Thermogenic and secondary biogenic gases, San Juan Basin, Colorado and New Mexico; implications for coalbed gas producibility

The San Juan basin is the most prolific coalbed gas basin in the world with 1992 production exceeding 440 Gcf* (12.4 billion m3), resources of approximately 50 Tcf (1.4 trillion m3), and proved reserves of over 6 Tcf (170 billion m3). Coalbed gas wells with the highest production (initial potential greater than 10 Mcf/day or 0.28 million m3/day) occur in the overpressured, north-central part of the basin. Hydrologic analysis indicates that overpressure in the Fruitland Formation is artesian in origin and represents repressuring that developed during the middle Pliocene. Highly permeable, laterally continuous coal beds override abandoned shoreline Pictured Cliffs sandstones and extend to the elevated recharge area in the northern basin to form a dynamic, regionally interconnected aquifer system. Coal rank and basin hydrodynamics control the composition of Fruitland coalbed gases, which varies significantly across the basin. Chemically dry gases in the north-central part of the basin coincide with meteoric recharge and regional overpressure. The consistency of methane delta 13C values across the basin, the presence of isotopically heavy carbon dioxide in coalbed gases and bicarbonate in formation waters, and biodegraded n-alkane distributions of some coal extracts indicate that coalbed gases in the north-central basin are a mixture of thermogenic (25-50%), secondary biogenic (15-30%), and migrated thermogenic (12-60%) gases. Migrated, conventionally and hydrodynamically trapped gases, in-situ generated secondary biogenic gases, and solution gases result in gas contents that plot on or above the coal sorption isotherm. Bacteria transported basinward in groundwater flowing from the elevated northern basin margins metabolized wet gas components, n-alkanes, and organic compounds in the coal and generated secondary biogenic methane and carbon dioxide subsequent to coalification, uplift, erosion, and cooling. These gases may be limited to basin margins, where shallow depths and structural deformation result in higher permeability, or may extend more than 35 mi (56 km) basinward from the recharge zone. The presence of appreciable secondary biogenic gas indicates an active dynamic flow system with overall permeability sufficient for high productivity. Basin hydrogeology, reservoir heterogeneity, location of permeability barriers (no-flow boundaries), and the timing of biogenic gas generation and trap development are critical for exploration and development of unconventional gas resources in organic-rich rocks.