Dudley D. Rice Charles N. Threlkeld April K. Vuletich Mark J. Pawlewicz
U.S. Geological Survey
Lakewood, Colo.
The San Juan basin is a major hydrocarbon producing province in the Rocky Mountains.
Cumulative production for both the New Mexico and Colorado parts of the basin, as of Jan. 1, 1983, was: 190 million bbl of oil, 14 tcf of nonassociated gas, 425 bcf of associated gas, and 60 million bbl of condensate. Ultimate recovery of nonassociated gas was estimated to be 23 tcf.
On the Four Corners platform and Chaco slope (Fig. 1), oil and associated gas occur in structural and stratigraphic traps. In the generally structureless central basin, nonassociated gas and condensate occur in underpressured sandstone reservoirs. Hydrodynamics and low permeability are responsible for trapping the gas in this unusual type of accumulation. Coal beds have been considered as the source rocks for much of this gas.
Recently there has been active exploration for gas from coal beds of the Upper Cretaceous Fruitland formation, with the coal beds serving as both the reservoir and source rocks. While coal is present in other units, the major resources are in the Fruitland formation.
Very large gas resources are estimated to be contained in these unconventional but promising reservoirs. It has been calculated that the Fruitland coal beds have generated about 26 tcf of gas in the thermally mature, north part of the basin. Some estimate that the Fruitland coal beds have in place gas resources of 31 tcf, while more recently others have said this volume is 50-56 tcf.
STRATIGRAPHY
The Cretaceous section is as thick as 6,000 ft in the San Juan basin and contains the source and reservoir rocks for most of the hydrocarbon reserves in the basin, especially the central basin (Fig. 2).
Widespread coastal sandstones have been the primary reservoirs for natural gas. Fractured marine shales and fluvial sandstones are less important reservoirs. However, coal beds are becoming an increasingly important exploration target for gas accumulations. Marine and nonmarine shales and coals are the main source rocks.
Hydrocarbon production in the Pictured Cliffs sandstone is mainly nonassociated gas with very little condensate from sandstone reservoirs along a northwest-trending belt located mainly within the New Mexico part of the central basin.
Low permeability caused by authigenic clay and hydrodynamics are the main trapping mechanisms.
The Fruitland formation contains the largest reserves of coal in the basin, 200 billion tons, and individual coal beds are thickest and most continuous in the lower part of the formation, where they intertongue with shoreline sandstones of the Pictured Cliffs sandstone.
Because of the stratigraphic rise of the Pictured Cliffs,
the basal coals become younger to the northeast. These thick, basal Fruitland coals are the primary target for gas exploration.
The Fruitland formation also produces small amounts of nonassociated gas from lenticular fluvial channel sandstones in stratigraphic traps. These sandstones are above the basal coal zone and intertongue with shale, carbonaceous in part, and coal.
In the central basin of Colorado, commingled production from the Pictured Cliffs sandstone and Fruitland formation is mainly structurally controlled.
GENERATION, COMPOSITION
Natural gas is generated by two distinct processes.
At shallow depths of burial and low temperatures, methane rich gas is generated during decomposition of organic matter by microorganisms. This gas is formed in environments free of dissolved oxygen and sulfate and is referred to as biogenic gas.
Natural gas is also generated by thermal degradation and cracking processes with increasing temperature and advancing geologic time. This type of gas is referred to as thermogenic.
Most gas produced from commercial oil and gas fields is probably of thermogenic origin.
Although the gases produced from the coalbed and sandstone reservoirs are similar in isotopic composition, they are quite different in chemical composition. The coal gas is mostly methane with as much as 6% CO2; the gas in the sandstone reservoirs contains as much as 15% C2+ components and less than 2% CO2.
Based on the chemical composition alone, it does not seem possible to have derived by migration the gas produced from the sandstone reservoirs from gas generated in the coal beds.
SUMMARY, CONCLUSIONS
The San Juan basin contains major reserves of hydrocarbons, particularly nonassociated gas.
This gas occurs mainly in the central basin, where a low permeability and hydrodynamics are the main trapping mechanisms.
Cretaceous rocks are the main reservoir and source rocks in the central basin. Coastal marine sandstones are the most important reservoir rocks, and enclosing marine and nonmarine shales and coals are considered as the major source rocks. Coal beds in the Fruitland formation are estimated to contain major resources of nonassociated gas and have been the target for recent exploration.
Within areas of current production from coal beds of the Fruitland formation, vitrinite reflectance values increase to the northeast from subbituminous to medium-volatile bituminous. The high level of thermal maturity in the north probably resulted from a combination of maximum depth of burial and a higher geothermal gradient during mid-Tertiary time due to emplacement of the San Juan batholith.
Gases analyzed from Cretaceous reservoirs throughout the central basin show a trend of becoming isotopically heavier and chemically drier with increasing depth of burial and thermal maturity. The gases are interpreted to be thermogenic in origin and were generated during the mature and post-mature stages of hydrocarbon generation. Scatter in the general trend is attributed to differences in the type of source rock.
Gases produced from the Pictured Cliffs sandstone are nonassociated and were generated during the mature stage of hydrocarbon generation.
The gas was probably generated from type III kerogen dispersed in the underlying, marine Lewis shale. Minor amounts of CO2 resulted from thermal decomposition of oxygen-containing functional groups in the kerogen.
Small quantities of nonassociated gas produced from sandstone reservoirs in the upper part of the Fruitland formation are similar in composition and origin to gases of the Pictured Cliffs sandstone, This gas was probably derived from dispersed type III kerogen in the enclosing, nonmarine shales.
Nonassociated gas produced from the Fruitland coal beds is different from that produced from adjacent reservoirs. The coal gas is composed mainly of methane with decreasing amounts of CO2 accompanying increasing levels of thermal maturity.
The gases are the product of devolatilization, and significant methane generation probably began at a rank of high volatile bituminous A. This distinct gas is generally confined to the coal beds and has not migrated to adjacent reservoirs, either above or below.
In addition to containing a distinct type of gas, Fruitland coalbed reservoirs are characterized by a different produced water composition and higher reservoir pressures than adjacent sandstone reservoirs. These factors indicate that the coal beds form a closed reservoir system.
The coal beds produce a bicarbonate-type connate water that probably resulted from geochemical processes associated with the early stages of coalification. Hydrocarbon generation is regarded as the principal mechanism for overpressuring in the Rocky Mountain region.
In the San Juan basin, higher reservoir pressures in the coalbed reservoirs are considered to be residual overpressuring caused by gas generation in the low permeability coal beds.
The adjacent sandstone reservoirs, such as Pictured Cliffs and Fruitland, have lower pressures because of uplift, erosion, and cooling since active hydrocarbon generation in mid-Tertiary time. However, the coalbeds have retained at least part of the higher pressures because the gas is retained in the coal's micropore structure. This storage phenomenon has prevented the gas from being expelled and migrating into adjacent reservoirs.
Occasionally, the coal beds and associated sandstones produce gas that is characteristic of the adjacent reservoir. As a result of drilling and completing wells, these gases are interpreted to have either migrated from or have drained from an adjacent reservoir.
The Fruitland coals are composed mainly of vitrinite and are capable of generating mostly methane-rich gas. Therefore, they can be considered as their own source. The coals have numerous voids that are filled with a hydrogen rich material that fluoresces and was probably generated in situ.
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