Concealed evaporite basin drilled in Arizona

Steven L. Rauzi
Arizona Geological Survey Tucson

The White Mountains of Arizona are a high forested plateau underlain by volcanic rocks of Late Pliocene and Quaternary age on the south margin of the Colorado plateau province. Elevations range from 6,000-11,590 ft, with winter snow and summer rain but ideal conditions for much of the year.

There was no evidence of a Permian evaporite basin concealed beneath the White Mountain volcanic field until 1993, when the Tonto 1 Alpine-Federal, a geothermal test well, was drilled alongside U.S. Highway 191 in 23-6n-30e, Apache County (OGJ, Jan. 3, 1994, p. 52).

This test did not encounter thermal waters, but it did encounter a surprisingly thick and unexpected sequence of anhydrite, dolomite, and petroliferous limestone assigned to the Supai (Yeso) formation of Permian age. The Tonto test was continuously cored through the Permian section, providing invaluable information that is now stored at the Arizona Geological Survey in Tucson.

Area geology

The evaporites and dolomites were originally deposited in a sea-margin sabkha environment similar to those currently seen in the Persian Gulf region. Protodolomite and chicken-wire anhydrite are very much in evidence, and short segments of missing core may have been salt. Over 100 ft of petroliferous sediments are present, the thickest petroliferous section known in Arizona. Brown oil bleeds from microfractures in some cores.

Several narrow mafic dikes were encountered in the well, none of which created any significant metamorphism. Visible evidence of contact metamorphism does not extend more than an inch from the dikes. The Tonto test was drilled to a depth of 4,505 ft and only penetrated 975 ft of the Permian section, which is estimated to be over 2,000 ft thick. Massive salt is probably present in deeper parts of the basin, and the writer believes that this salt is responsible for numerous salt springs in the Gila and Salt River drainage areas and could be the source of the great thickness of salt in Late Tertiary sediments near Phoenix (OGJ, July 22, 1991, p. 107).

A residual Bouguer gravity anomaly map¹ suggests the presence of an evaporite basin 70 miles long and 55 miles wide totally concealed beneath volcanic flows, tuffs, and agglomerates of the White Mountains (see figure [27716 bytes]). The bold line on the figure represents the -10 milligal contour line. The dashed line on the figure represents the -26 milligal contour line. The well-known Holbrook evaporite basin of Permian age lies north of the concealed basin. This concealed basin, along with the Holbrook basin, could be structurally and stratigraphically related to the Permian Basin of West Texas and New Mexico.

Concealed basin

The White Mountain volcanic field may be imposing to the eye, but it is only a few hundred feet thick, forming a veneer over underlying sedimentary rocks. The various igneous pipes and dikes have had little or no detrimental effect on the sedimentary section and may have, in fact, aided in the generation of oil and gas. For example, mid-Tertiary volcanic-intrusive activity has enhanced hydrocarbon production in northeastern Arizona at Dineh-bi-Keyah field, where more than 17 million bbl of oil have been produced from igneous reservoir rock intruded into Pennsylvanian carbonate units.

Much of the concealed White Mountain evaporite basin lies beneath the Apache National Forest of Arizona and New Mexico, whereas the remainder, to the west, is within the Fort Apache Indian Reservation. There are several parcels of privately owned land within the national forest boundaries.

In conclusion, the existence of the White Mountain evaporite basin can only be proved by drilling, but if it exists as is believed, it could provide a new and unexpected petroleum province.

Acknowledgment

Thanks to Edgar B. Heylmun for his help in preparing this article.

Reference

1. Lysonski, J.C., Aiken, C.L.V., and Sumner, J.S., Complete residual Bouguer gravity anomaly maps of Arizona, Arizona Geological Survey Open File Report OFR-81-24, 1981, scale 1:250,000.