Coal facies of the Olmos Formation in the Sabinas Basin, Northeastern Mexico

This work focuses on an organic petrographic study of the Olmos Formation coal sourced from five sub-basins within the Sabinas Basin (Mexico). The main goal was to determine the coal facies/depositional environment of these deposits. Organic petrographic studies revealed varying compositions across the sub-basins. The vitrinite content ranged from 73 % in the Sabinas Sub-basin to 89 % in the Adjuntas and Las Esperanzas sub-basins. The inertinite content varied from 8 % (Adjuntas Sub-basin) to 17 % (Sabinas Sub-basin). The liptinite content ranged from 2 % (Adjuntas, Las Esperanzas and San Patricio sub-basins) to 10 % (Sabinas Sub-basin). Vitrinite mean random reflectance values ranged from 0.92 % (San Patricio Sub-basin) to 1.38 % (Adjuntas Sub-basin). Gelification Index values recorded in this study varied from 4.2 (Sabinas Sub-basin) to 10.3 (Adjuntas Sub-basin), indicating relatively high wetness conditions during peat accumulation. Tissue Preservation Index values varied between 0.9 (Saltillito-Lampacitos Sub-basin) and 2.0 (Adjuntas Sub-basin), reflecting medium to well-preserved plant tissues. Groundwater Index values ranged from 0.4 (Adjuntas and Las Esperanzas sub-basins) to 1.1 (Saltillito-Lampacitos Sub-basin), suggesting a transition between ombrotrophic and rheotrophic hydrological conditions. The Vegetation Index varied between 1.0 (Saltillito-Lampacitos and San Patricio sub-basins) and 2.1 (Adjuntas Sub-basin), reflecting organic matter dominated by herbaceous vegetation. Four depositional sub-environments were identified: (i) a fen swamp sub-environment under limno-telmatic to telmatic conditions (Saltillito-Lampacitos Sub-basin), (ii) a fen swamp sub-environment under telmatic conditions (Sabinas and San Patricio sub-basins), (iii) a transitional sub-environment between fen and wet forested telmatic swamp (Las Esperanzas Sub-basin), and (iv) a wet forested telmatic swamp sub-environment (Adjuntas Sub-basin).