Coalbed Methane Resources of the Sabinas Basin, Coahuila, México

Abstract

The coal deposits of Mxico are located in the states of Sonora, Chihuahua, Coahuila, Hidalgo, Puebla, Oaxaca, and Tamaulipas. Particularly, the Sabinas Basin in Coahuila contains more than 5000 m of Upper Jurassic and Cretaceous rocks. The coal seams are at the top of the Upper Cretaceous section and occur from the present surface to a depth of about 900 m. They are identified on well logs by high resistivity, low density, and by lithology inferred from gamma-ray logs. Interpretation of logs from 12 wells in the Sabinas coal basin and 4 wells in the Ro Escondido coal basin, all with thick Upper Cretaceous sections, allow regional identification of the Austin and Upson Formations, four units in the San Miguel Formation, the Olmos Formation, and a thick sandstone in the Escondido Formation. A preliminary interpretation of this study is that several coal seams assigned to the Olmos Formation likely are contained in the lower one-third of the Escondido Formation, although it is possible that coal seams may occur at other stratigraphic levels in other areas. Coal in the Sabinas Basin was deposited in regressive, high-frequency, deltaic sequences characterized by overall lenticular geometries. A concentration of higher plants led to the accumulation of type III kerogen favorable for the in situ generation of methane. Coal in the Sabinas Basin is bituminous, with high-to-medium volatility. It has 45% fixed carbon, 14% volatile material, 40% ash content, and 1% sulfur and other impurities. Its caloric content is 1300 to 1400 Btu with a vitrinite reflectance of about 0.5, which places it between the zones of diagenesis and catagenesis with a maximum burial depth that may have been between 800 and 1200 m. By drilling 60 shallow wells (average depth of 700 m) at a cost of $US 21.6 million, approximately 18 million ft3 of gas per day could be produced with a payout of the investment in 18 months calculated at a price of $2.15 per thousand ft3. This area could potentially produce coalbed methane commercially for nearly 20 years, considering production increase though time as the coal seams desorb.