Table 1 Stratigraphy and hydrostratigraphy of the shallow subsurface in the vertical profile from southeastern Colorado to western and central Kansas.
Delineation of hydrostratigraphic units at the regional scale involves the application of:
Gamma-ray and sample logs of boreholes drilled for hydrocarbon exploration and production, sample logs of test holes and water wells, and cores were used to delineate the subsurface stratigraphy and the major lithofacies in the study area. The gamma-ray log records the natural gamma-ray radiation that emanates from geologic media. The natural gamma-ray radiation is measured by a tool that is passed along the length of the borehole. The logs that were used to delineate the subsurface hydrostratigraphy were calibrated to a standard in API units.
On the gamma-ray logs formation tops were delineated using the shapes of the log traces and used to correlate the subsurface stratigraphy between boreholes to produce maps and cross sections of subsurface geology. The gamma-ray log was also used for distinguishing between clayey strata and non-clayey strata in the subsurface and thus to differentiate between shaley rocks and sandstones and limestones (Doveton, 1986). Owing their vertical and lateral lithologic continuity, a lithostratigraphic approach to correlation and subsurface mapping is adequate for many of the geologic units in the subsurface.
However, within the heterogeneous Dakota aquifer system a strictly lithostratigraphic approach to its hydrostratigraphy is not appropriate because it is difficult to recognize the physical continuity of the deposits locally. Sequence stratigraphy is the study of genetically related units that are packaged into sequences (Van Wagoner et al., 1990). Each sequence is bounded above and below by subaerial unconformities and their correlative conformities. Subaerial unconformities result from lowering of sea level and subaerial exposure of the former sea floor. The areal extent of each unconformity depends largely on the amount of sea level lowering. Each sequence, therefore, is formed by a single cycle of transgression and regression during which a wedge of sediment is deposited that generally thickens and is more marine basinward. Given similar conditions from sequence to sequence, predictable patterns of facies distributions are expected. Sequence stratigraphic models can be extended into areas of low data density in order to predict facies patterns. Facies distributions may be helpful in developing a framework within which to correlate hydraulic properties with lithology (Anderson and Woessner, 1992).
Hydraulic properties data from the subsurface come primarily from the major aquifer units, and are unevenly distributed and data on the aquitard units in Kansas and eastern Colorado are unknown. However, permeability is often a function of rock type. The permeability of sandstone, limestone, and shale each vary over three or more orders of magnitude (Freeze and Cherry, 1979), but their permeability ranges frequently do not completely overlap. On the other hand, effective porosity, the percentage of interconnected void space, is directly related to the textural fabric which is related to lithology (Domenico and Schwartz, 1990). The effective porosity and permeability are usually much higher in sandstones than they are in shales or siltstones. In the case of sandstone and limestone their permeability ranges may completely overlap but frequently the nature of the permeability and porosity distribution are sufficiently dissimilar to distinguish one from the other.
Knowledge of the geologic history of the area aids in the delineation of hydrostratigraphic units by providing a framework for determining the effects of diagenetic processes and tectonics and the redistribution or alteration of initial depositional porosity and permeability of the sediment. Diagenesis is the process that a body of sediment undergoes including all of the physical, chemical, and biological changes, during the process of lithifciation after its initial deposition (Gary et al., 1972). Processes associated with burial and cementation reduce pore space and effective porosity and permeability in sandstones and shales (Schwartz and Longstaffe, 1988). Tectonic activity may increase or decrease effective porosity as a result of the imposed stress fields or failure by fracturing and faulting (Means, 1976). Uplift and subsequent erosion may substantially change the distribution of effective porosity and permeability by volume expansion from unloading or, in the case of carbonates and strata containing evaporites, dissolution of portions of the geologic framework and the formation of solution channels and fractures (Schwartz and Domenico, 1990).
Delineation of hydrostratigraphic units is aided immensely by the results of previous regional investigations. The turn of the century investigations reported by Darton (1905, 1906) provided the first insights into the ground-water flow system in the Dakota aquifer in the central Great Plains. He recognized the Dakota as a distinct and important regional aquifer system. He also realized that the shale aquitard above the Dakota was leaky and not "impermeable" to vertical flow. Belitz (1985), Belitz and Bredehoeft (1988), and Helgeson et al. (1993) demonstrated that the shale aquitards significantly isolate the Dakota and underlying aquifers from the water table in the Denver basin. Robson and Banta (1987) recognized the Dockum as a distinct aquifer unit in southeastern Colorado but did not consider the Morrison an aquifer unit. To the east in southwestern Kansas, Kume and Spinazola (1985) found irrigation wells producing from Morrison and other overlying aquifer units and considered the Morrison to be an aquifer unit of local significance.
Belitz, K., 1985, Hydrodynamics of the Denver basin: an explanation of subnormal fluid pressures: Ph.D. Thesis, Stanford University, Stanford, CA, 194 p.
Belitz, K., and Bredehoeft, J. D., 1988, Hydrodynamics of the Denver basin: explanation of fluid subnormal pressures: American Association of Petroleum Geologists Bulletin, 72(11), pp. 1334-1359.
Darton, N.H., 1905, Preliminary report on the geology and underground water resources of the central Great Plains: U.S. Geological Survey Professional Paper 32, 433 p.
Darton, N.H., 1906, Geology and underground water resources of the Arkansas valley in eastern Colorado: U.S. Geological Survey Professional Paper 52, 90 p.
Domenico, P.A., and Schwartz, F.W., 1990, Physical and chemical hydrogeology: New York, NY, John Wiley and Sons, 824 p.
Doveton, J.H., 1986, Log analysis in subsurface geology: New York, NY, John Wiley and Sons, 2nd edition, 273 p.
Freeze, R.A., and Cherry, J.A., 1979, Ground Water: Englewood Cliffs, N.J., Prentice Hall Inc., 604 p.
Gary, M., McAfee, R., Jr., and Wolf, C.L. (eds.), 1972, Glossary of geology: Washington, D.C., American Geological Institute, 857 p.
Helgeson, J.O., Leonard, R.B., and Wolf, R.J., 1993, Aquifer systems underlying Kansas, Nebraska, and parts of Arkansas, Colorado, Missouri, New Mexico, Oklahoma, South Dakota, Texas, and Wyoming--hydrology of the Great Plains aquifer system in Nebraska, Colorado, Kansas, and adjacent areas: U.S. Geological Survey Professional Paper 1414-E, 161 p.
Kume, J., and Spinazola, J.M., 1985, Geohydrology of sandstone aquifers in southwestern Kansas: Kansas Geological Survey Irrigation Series 8, 49 p.
Maxey, G.B., 1964, Hydrostratigraphic units: Journal of Hydology, 2, pp. 124-129.
Means, W.D., 1976, Stress and Strain: New York, Springer-Verlag, 339 p.
Robson, S.G., and Banta, E.R., 1987, Geology and hydrology of deep bedrock aquifers in eastern Colorado: U.S. Geological Survey Water-Resources Investigations Report 85-4240, 6 sheets.
Seaber, P.R., 1988, Hydrostratigraphic units: in Back, W., Rosenschein, J.R., and Seaber, P.R., Hydrogeology, The Geology of North America, Geological Society of America, Volume O-2, pp. 9-14.
Schwartz, F.W., and Longstaffe, F.J., 1988, Ground water and clastic diagenesis: in Back, W., Rosenschein, J.R., and Seaber, P.R., Hydrogeology, The Geology of North America, Geological Society of America, Volume O-2, pp. 413-434.
Van Wagoner, R. M., Mitchum, R. M., Campion, K. M., and Rahmanian, V. D., 1990, Siliclastic sequence stratigraphy in well logs, cores, and outcrops: Concepts for high-resolution correlation of time and facies, American Association of Petroleum Geologists Methods in Exploration Series, No. 7, 55.
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