Browsing by Subject "Subsurface characterization"
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Item Analysis of Areal Permeability Variations - San Andres Formation (Guadalupian): Algerita Escarpment, Otero County, New Mexico(1988-08) Kittridge, Mark Gerard; Lake, Larry W.This paper presents the results of an integrated outcrop and subsurface characterization study conducted on the San Andres Formation of the Permian basin. More than 1600 permeability measurements were obtained from an outcrop section located along the Algerita Escarpment in southeastern New Mexico using an experimental mechanical field permeameter (MFP). Subsurface core data (permeability and porosity) were available from ten closely spaced wells in the Wasson field on the adjacent Northwest Shelf of the Permian basin. Standard population statistics, contour plots and vertical profiles, and geostatistical techniques were used in an attempt to characterize the extremely heterogeneous formation. The outcrop permeability data were found to be log-normally to power-normally distributed, with 12 of 16 data sets having a negative p value. Mean permeability and variance was lowest in the fusulinid dolowackestones, while the highest mean was found in the dolopackstones and dolograinstone intervals. Permeability contour maps of the outcrop grid data typically revealed isolated 'pods' of high permeability in a generally low permeability matrix. The vertical transect measured displayed rapidly varying permeability, with values changing over a very short interval. Geostatistical analysis with the variogram predicted three distinct correlation lengths: 40 feet, 3 to 5 feet, and approximtely 0.25 feet, depending on the spacing of the data used. Predicted correlation length decreased with a decrease in sample spacing. The correlation length was found to be invariant with respect to direction, indicating that the formation is isotropic. Subsurface permeability and porosity data were analyzed in a similar manner. The permeability data was found to be log-normally distributed while the porosity data was power normal. The associated variance on the core plug data was much larger than on the whole core data. Vertical permeability and porosity profiles were similar to that observed from the outcrop vertical transect: alternating high and low values occurring over a very short distance. Variograms indicated a correlation length of approximtely 10.0 feet (vertically) for both permeability and porosity.Item Quantifying the influence of surface processes on subsurface geometry in deltaic environments(2019-05-07) Hariharan, Jayaram Athreya; Passalacqua, PaolaRiver deltas are densely populated and dynamically changing environments located at the boundary between land and sea. Population demands for water as well as rising sea levels are increasingly threatening aquifer water quality in deltaic regions. The rate at which aquifer contamination by salt water or other contaminants occurs is dictated, in part, by the arrangement of sediment within the subsurface. In this work, we examine the heterogeneity of the subsurface from a structural vantage to better understand how surface processes and geometry are linked to subsurface architecture. The numerical model, DeltaRCM, is applied to simulate delta evolution under a variety of input conditions. The resulting model outputs simulate 800 years during which the growing delta generates a subsurface volume that is over 40m deep. Surface channel properties and behavior, such as channel depths and channel planform decay rates are measured. Similarly, the structure of the sand bodies in the subsurface domain is evaluated. These different types of analyses, surface and subsurface, are ultimately compared to take a first-look at how channel properties in a deltaic environment may relate to subsurface structure and form. Broadly, expectations about channel trends and subsurface structure from the field of geomorphology are supported. Channel depths decrease with distance from the inlet, and as the input sand proportion increases. Similarly, the channelized fraction of the delta surface increases with higher input sand fraction values. In the subsurface, different types of channel behavior on the surface correspond to different structures. The sand bodies are larger when the surface channels are shallower and more mobile. In addition, the spatial continuity within strike sections (sections taken perpendicular to the inlet channel) increases with channel depth. Comparisons of the modeled subsurface with stochastically re-arranged replicates have confirmed the assertion that surface processes create unique subsurface structures. When the input proportion of sediment contains at least 40% sand by volume, the average size of the subsurface sand bodies follows a power-law relation with respect to surface channel depths and the average channelized fraction of the delta platform. The range of spatial entropy (disorder) also increases with channel depth. Within models, with increasing distance from the inlet both channel depths and spatial entropy ranges decrease. Changing the input sediment proportions over the course of the delta evolution provides mixed results. Some channel parameters like channel depth are indistinguishable from steady input cases, while others are influenced by the initial topographic setup. In the subsurface, variable sediment input proportions create vastly different sand body geometries depending on the rate of variation of the input sand proportion. When the input sand proportion is gradually increased, the average sand body size becomes very large; however when the sand input is abruptly increased, the mean sand body value is less than a steady sand input analog.