Report of Investigations No. 133 Three-Dimensional Ground-Water Modeling in Depositional Systems, Wilcox Group, Oakwood Salt Dome Area, East Texas

UT Librarie

Wilcox Group Facies and Syndepositional Salt Dome Growth, southern East Texas Basin

Shallow salt diapirs in the East Texas sedimentary basin are currently being considered as repositories of high-level nuclear waste (Kreitler, 1980; Kreitler and others, 1980; Kreitler and others, 1981). A crucial aspect of such an assessment involves understanding how fresh meteoric groundwater may impact long-term dome stability through dissolution. In the East Texas Basin, the zone of fresh meteoric groundwater includes the Wilcox Group, which encases or surrounds the upper 100 to 500 m (330 to 1640 ft) of most shallow diapirs. The distribution of Wilcox depositional facies may strongly influence flow directions and velocities of groundwater around salt domes. The Wilcox Formation exhibits extreme heterogeneity due to lateral and vertical variations in the distribution of highly permeable sands and other sandstones, mudstones, and lignites with low permeabilities. Variables affecting aquifer characteristics include thickness and hydraulic conductivity, which vary partly as a function of depositional facies and sand-body geometry, including sand body thickness, permeability, and interconnectedness. This report provides the basic facies and sand body data for computer modeling of groundwater flow around Oakwood Dome (Fogg, 1980a). Two main problems are addressed in this study: (1) determining the facies distribution and sand-body geometry of sandstones with high hydraulic conductivities and the greatest potential for possible dome dissolution, and (2) understanding how facies distribution is affected by syndepositional dome growth.Bureau of Economic Geolog

Modeling the Effects of Regional Hydrostratigraphy and Topography on GroundWater Flow, Palo Duro Basin,Texas

A cross-sectional ground-water flow model was constructed of the Palo Duro Basin to analyze available hydrogeologic data and better understand the causes of underpressuring below the Evaporite Aquitard and mechanisms of recharge and discharge to and from the Deep-Basin Brine Aquifer. Various effects of lithostratigraphy and topography on subhydrostatic conditions in the deep section were investigated in different simulations. The model indicates that the subhydrostatic pressures beneath the Evaporite Aquitard are caused by segregation of deep and shallow flow systems by the low-permeable evaporite section and drainage of the deep system by relatively permeable granite wash deposits. The Pecos River, which allows underflow of some groundwater recharging in the New Mexico area to the west, enhances underpressuring beneath the western half of the High Plains by serving as a discharge area for water that would otherwise move downdip into the Deep-Basin Brine Aquifer. In addition to this recharge, about 26% of the groundwater in the Deep-Basin Brine Aquifer originates from leakage through the evaporite section, assuming K2 = 2.8 x 10-4 md, the upper limit of aquitard permeability suggested by the model. The groundwater flow pattern within the Deep-Basin Brine Aquifer is governed by the spatial distribution of more permeable strata, particularly the granite wash deposits. In the cross-sectional model, most of the groundwater in the Deep-Basin Brine Aquifer discharges laterally through the eastern boundary and eventually by upward leakage in the easternmost part of the cross-section. Ground-water travel times through the Deep-Basin Brine Aquifer from the westernmost recharge area in New Mexico to the eastern boundary of the model range between 1.2 and 4 million years, depending on the flow path depicted by the streamtubes and average porosities of the different units.Bureau of Economic Geolog

Effects of Hydrostratigraphy and Basin Development on Hydrodynamics of the Palo Duro Basin, Texas

A two-dimensional groundwater flow model was developed along a cross-section through the Palo Duro Basin to understand regional groundwater flow paths and investigate factors influencing underpressuring below the Evaporite aquitard, as well as recharge and discharge mechanisms to and from the Deep-Basin Brine aquifer. Steady-state flow simulations were employed to examine the effects of lithostratigraphy and topography on groundwater flow. Additionally, transient flow simulations were used to describe changes in regional hydrodynamics resulting from various tectonic and geomorphologic processes. The groundwater flow pattern in the Palo Duro Basin is characterized by a shallow groundwater flow system primarily controlled by topography. Deeper flow regimes recharge in the New Mexico area and pass beneath the Pecos River into the deep section of the Palo Duro Basin. The Evaporite aquitard effectively separates the deeper flow regime from the more rapidly circulating shallow aquifer system, although leakage through the aquitard is significant and could contribute up to 27 percent of the water passing through the deep section. Within the Deep-Basin Brine aquifer, groundwater flow patterns are strongly influenced by the spatial distribution of more permeable strata, such as granite-wash deposits, which facilitate drainage of the deep aquifer system more readily than recharge occurs.Bureau of Economic Geolog

Hydrogeologic Characterization of the Saline Aquifers, East Texas Basin-Implications to Nuclear Waste Storage in East Texas Salt Domes

Groundwaters in the deep aquifers (Nacatoch to Travis Peak) range in salinity from 20,000 to over 200,000 mg/l. Based on their isotopic compositions, they were originally recharged as continental meteoric waters. Recharge probably occurred predominantly during Cretaceous time; therefore, the waters are very old. Because the basin has not been uplifted and faulting of the northern and western sides, there are no extensive recharge or discharge zones. The flanks of domes and radial faults associated with domes may function as localized discharge points. Both the water chemistry and the hydraulic pressures for the aquifers suggest that the basin can be subdivided into two major aquifer systems: (1) the upper Cretaceous aquifers (Woodbine and shallower) which are hydrostatic to subhydrostatic and (2) the deep lower Cretaceous and deeper formations (Glen Rose, Travis Peak, and older units), which are slightly overpressured. The source of sodium and chloride in the saline waters is considered to be from salt dome dissolution. Most of the dissolution occurred during the Cretaceous. Chlorine-36 analyses suggest that dome solution is not presently occurring. Salinity cross sections across individual domes do not indicate that ongoing solution is an important process. The major chemical reactions in the saline aquifers are dome dissolution, albitization, and dedolomitization. Albitization and dedolomitization are important only in the deeper formations. The high Na concentrations in the deeper aquifers system result in the alteration of plagioclase to albite and the release of Ca into solution. The increase in Ca concentrations causes a shift in the calcite/dolomite equilibrium. The increase in Mg results from dissolution of dolomite. The critical hydrologic factors in the utilization of salt domes for disposal of high-level nuclear waste are whether the wastes could leak from a candidate dome and where they would migrate. The following conclusions are applicable to the problem of waste isolation in salt domes: (1) Salt domes in the East Texas Basin have extensively dissolved. The NaCl in the saline aquifers is primarily from this process. Major dissolution, however, probably occurred in the Cretaceous time. There is little evidence for ongoing salt dome dissolution in the saline aquifers. (2) If there was a release to a saline aquifer, waste migration would either be along the dome flanks or laterally away from the dome. If there is a permeability conduit along the dome flanks, then contaminants could migrate to the fresh-water aquifers, provided an upward hydraulic gradient exists. Calculation of performance assessment scenarios must take into account whether there is potential for upward flow between saline aquifers at repository level and the fresh water aquifers. If an upward flow potential exists, upward leakage along the dome flanks should be used as the worst-case scenario.Bureau of Economic Geolog

San Andres/Grayburg Reservoir Characterization Research Laboratory

The Bureau of Economic Geology's Reservoir Characterization Research Laboratory project, "Characterization of San Andres and Grayburg Reservoirs," was initiated in September 1988 and has completed the first year of a proposed 2-year program. Substantial progress has been made toward the goals of this program, which are focused on development of advanced approaches to reservoir characterization for improving recovery efficiency of substantial remaining mobile oil resources in these prolific reservoirs. Key research results are in the areas of (1) quantitative description and geostatistical modeling of interwell and reservoir-scale heterogeneity from San Andres outcrops, and (2) preliminary studies on integration of the quantitative outcrop models with a geologic/engineering characterization of the Seminole San Andres Unit. Outcrop geologic studies were carried out at play, reservoir, and interwell scales along the Algerita Escarpment, Guadalupe Mountains, New Mexico. This 17-mile play-scale study area provides a dip-section framework for detailed investigations and serves as an analogous reservoir framework for comparison with producing San Andres fields. Reservoir-scale mapping of a 4-mile dip section of the upper San Andres with measured sections spaced 1,000 to 2,000 ft apart demonstrates the compartmentalization of individual grainstone shoal complexes on the scale of several thousand feet laterally and 50 to 100 ft vertically.Bureau of Economic Geolog

Hydrogeologic Characterization of the Saline Aquifers, East Texas Basin- Implications to Nuclear Waste Storage in East Texas salt Domes

Groundwaters in the deep aquifers (Nacatoch to Travis Peak) range in salinity from 20,000 to over 200,000 mg/L. Based on their isotopic compositions, they were originally recharged as continental meteoric waters. Recharge probably occurred predominantly during the Cretaceous time; therefore, the waters are very old. Because the basin has not been uplifted, there are no extensive recharge or discharge zones. The flanks of domes and radial faults associated with domes may function as localized discharge points. Both the water chemistry and the hydraulic pressures for the aquifers suggest that the basin can be subdivided into two major aquifer systems: (1) the upper Cretaceous aquifers (Woodbine and shallower) which are hydrostatic and (2) the deep lower Cretaceous and deeper formations (Glen Rose, Travis Peak, and older units), which are slightly overpressured. The source of sodium and chloride in the saline waters is considered to be from salt dome dissolution. Most of the dissolution occurred during the Cretaceous. Chlorine-36 analyses suggest that dome solution is not presently occurring. Salinity cross sections across individual domes do not indicate that ongoing solution is an important process. The major chemical reactions in the saline aquifers are dome dissolution, albitization, and dedolomitization. Albitization and dedolomitization are important only in the deeper formations. The high Na concentrations in the deeper aquifer system result in the alteration of plagioclase to albite and the release of Ca into solution. The increase in Ca concentrations causes a shift in the calcite/dolomite equilibrium. The increase in Mg results from dissolution of dolomite.Bureau of Economic Geolog

Review of GroundWater Quality Monitoring Network Design

Ground-water quality monitoring network design is defined as the selection of sampling sites and (temporal) sampling frequency to determine physical, chemical, and biological properties of ground water. The main approaches to ground-water quality monitoring network design were identified as hydrogeologic and statistical. The various methods for network design available in the hydrologic literature have been evaluated by considering the spatial scale of the monitoring program, the objective of sampling, data requirements, temporal effects, and range of applicability. Considerable advance has been made over the last two decades that now permit the application of methodical and testable approaches to ground-water quality monitoring network design, although they mostly serve for preliminary analysis and design. The opinion of the Task Committee on Ground-Water Quality Monitoring Network Design is that as there continues to be advances in hydrogeochemistry, ground-water hydrology, and risk and geostatistical analysis, methods for ground-water quality monitoring network design will be improved and refined, and they will become ever more useful in the important mission of environmental protection. © ASCE

Characterization of Facies and Permeability Patterns in Carbonate Reservoirs Based in Outcrop Analogs

More than 13 billion barrels (Bbbl) of mobile oil and 17 Bbbl of residual oil will remain in San Andres and Grayburg reservoirs at abandonment under current development practices. Through the development and application of new recovery technology, a large part of this resource can be recovered. This report focuses on research for the development and testing of new techniques for improving the recovery of this resource. Outcrop and subsurface geologic and engineering data are utilized to develop new methodologies through the integration of geologic observations and engineering data for improving numerical models that predict reservoir performance more accurately. Extensive regional mapping of the 14-mile by 1,200-foot San Andres outcrop, located on the Algerita Escarpment, Guadalupe Mountains, New Mexico, demonstrates that the San Andres carbonate-ramp complex is composed of multiple depositional sequences that have significant basinward shifts in reservoir-quality facies tracts occurring across sequence boundaries. Detailed geologic and petrophysical mapping of three reservoir-quality facies tracts demonstrates that the fundamental scale of geologic description for reservoir characterization is the parasequence and its component rock-fabric-based facies. Descriptions of cores from the Seminole San Andres Unit illustrate that the parasequence is also the fundamental geologic scale for reservoir mapping in the subsurface.Bureau of Economic Geolog

Characterization of Reservoir Heterogenity in Carbonate-Ramp Systems, San Andres/Grayburg Permian Basin

This report summarizes research carried out by the Bureau of Economic Geology's San Andres/Grayburg Reservoir Characterization Research Laboratory (RCRL) from September 1988 through September 1990. The goal of the RCRL program was to develop advanced approaches to reservoir characterization for improved recovery of the substantial remaining mobile oil in San Andres and Grayburg reservoirs. Emphasis was placed on developing an outcrop analog for San Andres strata that could be used as (1) a guide to interpreting the regional and local geologic framework of the subsurface reservoirs and (2) a data source illustrating the scales and patterns of variability of rock-fabric facies and petrophysical properties, particularly in lateral dimensions, and on scales that cannot be studied during subsurface reservoir characterization. Areas selected for study were the San Andres exposures of the Algerita Escarpment in the northern Guadalupe Mountains and the Seminole San Andres Unit on the northern margin of the Central Basin Platform. The outcrop-analog research was emphasized because it had received little attention before this study by either industry or academe. Reports in this summary involve (1) outcrop and subsurface geological characterization of the Algerita Escarpment San Andres and the Seminole San Andres Unit (Kerans), (2) correlation of detailed outcrop mapping in order to research cored wells at Lawyer Canyon, Algerita Escarpment (Nance), (3) diagenetic/petrographic analysis of selected upper San Andres facies focusing on the origin of moldic porosity (Hovorka), (4) geologic engineering description of the upper San Andres carbonates at Lawyer Canyon and the upper producing interval at Seminole (Lucia), (5) geostatistical analysis of permeability patterns and stochastic-based finite-difference modeling of the upper San Andres parasequence window (Senger and Fogg), and (6) deterministic finite element modeling of the upper San Andres parasequence window (Kasap). Availability of basic data for these studies is summarized in the appendix.Bureau of Economic Geolog