Basin and Range province.

Secondary horsts yielded Paleozoic discoveries in 1982 (Blackburn field) and 1983 (Grant Canyon field). Grant Canyon includes a well flowing nearly 4,000 BOPD. The 1986 Kate Spring discovery is probably also a secondary horst. Foster's geomorphic techniques can be modified and used to explore for both secondary horst and secondary graben traps.

AAPG Search and Discovery Article #91034©1988 AAPG Southwest Section, El Paso, Texas, 21-23 February 1988.