SIGDA, J.M., MOZLEY, P.S., and GOODWIN, L.B., Dept. Earth & Environmental Sciences, New Mexico Tech, Socorro, NM 87801, sigda@nmt.edu, mozley@nmt.edu, lgoodwin@nmt.edu; HANEBERG, W.C., New Mexico Bureau of Mines, 2808 Central Avenue SE, Albuquerque, NM 87106, haneberg@nmt.edu.
Deformational and diagenetic changes in original permeability distributions are less well understood for poorly consolidated sediments, such as those that are common along the Rio Grande rift of New Mexico, than for well-indurated rocks. We used air and gas mini-permeameters to measure the single-phase permeability of both deformed and undeformed sands near small faults (< 1 to 2 m displacement) at two outcrops within the rift. Small faults were chosen because they commonly separate lithologically similar sands, eliminating complications resulting from juxtaposition of different lithologies. Point counts on thin sections of epoxy impregnated samples were used to determine porosity, grain size, and mineral composition. The permeability of deformed sand can be two to three orders of magnitude less than that of undeformed sands. Fault zone permeability is typically more variable than undeformed sand permeability, which is on the order of 10 darcies at the outcrops that we studied. Thin section petrography showed a four- to five-fold increase in the clay-sized fraction, large reductions in porosity and grain size, and reorientation of grains within the fault zone. These observations can be explained by a combination of grain comminution, weathering of feldspars, and perhaps the mechanical infiltration of clays along the fault. Calcite cements were negligible in both deformed and undeformed samples. Our results show that both deformation and diagenesis can significantly reduce the original permeability of poorly consolidated sands.
key words-- faults, permeability, New Mexico, diagenesis