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

RI0165. Effects of Hydrostratigraphy and Basin Development on Hydrodynamics of the Palo Duro Basin, Texas, by R. K. Senger, G. E. Fogg, and C. W. Kreitler. 48 p., 30 figs., 5 tables, 2 appendices, 1987. ISSN: 0082335X: Print.


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ABSTRACT
Deep brine aquifers of the Palo Duro Basin, Texas Panhandle, ranging in depth from about 1,000 to 3,000 m below land surface, are underpressured compared with the shallow Ogallala aquifer. Two-dimensional simulations of flow in an east-west regional cross-sectional model through the basin permitted us to characterize regional ground-water flow, to investigate causes of underpressuring below the Evaporite aquitard, and to evaluate mechanisms of recharge and discharge to and from the Deep-Basin Brine aquifer. Employing steady-state flow simulations, we investigated various effects of lithostratigraphy and topography on groundwater flow. We also used transient flow simulations to illustrate changes in regional hydrodynamics caused by different tectonic and geomorphologic processes.

The regional ground-water flow regime in the Palo Duro Basin consists of a shallow ground-water flow system governed primarily by topography and a deeper flow system recharging in the New Mexico area and passing deep beneath the Pecos River into the deep section of the Palo Duro Basin. The low-permeability Evaporite aquitard limits flow between the shallow and deep systems, although leakage through the aquitard is important on a regional scale and could contribute a significant part of the water in the deep system. The ground-water flow pattern within the Deep-Basin Brine aquifer is strongly controlled by the spatial distribution of more permeable strata such as granite-wash deposits, which drain the deep aquifer system more easily than it is recharged.

Repeated simulations with the model demonstrate that chief causes of underpressuring are regional topography and geology. Specifically, the Ogallala water table is relatively high owing to the elevation of the High Plains, the potentiometric surface of the Deep-Basin Brine aquifer is relatively low owing to the draining effect of permeable granite wash and very low recharge rates, and the low-permeability Evaporite aquitard effectively segregates the two aquifer systems, thereby maintaining the large head differential. The Pecos River valley serves as a discharge area for some ground water from the west that would otherwise move downdip into the deep aquifer; thus, capture of recharge water enhances underpressuring in the deep system beneath the western half of the High Plains.

Simulations of the hydrodynamic development of the Palo Duro Basin as a result of successive tectonic and geomorphologic events indicate that Cenozoic uplift and tilting of the basin (10 to 15 mya) caused considerable increase in ground water flow rates. The timing of the erosional events implies that significant underpressuring (hydraulic-head difference between shallow and deep aquifers in excess of 175 m) occurred within the last 1 to 2 m.y. as a result of the erosional retreat of the Eastern Caprock Escarpment. The drastic change in hydraulic heads during that period suggests a significant effect on the overall ground-water flow pattern in the deep section within the last 1 to 2 m.y. Modeling also indicates that erosional unloading associated with the retreat of the Eastern Caprock Escarpment was ineffective in creating large-scale underpressuring in the Deep-Basin Brine aquifer. Only in the vicinity of the escarpment is it possible that erosional unloading and the resulting expansion of the underlying aquitard could create subhydrostatic conditions within the aquitard. Similarly, hydrocarbon production along the margins of the basin and the concomitant reduction in reservoir pressure affect hydraulic heads only locally and do not influence regional ground-water flow in the Deep-Basin Brine aquifer.

Keywords: ground-water modeling, basin hydrodynamics, regional underpressuring, geologic processes, hydrogeology, Evaporite aquitard, Palo Duro Basin, Texas Panhandle, TExas

CONTENTS

Abstract

Introduction

Scope

Hydrogeology

Geologic setting

Physiography and climate

Recharge and discharge

Mechanisms of Underpressuring

Data Base

Hydraulic head and pressure

Hydrologic properties

Ogallala Formation

Dockum Group

Permian evaporite strata

San Andres unit 4 carbonate

Lower Permian and Pennsylvanian strata

Salt-dissolution zones

Specific storage

Computer Programs

Modeling Procedure

Finite element mesh

Boundary conditions

Limitations of the Model

Simulation Strategy: Steady-State Model

Results and Discussion

Granite-wash permeability

Simulation A-1

Simulation A-2

Simulation A-3

Role of the Evaporite aquitard

Simulation B-1

Simulation B-2

Simulation B-3

Interconnection of Ogallala and Dockum aquifers

Simulation C

Effect of the Pecos River

Simulation D

Ground-water flow rates and travel times

Simulation Strategy: Transient Model

Results and Discussion

Simulation of hydrodynamic development of the basin

Pre-uplift conditions

Basin uplift and tilting

Deposition of the Ogallala Formation

Erosion of the Pecos River

Erosion of the Eastern Caprock Escarpment

Effect of erosional unloading

Effect of hydrocarbon production

Summary and Conclusions

Acknowledgments

References

Appendix A. Effect of node-spacing differences in horizontal and vertical direction on the hydraulic-head solution

Appendix B. Simulating the effect of erosional unloading

FIGURES

1. Structural features of the Texas Panhandle and adjacent areas

2. Regional east-west cross section illustrating spatial relationships of the major depositional systems in the Palo Duro Basin

3. Fluid-pressure changes as a result of (a) epeirogenetic movement and (b) erosion and deposition

4. Fluid·pressure changes as a result of erosional unloading with pore·volume expansion of shale

5. Plot of pressure·depth drill-stem·test data from selected wells in the Deep·Basin Brine aquifer using high-quality class A data

6. Potentiometric-surface map of the Ogallala aquifer on the High Plains

7. Potentiometric-surface map of the Deep-Basin Brine aquifer constructed from kriged estimates of fresh-water hydraulic head

8. Difference between fresh·water hydraulic heads in the Ogallala and Deep-Basin Brine aquifers

9. Finite element mesh representing the major hydrologic units

10. Pressure-depth data from the Deep-Basin Brine aquifer, Jackson County, Oklahoma

11. Flow net from Simulation A-1 using hydraulic conductivities from table 4

12. Flow net from Simulation A-2 with increased permeability of proximal granite wash

13. lsolith map of Pennsylvanian and Permian (Wolfcampian) granite wash in the Texas Panhandle

14. Flow net from Simulation A-3 with increased hydraulic conductivities for granite wash compared with Simulation A-2

15. Flow net from Simulation-1 with increased vertical permeability of the Evaporite aquitard

16. Flow net from Simulation B·2 indicating a relatively small decrease in hydraulic heads in the deep section

17. Flow net from Simulation B-3 with decreased vertical permeability of the Evaporite aquitard

18. Flow net from Simulation C accounting for decreased vertical permeability within the shallow aquifer system

19. Flow net from Simulation D with modified finite element mesh

20. Hydraulic·head distribution from Simulation T-1 using hydraulic conductivities from table 2

21. Hydraulic-head distribution from Simulation T-2 representing hydrodynamic conditions 10 m.y. after the beginning of uplift

22. Flow net from the steady·state simulation equivalent to Simulation T-2

23. Hydraulic·head distribution from Simulation T-3 representing hydrodynamic conditions 10 m.y. after the beginning of Ogallala deposition

24. Hydraulic-head distribution from Simulation T-4 representing hydrodynamic conditions 5 m.y. after the onset of Pecos River valley erosion

25. Difference in hydraulic heads before and after Pecos River valley erosion

26. Hydraulic-head distribution from Simulation T-5 representing hydrodynamic conditions 1 m.y. after the gradual retreat of the Eastern Caprock Escarpment

27. Difference in hydraulic heads before and after escarpment retreat

28. Hydraulic·head distribution from Simulation P-1 showing possible effects of erosional unloading conditions 800,000 yr after gradual retreat of the Eastern Caprock Escarpment

29. Interval transit times of two mudstone marker beds near the Eastern Caprock Escarpment

30. Hydraulic-head distribution from Simulation H·1 showing effects of local hydrocarbon production in the lower Permian strata 100 yr after production began

TABLES

1. Generalized stratigraphic column of the Palo Duro Basin

2. Permeability of individual hydrogeologic units of Permian, Pennsylvanian, and pre-Pennsylvanian strata, Palo Duro Basin

3. Summary of steady-state simulations of the cross-sectional ground-water flow model

4. Assigned hydraulic-conductivity values for the major hydrologic systems

5. Summary of transient simulations representing the different tectonic and geomorphologic events

APPENDIX FIGURES

A1. Simplified model to test the effect of node-spacing differences in horizontal and vertical directions

A2. Diagram showing the differences in computed hydraulic heads at the corresponding node locations with respect to the 4-element case

B1. One-dimensional finite element model representing a uniform aquitard 800 m thick

B2. Comparison of analytical and numerical solutions for the effect of erosional unloading in an aquitard

APPENDIX TABLE

A1. Performance of the model at different conductance ratios



Citation
Senger, R. K., Fogg, G. E., and Kreitler, C. W., 1987, Effects of Hydrostratigraphy and Basin Development on Hydrodynamics of the Palo Duro Basin, Texas: The University of Texas at Austin, Bureau of Economic Geology, Report of Investigations No. 165, 48 p.