New Mexico Geological Society Annual Spring Meeting — Abstracts

Outcrop studies allow valuable observation and measurement of the characteristics of similar deposits occurring in the subsurface, including permeability measurements, relationships among facies, continuity of facies, scale of bedding, and sedimentary structures. Aquifer related sediments crop out at the surface at a number of locations in the Albuquerque basin. As part of an investigation of natural and artificial recharge in the basin, 12 outcrops in the municipal area were examined. Three outcrops of upper Santa Fe Group were selected, along with proximal and medial Tijeras Arroyo facies, two recent outcrops in Bear Canyon Arroyo, and distal Embudo fan facies. Late Pleistocene river-terrace deposits, the Los Duranes and Edith Gravel type sections, and young Rio Grande channel deposits were also sampled. Permeability of surface exposures were measured with an air-minipermeameter. In contrast to the compressed gas type permeameters, the air-minipermeameter uses a falling piston to induce air flow. It is highly portable, and was designed to measure the range of permeabilities common to poorly to moderately consolidated sandy and silty deposits occurring in the Albuquerque basin. Permeability was measured in situ, porosity of the deposit was determined, and a soil sample taken from the point of measurement. Grain size distributions were determined by mechamcal sieving. Permeability is correlated with porosity, lithification and a number of grain size distribution parameters of the samples. A strong correlation is observed with the log₁₀ of measured permeability and mean particle size. Correlation of permeability with the 10 and 20 percent passing sieve diameters is also high. A weak correlation is found with porosity and permeability, explained in part by cementation and sorting of the samples. Multiple regression analysis was used to formulate predictive
permeability equations based on grain size distribution parameters. These equations differ from most published permeability equations by excluding a value for porosity, which is difficult to obtain. Samples were divided into the following bedding types: cross beds, channels, horizontal beds, scour and fill structures, and structureless deposits. Moment measurements were used to calculate sorting coefficients. Log-hyperbolic plots were employed as an effective way to graphically represent and compare grain size distributions. Compared to the common cumulative percent plots, log-hyperbolic plots provide high resolution of the finest grains, which are generally less abundant but have a large influence on permeability. Scatter plots and box plots were used to make visual comparisons between bedding types. Kolmogorov-Smimov tests were used to quantify differences between facies. Comparison of outcrop samples to well core and cuttings allows estimation of the permeability of sediments at depth, based on grain size distribution. Quantitative analysis of geophysical logs provide a check for the effectiveness of the technique. The uncertainty associated with the use of grain size analysis of well cuttings, subject to rapid vertical facies changes and mixing of material in the well bore, is reduced when used in conjunction with geophysical logs. Geophysical logs and the analysis of core and cuttings may be used to evaluate permeability on a scale that meets or exceeds the resolution of many groundwater models.