A GEOMORPHIC APPROACH FOR PREDICTING DIMENSIONS OF HIGH-SINUOSITY FLUVIAL SAND BODIES IN THE LOWER WILLIAMS FORK FORMATION, SOUTHWEST PICEANCE BASIN, COLORADO

Fluvial sand bodies in the lower 300 to 700 ft of the Williams Fork Formation (Late Cretaceous) are major natural gas reservoirs in the southern Piceance Basin. Lateral and vertical connectivity is very poor because of the small dimensions of the sand bodies and the abundance of interstratified mudrock. Thus, closely spaced wells (10 to 20 acres) are often needed for economic gas recovery. Individual reservoir elements penetrated in these wells are thought to be isolated point bars or point-bar complexes deposited by high-sinuosity fluvial systems. Because the subsurface dimensions and volumes of these sand bodies are poorly understood, an investigation of surface analogs was conducted in Coal Canyon, near Palisade, Colorado. In this study, variations in thickness and width for 85 sand bodies were defined, along with their sedimentological characteristics. For this population, the average sand-body thickness ranged from 4.1 to 29.0 ft (avg. = 12.1 ft) , whereas the widths ranged between 112 and 2,791 ft (avg. = 706 ft). Because of the two-dimensional nature of the outcrops, the sand-body lengths do not necessarily represent the actual sizes of a given point-bar. Thus, it is not possible to directly determine the areal dimensions of lower Williams Fork sand bodies and their reservoir volumes from outcrop data.

To gain a better understanding of point-bar dimensions, a data set was created from selected stretches of four present-day, high-sinuosity rivers in Colorado (Animas, Rio Grande, North Platte, and Illinois). Point bars in these rivers can be modeled as partial ellipses and, therefore, characterized by their long and short axes. Using aerial photographs, satellite imagery, and topographic maps, the long and short dimensions of 138 point bars were measured. For the total population, the point-bar long axes ranged from 106 to 2,534 ft (avg. = 890 ft), and the short axes ranged from 53 to 1,320 ft (avg. = 635 ft). The corresponding point-bar surface areas ranged 0.1 to 44 acres (avg. = 9.9 acres). Obviously, the rivers used to generate this data set operate in different tectonic, geomorphic, and climatic settings than those in the Late Cretaceous; however, the linear dimensions of the modern point bars are surprisingly similar to the sand-body widths recorded from the lower Williams Fork in Coal Canyon.