WATER BUDGET ANALYSIS
The water budget analysis conducted on the North Fork of Horse Creek was an accounting of all inflow to and outflow from the reach which flows across outcrops of the Casper Formation. The water budget can be summarized as
Qin + P + GWin = Qout + E + ET + GWout (1) where: Qin and Qout are stream flow into and out of the study reach, respectively, P is the water gained by the study reach due to precipitation, E and ET are water lost by evaporation from the stream and from evapotranspiration from the phreatophyte zone, respectively; GWin and GWout are discharge from the aquifer to the stream and recharge to the aquifer form the stream, respectively.For the purposes of this study, GWin and GWout were combined into a single term, net GW flux, for which equation (1) was then solved:
net GW flux = Qin + P - Qout - E - ET. (2)Net groundwater flux is positive if the aquifer is being recharged. The net groundwater flux was computed for each month based on estimates for the five remaining variables: Qin, Qout, P, E, ET.
Approximations of Qin and Qout were based on stream flow data collected from the east and west stream gauging stations, respectively. These two stations were established at each end of the study reach as shown on Figure 3. Continuous records of stream stage were obtained with a U.S. Geological Survey type bubble gage servo-manometer. The average stage for each day was estimated from these records.
Standard staff gages and Parshall flumes were used to obtain weekly paired observations of stage and discharge at the east and west stream gaging sites. These paired observations were used to develop a log-log regression between stage and discharge for each site. The regression was then used to convert daily average stage to daily average discharge. The total volume of stream flow which passed each gaging station was then computed on a daily basis and summed to obtain the monthly totals of inflow to and outflow from the study reach.
Figure 3. Location of stream gauging stations on the North Fork of Horse Creek, Laramie County, Wyoming.
P was estimated by applying the local monthly accumulation of precipitation over the surface area of the stream. Precipitation was measured by a Belfort, weighing-bucket type recording gage which was located on a hill near the east stream gaging station. The gage had an alter type windshield around the collector orifice.
Contributions to the gauged reach resulting from runoff associated with precipitation events were not considered for two reasons. First, no evidence of overland flow entering the stream was observed even during large precipitation or snowmelt events. Second, it was believed that any significant runoff reaching the phreatophyte zone would infiltrate and be accounted for by the groundwater flux term.
Estimates of E and ET rates for the North Platte River drainage basin were obtained from Lewis (1978) and Van Klaveren (1975), respectively, for each month. Volumes were computed by applying the evaporation rate over the surface area of the stream and by applying the evapotranspiration rate over the area of the phreatophyte zone along the stream. The rate of evapotranspiration is zero for the non growing season months of October through April.
In order to understand where the recharge and discharge is occurring and to further isolate the part of the gauged reach which flows over the limestones of the Casper Fm., the study reach was divided into three segments by two three-inch Parshall flumes. Stream flow losses for each of the three segments were calculated by subtracting the weekly flow rate at the downstream flume from the flow rate at the upstream flume. Precipitation, evaporation and evapotranspiration were considered negligible for this analysis.
Gauging of additional streams in the region, to verify that the results of the North Fork of Horse Creek study are applicable to all streams along the east flank of the Laramie Range, was considered unnecessary because these streams flow across outcrops of the Casper Formation which are hydraulically severed from the rest of the basin by thrust faults. These faults are impermeable boundaries which effectively prohibit any recharge occurring along these streams from reaching the basin interior. The two streams which do flow through parts of the recharge area which may be in hydraulic communication with the rest of the aquifer are the North Fork of Horse Creek and Mill Creek. The North Fork of Horse Creek was gaged in detail for the water budget analysis. Mill Creek, which is the next stream to the south of the North Fork of Horse Creek, flows over Paleozoic rocks which were heavily disrupted by the limestone mine on the adjacent hogback. Mining practices have altered the hydrologic characteristics of these rocks to such an extent as to render the results of any gauging of this stream unique to this one circumstance and therefore, of limited interest to this study.
EXAMINATION OF THE GEOLOGIC FRAMEWORK
The geologic framework through which recharge must flow was examined by identifying any variations in permeability which could impact groundwater circulation patterns. These features included tectonic structures which deform the aquifer and hydrologically isolated compartments which are isolated within the aquifer by zones of small permeability.
TECTONIC MAPS. Tectonic structures were identified on two different scales. A structure contour map of the Muddy Sandstone was prepared on a scale of 1:125,000 and is presented on Plate I. Tectonic structures along the east flank of the Laramie Range, where the Paleozoic and Mesozoic rocks crop out, were mapped on a 1:24,000 scale and are presented on Plates II, III and IV. Lithologic descriptions of the geologic units present in this area are listed in Table 1.
Table 1. Lithologic Descriptions of the Geologic Units Present Along the East Flank of the Laramie Range, Laramie County, Wyoming, (from Grey, 1947).
GEOLOGIC AGE NAME OF UNIT DESCRIPTION OF UNIT Quaternary Quaternary Floodplain alluvial Alluvium deposite. UNCONFORMITY Oligocene White River Brule Fm.: Group tough sandy clay, 200 ft. Chadron Fm.: Interbedded red and green sandy clay, with arkosic gravel and light brown, poorly cemented, arkosic conglomerates, 20 to 200 ft. UNCONFORMITY Cretaceous Fox Hills light brown to grey sandstone, Fm. with tan and dark grey shales, 360 ft. Pierre Fm. Succession of shales and sandstones, 3000 ft. Niobrara Fm. Calcareous shales and sandstones, 420 ft. Frontier Fm. Black sandy shales, with some sandstones, 165 ft. Mowry Shale Black siliceous shales which weathers to silver-grey, 150 ft. Thermopolis Upper: dark ferruginous Fm. shale, 50 to 60 ft. Muddy Sandstone: siliceous sandstone, 50 to 75 ft. Lower: black shale, 100 ft. Cloverly Fall River Sandstone, 25 ft. Group Fuson Shale, 50 ft. Lakota Sandstone, 27' ft. UNCONFORMITY Jurassic Morrison Fm. Variegated shales, 200 ft. Sundance Fm. Grey to buff, fine to medium grained sandstone, with orange poorly indurated sandstones at the base, 100 to 165 ft. UNCONFORMITY Triassic Chugwater Fm. Red shales and sandy shales Permian with, two thin limestones at the base, 600-700 ft. Opeche- Minnekahta Limestone: Minnekahta Pink to purple interbedded Succession limestones and siltstones, 22 ft. Opeche Shale: Red shales and sandstones, 89 ft. Pennsylvanian Fountain- Casper Fm.: Casper Upper: red shales and sandstones, 400 ft. Middle: interbedded shales limestones and sandstones, 660 ft. Lower: red, coarse-grained arkosic sandstones, 200 ft. Fountain Fm.: red coarse grained, arkosic sandstones and conglomerates, 30 ft. UNCONFORMITY Precambrian Sherman Pink, coarse grained, arkosic Granite granite.A structure contour map of the Muddy Sandstone, Plate I, was made by contouring depth-to-formation data obtained from library files which are open to the public at the Wyoming Oil and Gas Commission and from Petroleum Information Cards at the Wyoming Geological Survey. Depth-to-formation data were based on well logs including both mud logs and geophysical logs obtained during oil and gas exploration. Among the geophysical logs used were conductivity, resistivity, spontaneous potential and gamma ray logs. This map was then used to identify tectonic structures which could potentially enhance or inhibit groundwater flow in the basin. Depth to formation data is listed in Appendix B and the spacial distribution of these data points is shown on Plate V.
Tectonic structures which deform the Paleozoic and Mesozoic rocks along the east flank of the Laramie Range were identified and mapped using previously published maps by Gray (1946) and Brady (1949), stereo aerial photographs, and field observations, where access was permitted. The purpose of these maps was to identify any tectonic features which impede or enhance the flow of groundwater from the recharge area to the basin interior.
It was not possible to map the structure of the basin interior in as much detail as was possible along the east flank of the Laramie Range. Paleozoic rocks in the basin interior and the structures which deform them are unconformably buried by the Oligocene White River Group.
Use of the Muddy Sandstone. Although the Muddy Sandstone is not part of the Paleozoic aquifer, it was necessary to use it as the source of data for much of the analysis done for this thesis because there is virtually no data available for the Paleozoic aquifer in this area.
The Muddy Sandstone was chosen because the circulation patterns which describe groundwater flow through the Muddy Sandstone closely mimic the patterns which describe groundwater flow through the Paleozoic aquifer. This occurs because the basic geologic framework for both formations is the same including common basin boundaries, common tectonic deformation of the strata and common overall basin geometry. Copeland (1984), Gray (1946), Brady (1949) and I have shown through our mapping that all of the stratigraphic units from the Pennsylvanian Fountain Formation to the Late Cretaceous Fox Hills Formation are involved in the same major geologic structures. There are no angular unconformities in the stratigraphic sequence from the Pennsylvanian Fountain Formation to the Late-Cretaceous Fox Hills Formation that would indicate any deformation of the Paleozoic aquifer which did not also involve the Muddy Sandstone.
The premise that the groundwater circulation patterns will be the same in the Paleozoic aquifer as they are in the Muddy Sandstone, because the geologic framework is the same, is substantiated in work done by Belitz (1985). This work shows structure contour and potentiometric maps for several stratigraphic levels from pre-Cambrian to late Cretaceous in the Denver-Julesburg Basin. Comparison of these maps shows that the Middle Cretaceous units, including the Muddy Sandstone, and the Paleozoic units, including the Casper Formation have the same geologic framework and the same groundwater circulation patterns.
Unlike data for the Paleozoic aquifer, data for the Muddy Sandstone is readily available because, locally, it is .a major target for petroleum exploration and development. In fact, there are two fields in the study area which are currently producing from the Muddy: the Horse Creek and the Borie fields.
Hydraulically Isolated Compartments. Hydrologically isolated compartments within the aquifer were identified by locating zones of anomalous fluid pressure within the basin. A zone has anomalous fluid pressure if the fluid level, or hydraulic head, in a well completed in that zone is not within a few hundred feet of the land surface (Belitz, 1985). Zones in which the hydraulic head is significantly below the land surface, that is, not within a few hundred feet, are under pressured. Zones in which the hydraulic head is significantly above the land surface are over pressured.
Zones of anomalous fluid pressure were located by plotting the greatest recorded shut-in pressure against depth of measurement for all of the drill stem test (DST) data available for the area. This plot is presented as Figure 4 and the DST data is listed in Appendix A. DST data was obtained from library files at the Wyoming Oil and Gas Commission and from Petroleum Information Cards at the Wyoming Geological Survey.
Figure 4. Pressure as a Function of Depth Below the Land Surface
as Measured in Drill-stem Tests, Laramie County, Wyoming.
There is a linear increase of pressure with depth in hydraulically connected zones of an aquifer that follows the equation
P = gd, (3) where: P = pressure, = density of formation fluids g = gravitational acceleration, and d = depth below the water in the saturated zone.This relationship appears as a line on pressure-depth plots such as Figure 4, where the slope depends on the density of the fluid. Slopes are steeper for less dense fluids and, gentler for more dense fluids.
This line is called the normal pressure line for a fluid of a given density. The line shown on Figure 4 is the normal pressure line for fresh water. Abnormally pressured parts of the basin produce data points which plot significantly to the left (underpressured) or to the right (overpressured) of this line. Data from several depths within a given abnormally pressured zone produce clusters of points which fall on a line that lies roughly parallel to the normal pressure line
Hydraulic head is expressed in the Bernoulli equation as
h = P/ g + z, (4) where: h = hydraulic head, g = gravitational acceleration = density of formation fluids z = elevation of the point of measurement. If equation (3) is substituted into equation (4), h = d + z. (5)It follows that h is a constant for normally pressured parts of a basin which is fully saturated, and which has a reasonably flat land surface. Similarly, the heads within an abnormally pressured zone will also be a constant, but that value will be greater than (overpressured) or less than (underpressured) the value obtained for the normally pressured parts of the basin. Obviously, under- and overpressuring implies that the zone in question is not in good hydraulic connection with the normally pressured parts of the basin.
The potentiometric surface was mapped by contouring hydraulic head data obtained from drill stem tests preformed by the petroleum industry throughout the basin. DST's are transient formation pressure tests which are used by the petroleum industry to evaluate the production potential of a specific stratigraphic interval (Jarvis, 1986). The test is performed by isolating a specific stratigraphic interval and allowing the fluids in that interval to flow into the well and then allowing pressure to build up. The changes in pressure are recorded for two or four alternating periods during which the well is either shut-in or open. The shut in periods are intended to allow the measured pressure to equilibrate with formation pressure as closely as possible (Bair and others, 1985).
Complete DST records include a continuous record of the fluid pressure changes during the entire test, the volume of fluid recovered during the shut-in periods, a chemical and thermal analysis of the fluids recovered, the reference elevation, and the gauge depth (Jarvis, 1986). The complete pressure record can be used to extrapolate the undisturbed formation pressure as demonstrated by Bredehoeft (1965). These calculations involve a curve matching technique for radial fluid flow to a producing well which was adopted from Theis (1935).
Complete DST records were not readily available for the project area because they are proprietary. Incomplete DST records are, however, routinely filed with state agencies such as the Wyoming Oil and Gas Commission. These incomplete records were used to compute the hydraulic head values listed in Appendix A and used to map the potentiometric surface on Plate I. The spatial distribution of these data points is shown on Plate V. The incomplete DST records most often included the reference elevation, the interval tested, the gauge depth, the volume and type of fluid recovered during the shut-in periods, discrete measurements of hydrostatic and shut-in pressures, and the length of time which elapsed during each shut in and flow period.
Numerous hydrogeologic studies have made use of incomplete DST data including: Miller (1976) in the Madison Group in Montana; Bair and others (1985) in the Palo Duro Basin of Texas and New Mexico; and Jarvis (1986) and Doremus (1986) in the Big Horn Basin of Wyoming.
Murphy (1965) developed the following equation to compute hydraulic head from DST data:
PE = RE - GD + (2.319 * Ps) (6) where: PE = elevation of the potentiometric surface; RE = reference elevation, usually derrick floor, rotary bushing, or ground level; GD = gauge depth, as measured from RE; Ps = extrapolated static pressure, highest shut-in pressure is often substituted for this value; 2.319 = constant for converting pounds per square inch to feet of head.The assumptions built into this equation include: (1) the density of formation fluids is equal to that of fresh water, (2) the temperature of the fluid is approximately 35 degrees celsius, and (3) the shut-in period which is used to measure shut-in pressure (SIP) is long enough to closely approximate the stabilized formation pressure.
Extrapolated static pressure refers to approximation of the undisturbed formation pressure made from continuous pressure data recorded during a DST (Bredehoeft 1965). For this thesis, the greatest shut-in pressure reported in the incomplete DST record was used instead of the extrapolated static pressure because of the lack of complete DST data. Bair and others (1985) preformed an analysis of the differences which result from substituting greatest SIP for extrapolated formation pressure and concluded that computations using these two values should not be mapped together, because the use of SIP resulted in significantly lower head values than did the use of the extrapolated formation pressure. Bair and others further concluded that the consistency of this error allowed for reasonable accuracy in a potentiometric map which was constructed from SIP's exclusively. The value of the greater number of data points available if the SIP's are used was considered to outweigh the value of greater accuracy for a few points.
Inconsistencies in the quality of data reported by incomplete DST records prompted Jarvis (1986) to develop a data quality ranking system. This system ranked each calculated hydraulic head according to the number of data quality criteria met by the DST record. The data quality criteria include (1) 10%, or less, difference between the hydrostatic pressures measured at the beginning and at the end of the test, (2) 25%, or less, difference between the shut-in pressures measured at the beginning and at the end of the test, (3) one of the shut-in periods lasted 30 minutes or longer, and (4) two of the shut-in periods lasted 30 minutes or longer.
The hydraulic head data computed for this thesis was ranked accordng to this system and the potentiometric surface (Plate I) was then mapped by contouring all of the hydraulic head data, giving weight to the head values with higher data quality rank. The resulting potentiometric map was overlaid onto the structure contour map so that the direction of groundwater flow could be deduced assuming increases or decreases in permeability parallel to the strike of folds and faults.
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