Devonian Shelf to Basin Facies Distributions and Preliminary Shale Geochemistry for South-central and Southwestern New Mexico

By

William D. Raatz New Mexico Bureau of Geology and Mineral Resources, a Division of New Mexico Tech  801 Leroy Place,  Socorro, NM  87801  [email protected]

The often poorly exposed Devonian section in southern New Mexico contains complex vertical and lateral ramp-to-basin facies changes that culminate in an  elongate trough filled with up to 76 m of black shale. The trough trends east-west for over 350 km, and ranges from a width of ~30 km near Las Cruces to ~100 km near Deming. To the north, Middle-Late Devonian-aged ramp carbonates, sandstones, siltstones, and shales of the Oñate (Givetian), Sly Gap (Frasnian), and Contadero (Frasnian-Fammenian)  formations outcrop in the San Andres and Sacramento mountains. These formations grade southward into the Percha Formation black shale facies. Due to the largely barren nature of the shales, it is difficult to determine exact correlations. The Percha Formation is divided into two members, the black, fissile, barren Ready Pay and the more calcareous and fossiliferous Box. Fossils from the upper Box Member indicate a Fammenian age.  South of the trough, the oldest Devonian units comprise the cherty shelf carbonate Canutillo Formation (Middle-Late Devonian) which is both overlain by and apparently a partial lateral facies equivalent to the Percha black shales.

Existing 1960’s vintage isopach and facies maps are updated with more recent outcrop and subsurface data, and integrated with geochemical data and basin analysis models to better characterized the aerial extent, volume, richness, and maturity of the black shale facies. Public domain geochemical data show potential for the Devonian shales in south-central and southwestern NM to act as source rocks:  TOC values locally reach 3.7% and thermal maturities are consistently in the mature to very mature range.

Three goals exist for this ongoing project:  better constraints on formation correlations and facies relationships;  improved characterization of Devonian shales for hydrocarbon source rock potential (richness and maturity trends, kerogen types, and expulsion timing);  and description of the shales for shale gas reservoir potential.

AAPG Search and Discovery Article #90010©2003 AAPG Southwest Section Meeting, Fort Worth, Texas, March 1-4, 2003