Abstract Coal resources have been known to exist in the northeastern Coahuila region (Fig. 1) since the last century when there was small-scale mining across the border in Texas at Eagle Pass (Maverick County), St. Thomas District (Webb County), and San Carlos (Presidio County), among others (Evans, 1974). With the aim of diversifying energy sources for power generation, the CFE (Comisión Federal de Electricidad) under Manuel Moreno Torres launched the coal development program in 1960; since then, six geological exploration programs have been carried out over what is now known as the Fuentes-Rio Escondido Basin (Fig. 2) as follows: I. 1960 to 1961; II. 1963 to 1964; III. 1967 to 1968; IV. 1973 to 1976; V. 1977 to 1979; VI. 1980. By 1961,12 million tons of thermal coal had been evaluated for the Venustiano Carranza Thermoelectric Project in Nava capacity of 37.5 MW, consuming 140,000 tons/year of coal from the Rio Escondido Mine developed by the CFE. By 1976, 192 million tons of evaluated probable reserves justified the Jose- Lopez Portillo Thermoelectric Power Station Project, with 1,200 MW of installed capacity in four 300-MW units fed by a total 12,000 tons of coal per day for 30 years. The thermoelectric station is located 25 km south of the town of Piedras Negras in Coahuila (Fig. 2). To date an evaluated 600 million tons of thermal coal represents 45 percent of the tonnage required to satisfy demand to the year 2,000, based on the installed capacity of 18,683 MW for the NOINE

ABSTRACT Upper Cretaceous sandstones of northern Mexico have similar framework composition for a distance of 400 km south of the Rio Grande but differ markedly in reservoir quality in the north versus the south chiefly because the basins in which the sandstones were deposited experienced different post-depositional histories. The sandstones are composed largely of detritus from volcanic and intrusive igneous rocks and were deposited in paralic and fluvial environments. Sandstones in the north were never buried more than 1500 m and were subjected to slow, gentle, basinward downwarping, followed by gentle uplift and denudation. They underwent a complex diagenetic history of cementation by chlorite, quartz, calcite, ferroan carbonate and kaolinite; the development of modest secondary porosity; and they form hydrocarbon reservoir rocks of moderate quality ( ϕ = 2 to 18%, k = 0.2 to 20 md). Sandstones to the south were buried rapidly by 1000 to 4000 m of younger strata and immediately thereafter underwent strong compressional folding and local thrustfaulting during the Laramide orogeny. These sandstones lost from 20 to 35% porosity by compaction and the remainder of the porosity by cementation with calcite and minor chlorite and quartz. They are tight, did not develop secondary porosity, and have no shows of hydrocarbons. During slow subsidence of sandstone-shale sequences in the north, the normal maturation of shale and associated organic matter occurred. Shale water was probably expelled in stages, organic matter evolved to produce some hydrocarbons, and acid formation water was generated. Diagenesis of the associated sandstones followed a typical sequence found in many sedimentary basins and included the development of secondary porosity by the dissolution of framework grains and cement. To the south, Laramide compressive forces caused strong compaction and more rapid than normal de-watering of the Upper Cretaceous shales. Most water present in the shales was expelled prior to development of conditions favorable for hydrocarbon maturation. Thus, the rapid and severe compaction precluded the opportunity for the development of typical acidic formation waters that might have contributed to dissolution of some labile constituents. Close to the Sierra Madre front, rapid and early expulsion of water produced a strong fracture cleavage in shale and siltstone.