Basin-wide Architecture of Sandstone Reservoirs in the Fort Union Formation, Wind River Basin, Wyoming

FLORES, ROMEO M., C. W. KEIGHIN, and WILLIAM R. KEEFER, U.S. Geological Survey, Denver, CO

Architecture of hydrocarbon-bearing sandstone reservoirs of the Paleocene Fort Union Formation in the Wind River basin, Wyoming, was studied using lithofacies, grain size, bounding surfaces, sedimentary structures, internal organization, and geometry.

Two principal groups of reservoirs, both erosionally based and fining upward, consist of either conglomeratic sandstone or sandstone lithofacies. Two types of architecture were recognized in conglomeratic sandstone reservoirs: (I) heterogeneous, multistacked, lenticular and (II) homogeneous, multiscoured, wedge-sheet bodies. Three types of architecture were recognized in sandstone reservoirs: (III) heterogeneous, multistacked, elongate; (IV) homogeneous, multilateral, lenticular; and (V) homogeneous, ribbon-lensoid bodies. Homogeneity of reservoir types II, IV, and V, characterized by vertical and lateral uniformity of bed forms and less compartmentalization, make them better reservoirs than internally heterogeneous types I and III reservoirs. Type I reservoirs occur commonly along th southern part of the basin, and type II reservoirs are locally found in the southwest. Types I, III, IV, and V reservoirs developed in the northwestern part of the basin where types III and IV reservoirs are coeval and are overlain by types I and V reservoirs.

Conglomeratic sandstone reservoirs in the southern and southwestern parts of the basin suggest deposition in gravel-bedload fluvial systems influenced by provenance uplift of the Granite and southern Wind River mountains. Type II reservoirs represent deposits of eastward-flowing braided streams aggrading an alluvial valley in response to base level rise. Type I reservoirs reflect downstream-continuum deposits of northeastward-flowing, low-sinuosity streams aggrading an alluvial plain near base level. Sandstone reservoirs in the northwestern part of the basin indicate deposition in eastward-flowing fluvial systems consisting of sandy- to gravelly bedload braided streams (types I and III) and mixed-load, low-sinuosity streams (types IV and V) controlled by basin subsidence and accompani d by provenance uplift of the northern Wind River Mountains.

Thus, to determine basin-wide architecture of reservoirs requires understanding the interplay between base level conditions, basin subsidence, and provenance uplift. These interrelated factors, in turn, control differences in hierarchies of fluvial systems throughout the basin.

AAPG Search and Discovery Article #91010©1991 AAPG Rocky Mountain Section Meeting, Billings, Montana, July 28-31, 1991 (2009)