W.R. Kaiser

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 (FOOTNOTE *) (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 he 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 ^dgr13C 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 content 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 devel pment are critical for exploration and development of unconventional gas resources in organic-rich rocks.

Defining Coalbed Methane Exploration Fairways: An Example from the Piceance Basin, Rocky Mountain Foreland, Western United States

A basin-scale coalbed methane producibility and exploration model has been developed on the basis of research performed in the San Juan, Sand Wash, Greater Green Rivers, and Piceance Basins of the Rocky Mountain Foreland and reconnaissance studies of several other producing and prospective coal basins in the United States and worldwide. The producibility model indicates that depositional setting and coal distribution, coal rank, gas content, permeability, hydrodynamics, and tectonic/structural setting are controls critical to coalbed methane production. However, knowledge of a basin’s geologic and hydrologic characteristics will not facilitate conclusions about coalbed methane producibility because it is the interplay among geologic and hydrologic controls on production and their spatial relation that govern producibility. High producibility requires that the geologic and hydrologic controls be synergistically combined. That synergism is absent in the marginally producing, hydrocarbon-overpressured Piceance Basin. As predicted from the coalbed methane producibility model, significant coalbed methane production (greater than 1 MMcf/d [28 Mm3/d]) may be precluded in many parts of the hydrocarbon-overpressured Piceance Basin by the absence of coalbed reservoir continuity, high permeability, and dynamic groundwater flow. The best potential for coalbed methane production may lie in conventional and compartmentalized traps basinward of where outcrop and subsurface coals are in good reservoir and hydraulic communication and/or in areas of vertical flow potential and fracture-enhanced permeability. In the low-permeability, hydrocarbon-overpressured Piceance Basin, exploration and development of migrated conventionally and hydrodynamically trapped gases, in-situ-generated secondary biogenic gases, and solution gases will be required to achieve high coalbed methane production.