Coal deposits in the state of Sonora are distributed mainly in two regions. One is the Cabullona region, in the northeast, where the coal is contained in the Cintura Formation of Early Cretaceous age; its physical and chemical characteristics place it in the hypo-bituminous rank. The other region is in La Barranca, east-central Sonora, where the coal is contained in the Santa Clara Formation of Triassic-Jurassic age; there the coal ranks as anthracite. This region has been divided into the San Marcial, San Javier, Santa Clara, and San Enrique areas. The Cabullona region is currently in the regional exploration stage, whereas in the La Barranca region, although there has been previous exploitation, exploration has not been completed. Therefore, the coal potential of Sonora has not yet been fully determined.

Most of the coal in Mexico is concentrated in three widely separated regions Coahuila on the northeast, Sonora on the northwest, and Oaxaca on the south-southeast. In the Coahuila region, bituminous coking coals are present in Upper Cretaceous rocks. The Sonora region contains deposits of Upper Triassic anthracitic coal, altered by probable thermal metamorphism. The Oaxaca region contains anthracitic and some bituminous coking coal in Lower(?) to Middle Jurassic sequences. Through 1973, only the coal deposits of the Coahuila region had been developed commercially. The coals of the Sonora and Oaxaca regions are undeveloped because of unfavorable geo-economic conditions and lack of exploration. Through 1973, the Coahuila region had yielded about 110 million metric tons of coal, most of which was used to produce coke. Present production is 4,250,000 metric tons per year. The largest mines include one underground mine (Sabinas no. 2) and one openpit mine (La Florida), which produce 40,000 metric tons per month. Only reconnaissance-geologic investigations, complemented locally by a few limited drilling projects, have been made in the Sonora and Oaxaca regions. Exploration has been more extensive in the Coahuila region. There are many totally unexplored areas with potential coal resources. The estimated resources in the Sabinas area of the Coahuila region total 1,781 million metric tons classed as reserves in situ, and 1,180 million metric tons classed as potential but not commercially important at present. North of the Sabinas area, in the Río Escondido basin, the estimated resources total 120 million metric tons classed as measured. In the Sonora region the estimated resources total 11 million metric tons classed as reserves in situ for the Santa Clara coalfield and 4 million metric tons classed as measured, 9 million metric tons classed as inferred, and 18 million metric tons classed as possible for the San Marcial district. In the Mixtepec and Tezoatlán coal fields of the Oaxaca region, the estimated resources total 24 million metric tons classed as inferred and 26 million metric tons classed as possible.