SUBSURFACE MAPPING OF MORRISON FORMATION SAND DISTRIBUTION IN THE SAN JUAN BASIN, NEW MEXICO

Fluvial deposits of the Morrison Formation in the San Juan Basin, New Mexico are exposed in outcrops and recorded in several hundred wells throughout the basin, providing the opportunity to carry out a broad scale analysis of the succession. With this subsurface data, resistivity, gamma ray, sonic, and density-neutron logs can be used to determine formation tops, lithology, and thickness of individual members within the Morrison Formation. These measures allow the sand percentage to be calculated and mapped across the area to establish trends in the fluvial units. On the basis of these data and integration with published outcrop information the pattern of fluvial channel and overbank facies can be established. Deposition by a distributive fluvial system (DFS) has been suggested and the subsurface mapping will establish whether the fluvial deposits show the down-system decrease in grain size, decrease in channel body thickness, increase in overbank facies and a radial channel paleoflow pattern that is predicted by the DFS model. This study also has significance in potentially mapping the distribution of uranium deposits in the Morrison Formation and can serve as a guide in identifying petroleum reservoirs in other fluvial formations.