New Mexico Geological Society Annual Spring Meeting — Abstracts

Petrographic studies of cuttings and core from the Shell Isleta No. 2 oil exploration well provide data on the stratigraphy and thickness of Santa Fe Group and older Tertiary deposits within the Albuquerque Basin. This well, located about 12 mi southwest of Albuquerque, was drilled to a depth of 21,266 ft without penetrating Cretaceous bedrock.

Point counts indicate that a significant change in sandstone composition occurs at a depth of 14,200 ft. Above this depth, well samples contain a greater percentage of volcanic lithic grains and a smaller percentage of quartz grains than samples from below 14,200 ft. The deeper samples show a general trend of increasing percentage of quartz grains with depth. The 18,900 ft sample contains abundant ash-flow tuf f fragments. This is believed to be the tuff that Shell encountered at about 19,000 ft and dated at 34-36 Ma.

The change at 14,200 ft represents the transition from Santa Fe Group deposits to older deposits and indicates the approximate thickness of the Santa Fe in the central part of the Albuquerque Basin. This interpretation is supported by comparing samples from above 14,200 ft with samples of Santa Fe Group deposits from outcrops and other wells. Samples from below 14,200 ft are probably from a late Eocene-Oligocene unit that is at least partially equivalent to Datil Group and post-Datil Group rocks because of the ash-flow date. However, samples from below 14,200 ft differ from Datil rocks in that they do not contain as much volcanic material. This indicates that during Oligocene time large amounts of volcanic-rich sediments (other than a major ash flow) were not reaching this portion of the Albuquerque Basin. Samples from below 20,000 ft may be Baca Formation equivalents due to their high quartz percentages.