GEOLOGIC MODEL FOR CHARACTERIZING HYDROCARBON ACCUMULATIONS IN CLASTIC RESERVOIRS OF THE PARADOX BASIN

The sedimentary section of the Paradox Basin, which lies in eastern Utah and western Colorado, includes many clastic reservoirs that contain hydrocarbons. Reservoir ages range from Pennsylvanian to Jurassic, and a single well can produce from multiple reservoir zones. In the Paradox Basin most hydrocarbons are produced from sandy carbonates of the Upper Pennsylvanian Honaker Trail Formation and from continental facies of the Upper Pennsylvanian-Lower Permian Cutler Formation.

The hydrocarbon source for these accumulations is the Pennsylvanian Paradox Formation, which was deposited over much of the Paradox Basin. Oil-prone kerogen of the Paradox Formation was the source for oil in the western portion of the basin, near Bluff, UT. In the eastern part of the basin, greater depths of burial resulted in higher degrees of thermal maturity, and most oil has been cracked to gas.

Some vertical migration of oil into clastic reservoirs predates Paradox Formation gas generation, as shown by the presence of oil in these reservoirs. This was followed by similar migration of Paradox Formation-sourced gas, which is the dominant produced hydrocarbon in the eastern part of the basin. Vertical migration likely occurred through faults related to the movement of Paradox Formation salt. This salt also created structures and traps associated with gas accumulations. Oils found in reservoirs to the west are in traps created by Laramide structures, and tend to be in Upper Pennsylvanian and Lower Permian rocks, suggesting that oil migration may have occurred mostly through adjacent permeable formations, such as the Honaker Trail, rather than vertically though faults.

Gas-oil-ratios (GOR) sampled across the basin show gas in the east, and medium- to light-weight oils in the west, with a fairly strong bounding line; this line runs essentially northwest to southeast, and lies just to the west of Moab, UT and Farmington, NM. Comparing clastic-reservoir oil and gas wells in the basin to the GOR bounding line (which likely approximates the oil-gas window transition of the Paradox Formation), suggests that little if any lateral (east-west) migration has taken place.