AN INTEGRATED PETROLEUM EVALUATION OF NORTHEASTERN NEVADA
BOTTOMHOLE THERMAL GRADIENTS
The following discussions are by valley or area, and detail the general interpretation of the gradient data for 33 specific locales within the evaluation area.
All six points shown on the Adobe Range temperature/depth plot are from log runs in the Diamond Shamrock Kimbark #1-28. With the exception of the shallow depth point, the gradients are in the lower portion of the Medium classification.
BULL RUN AREA
Located in the vicinity of Township 43 North and Range 52 East, the 2 points on the Bull Run Area temperature/depth plot represent TD and an intermediate point from the Arco K.D. Scott #1. Both points fall in the Medium range with 11.5 deg. F./K.Ft. gradients. These similar gradients show that the baseline of 60 degrees fahrenheit is a reasonable estimate with the assumption of linear increase in temperature with depth. This contrasts with the Deeth and Mary's River areas further to the east. These areas show much higher gradients and suggest a much higher baseline should have been used for the gradient calculation.
The three points that represent Butte Valley on the upper temperature/depth plot are located in the southern portion of the valley and in the Butte Mountains. The Very Low gradient values of 5.3 and 5.4 deg. F./K.Ft. for the well which spudded in surface Permian sediments in the Butte Mountains, suggests a very low heat flow through the Paleozoic rocks in the area. The wells drilled within Butte Valley show Medium to High thermal gradients.
Very few wells have been drilled in the ranges within the evaluation area. Of the few wells drilled in the ranges, the Standard - White Springs Unit #1 in the Butte Mountains and those drilled in the nearby Illipah Anticline area show the same phenomena of low thermal gradients at very deep depths. This suggests that either the addition of Tertiary sedimentary or volcanic section is required to raise the thermal gradient, or that the major normal faults responsible for valley development create heat flow conduits that do not penetrate the ranges.
The four points on the Cave Valley temperature/depth plot range from Low gradients in the southern portion of the valley to Medium gradients at the northern end of the valley. The Cave Valley and White River Valley bottom hole thermal gradients are similar with Cave Valley showing Low and low Medium values and no gradients in the High or Very High classification.
The American Quazar - Adobe Federal #1-19 contributes the 5 points seen on the temperature/depth plot for Coal Valley. The two shallow points with a gradient of 19.6 deg. F./K.Ft. are probably the result of a local shallow thermal anomoly, or may be a result of the 60 degree baseline being too low to accurately calculate this gradient. The value of 7.5 deg. F./K.Ft. at TD is the value more likely to represent the regional gradient for this area and was therefore used in making the Bottonhole Thermal Gradient Overlay.
The lower temperature/depth plot for the Deeth Area shows the effects of elapsed time since circulation. This plot also contains points picked from a temperature log that was run much later in the logging sequence. The points from the temperature log show a nonlinear relationship with depth.
The histogram of the gradients in the Deeth Area shows a distribution in the High to Very High bracket. The maxima is at 23 deg. F./K.Ft. with the majority of the points falling between 20 and 30 deg. F./K.Ft. This area of high heat flow is associated with the thermally complex region surrounding the Ruby Range metamorphic core complex and lies along the Humboldt River Trend which represents a major tectonic boundary.
Most of the points on the lower temperature/depth plot of Diamond Valley are from the temperature log run in the Hunt - W.L. Plaskett #1. In comparing the two graphs, it can be seen that most of the points were measured after 5.0 hours had elapsed since circulation. Even with the longer time of equilibration, most of the points in Diamond Valley represent a lower gradient than those present in adjoining valleys.
The histogram for Diamond Valley shows the maxima to be in the range of 9 or 10 deg. F./K.Ft. The three points that are above 15 deg. F./K.Ft. came from the shallow section of the temperature log. Otherwise, the data fall primarily in the Medium gradient range.
Dixie Valley is typical of many valleys in the evaluation area in that it has had very little in the way of deep exploration. The two points on the temperature/depth plot come from a single well, the Texaco Crane Springs Unit #19-1, and show the gradients to be in the Medium range. These points suggest that the 60 degree baseline may be too high for the area, but without more data no definitive conclusions can be drawn.
The three points on the Elko Area temperature/depth plot are from the Ladd-Nevelko #1. The resulting gradients fall within the Low range and because the points are from a moderate depth of 5680 feet, can be considered representative of the area. Of interest is the location of this well in relation to the location of the nearby Humboldt-Wells lineament. The Elko Area does not show the high gradients observed along this trend to the northeast.
The single point of the upper temperature/depth plot in Goshute Valley shows a High gradient. The lower plot and histogram shows all the points to be in the High gradient range. These points show a systematic increase in temperature with depth as a result of the well bore approaching thermal equilibrium.
Hamlin Valley is the site of the 12,994 foot Commodore Resources well. The Outlaw Federal #1, located in Section 1 of Township 10 North, Range 70 East, shows gradients that are in the Low range both at intermediate depths and at total depth. These Low gradients do not support the oil and gas shows rumored to be present in the Commodore well.
HOT CREEK VALLEY
Hot Creek Valley is the next major valley to the west of southern Railroad Valley. The close proximity of this valley to production in Railroad Valley entices speculation about the similarities between the two. The two wells with thermal data in Hot Creek Valley indicate borderline Medium/High gradients at TD and compare quite favorably with the gradients found in Railroad Valley.
Huntington Valley is penetrated by a number of very deep wells that have teased the industry with significant gas shows but no commercial production. The data points on the lower graph of temperature/depth show the effect of time since circulation as the drilling fluid comes to equilibrium. The highest recorded Study area temperature of 344 deg. F. was recorded in the central portion of Huntington Valley. It was logged at the Wexpro Cord #24-1, 27 hours after mud circulation stopped. Huntington Valley lies immediately to the west of the Ruby Range metamorphic core complex which helps explain the high bottom hole temperatures.
The gradients of Huntington Valley fall in the High and Very High classification. The maxima is 22 deg. F./K.Ft. with a narrow distribution of values from 17 to 25 deg. F./K.Ft. Huntington Valley is at the other thermal extreme from areas such as White River and Cave Valleys; there are no points below the High range.
The Illipah Anticline histogram shows a very diffused collection of gradients. Although data points from various depths in two wells are represented on the temperature/depth plot, the histogram is showing a grouping of gradients by depth rather than by well as is commonly the case.
The three lowest gradients found on the histogram represent TD thermal measurements. The four higher gradients are at intermediate depths. A transition from Low to High gradients occurs from deep to shallow depths in the Illipah area. This transition with depth suggests that a local thermal system may be operating at shallow depths. Less likely is the possibility that the baseline of 60 degrees fahrenheit is much too low to calculate an appropriate gradient.
The Depco Ventrose Unit #42-13, located in Township 34 North and Range 64 East, supplies the only value for Independence Valley of 17.9 deg. F./K.Ft. This value is in the High gradient bracket and fits into the regional picture of High values near the Ruby Range metamorphic core complex to the west and regionally metamorphosed rocks in the northern Pequop Range to the east.
The temperature/depth plot and gradient histogram for Jakes Wash show that the Jakes Wash area has gradients which vary from Medium to Very High. Five of the samples in the Medium classification are multiple readings from one well as it reached thermal equilibrium. For this reason, the maxima at 12 deg. F./K.Ft. is not as significant as depicted by the histogram of all data points.
Kobeh Valley is represented by three temperature points from the Depco - Silver State Fed. #33-18. The three points are TD measurements which were taken over an extended period of time. The lower plot shows the difference in temperature measured between the first reading and the last temperature reading 22.5 hours later. The gradients in Kobeh Valley fall in the lower portion of the Medium range.
Four wells contribute seven points to the temperature/depth plots and histogram for Long Valley. In general, the deeper points have a higher gradient and demonstrate the same phenomena as seen in Railroad Valley with the gradient increasing after the Tertiary section has been penetrated. The gradients in Long Valley are Medium and High. The spread in the gradient range for Long Valley is very similar to Railroad Valley, but is more tentative due to the limited amount of data in Long Valley.
MAGGIE CREEK AREA
The Arco - Rabbit Creek #1 is the only well in the Maggie Creek Area. The gradient is High and decreases with depth in this well. The decrease in gradient is the result of the value chosen for the baseline. Graphically, the straight line nature of the recorded points suggest that the baseline of 60 degrees fahrenheit is about 20 degrees too low for the Maggie Creek Area. Since an 80 degree fahrenheit baseline is above an average annual ambient air temperature, this well appears to have been drilled in a currently active thermal system.
MARYS RIVER AREA
The fourteen data points in the Marys River Area are from the Gulf - Marys River #1 and Hunt Oil - Marys River Fed. #1-8. The plot of temperature/depth shows two entirely different thermal regimes within this area. The lone point from the Gulf well has a Medium gradient of 11.9 deg. F./K.Ft. whereas the Hunt well has Very High gradients ranging from 25 to 350 deg. F./K.Ft. using the 60 degree baseline. The linearly extrapolated surface temperature of the Hunt well is 120 degrees fahrenheit. The Hunt well is the closer of the two wells to the Wells area which also shows High and Very High gradients.
The three thermal points in Monitor Valley are taken from the Depco-Willow Wash well. The shallowest point has a High gradient of 20.2 deg. F./K.Ft., with the deeper points having low Medium gradients of 8.8 and 10.8 deg. F./K.Ft. These gradients correlate with those present in valleys with known hydrocarbon occurrences, but the general lack of data makes a meaningful interpretation difficult for Monitor Valley.
Two wells contribute the two data points for the Newark Valley plot. The two points fall within the Medium and High gradient range. With only two points from two different wells, it is difficult to determine a definitive gradient trend for the area. The two points from Newark Valley show values which certainly lie within the range of thermal gradients responsible for the occurrence of hydrocarbons in Railroad and Pine Valleys.
The two points of the lower temperature/depth plot for the Pilot Creek Area lie on top of each other. The 11.9 deg. F./K.Ft. gradient calculated is in the Medium range and is derived from the Gulf Wilkins Ranch Nos. 1 and 2 in Section 19, Township 39 North, Range 69 East. The repeatability of the data is significant, but is of limited use since the same temperature readings are from nearly the same depth.
The Pine Valley area has been the subject of recent exploration activity by Amoco and Getty, which led to the discovery of the Blackburn Oil Field in 1982. Still, Pine Valley has relatively few well penetrations in relation to the more mature producing areas within Railroad Valley. The lower temperature/depth plot for Pine Valley shows the paucity of temperature points, but does show that there is a very even distribution of points over various depths. This distribution is a function of the exploration objectives and the variability in the thickness of the Tertiary fill within Pine Valley.
The thermal gradient histogram for Pine Valley shows a spread of gradients that primarily fall within the classifications of Medium and High. There is not a clear maxima for these 10 gradients, but the gradient trend is in the High range.
Nearly half the wells drilled in the study area occur in Railroad Valley making it one of the few valleys that has enough sampling to show the variability of rock properties between the Tertiary and Paleozoic rocks. Isolated areas demonstrate elevated temperatures that are related to igneous activity within and along the margins of the valley. This can be seen by comparing the higher gradients on the Bottomhole Thermal Gradient Overlay with nearby geologic outcrops on the Geologic Map. The igneous activity has had a strong influence on the overall thermal regime but is difficult to quantify or seperate from the effects of depth of burial and variable and generally unknown rock conductivities.
In general, the temperature/depth plot shows that the Tertiary fill, commonly 5000-6000 feet thick in Railroad Valley, shows differing gradients than those in the underlying Paleozoic section, probably as a result of differing conductivities.
The gradients in Railroad Valley are dominantly in the Medium and High range with a number of scattered points representing Very High gradients. Various points in the Very High range are associated with wells that have been drilled near thermal springs (Duey, 1985).
REESE RIVER VALLEY
Very few wells have been drilled in the northwest portion of the evaluation area. The Blalock - Voorees #1 in Township 32 North, Range 45 East is the only well available in Reese River Valley, and falls within a Very High gradient classification with a gradient of 32.3 deg. F./K.Ft. The Very High gradient is related to the Battle Mountain Heat Flow High (Blackwell, 1983). The Reese River Valley area lies within the structural and thermal influence of the Late Miocene Northern Nevada Rift.
The five data points on the temperature/depth plot for Ruby Valley are from various depths in the Pan American - Franklin #1 and represent temperatures taken at various times since circulation. These points graph as a nearly linear function and show a borderline classification of Medium to High thermal gradient. The relatively low gradients are surprising considering their proximity to the Ruby Range metamorphic core complex and the 13,100 foot depth of two data points.
Although the Deeth, Huntington Valley, and Wells Area to the north and west of the Ruby Range have High to Very High thermal gradients, areas to the southeast such as Ruby and Butte Valleys show Medium and marginally High gradients. This thermal variation from southeast to northwest across the Ruby Range must somehow reflect the geometry of the thermal halo surrounding this core complex and the thermal effect of the Humboldt-Wells lineament.
The temperature data for Spring Valley consists of two points which were derived from two wells. The northern well, the Dome - Bastian Creek #1 shows a High gradient of 20.6 deg. F/K.Ft. whereas the southern Amerada - Fed. #1-20 well gives a Low gradient of 6.6 deg. F/K.Ft.
Both wells were drilled within the low-angle normal fault complexes in the Schell Creek and Snake Ranges and penetrate nearly the same thickness of Tertiary fill. The Dome well appears to be in the lower plate of Cambrian and Precambrian rocks while the Amerada well appears to be within a relatively intact upper plate. The structural attenuation of several thousand feet of section between the Dome and Amerada wells probably resulted in a higher heat flow in the area of the Dome well. This may explain the High gradients in the north and Low gradients to the south in Spring Valley.
There is a wide distribution of data points in Steptoe Valley which span about 130 miles in a north-south direction. The thermal gradient histogram dramatically demonstrates the spread of thermal gradients in Steptoe Valley. The gradients appear to be generally related to the intensity of structural deformation and to a lesser degree the relative thickness of valley fill.
In general, those wells that are in close association with the lower plate of low-angle normal faults in the northern Egan-southern Cherry Creek, and northern Schell Creek Ranges show High to Very High gradients. These include the Dome Grassy Springs #1, Gulf Nevada Federal #A-1, Dome Duck Creek #1, L.L. and E. USA Steptoe Valley #1, Placid Steptoe Federal #17-14, and the Shell Steptoe #1 wells.
In the northern and less structurally complicated portion of the valley near the Goshute and southern Pequop Mountains, the gradients fall in the Medium classification. This includes the Shell Goshute #1 and the Gulf Dolly Varden #1 wells.
The Gulf - Nevada Fed. #2 at the southern end of the valley penetrated a relatively thin Tertiary valley fill of about 2100 feet and is in a relatively intact upper plate structural position. The well shows a low gradient of 7.6 deg F./K.Ft.
TOANO DRAW AREA
The Toano Draw area, in the vicinity of Township 39 North and Range 66E, shows High to Very High gradients from the Sun Southern Pacific #1 and Sun Toana wells. The temperature/depth plot shows the results of a temperature log run and includes multiple picks for the same TD at various times. The thermal gradient histogram shows that the deeper data cluster in the Medium and High gradient range and the shallow points range Very High to "off scale."
The Toano Draw Area appears to be an active geothermal area similar to the nearby Wells Area and is also thermally related to the Humboldt-Wells lineament.
The Wells Area thermal gradient histogram shows a bimodal distribution of gradients. The High gradient data is from the Gulf - Wilkins Ranch #1 and the Shell - Marys River Fed. #1. These wells are three miles apart and have a 1400 foot difference in TD, and still show the same relative gradients with time of equilibrium. The Shell well gradients for the shallow data points agree quite closely with those calculated for the deeper points. This agreement indicates that the baseline of 60 degrees is valid for this local area.
The Very High gradients are from the temperature log and other log runs in the Atlantic - Vernon Dalton #1. This well lies within the Humboldt-Wells lineament and is just north of the Ruby Range and Wood Hills regional metamorphic terrane.
WHITE HORSE FLATS
The White Horse Flats area is located in the vicinity of Township 26 North and Range 70 East. The gradients in the area are in the Medium to High range but show a trend on the temperature/depth plot that suggests a higher baseline temperature may be more valid. The deeper points yield identical gradients however, which argues against a higher baseline and suggests that shallow thermal conditions may be influencing the gradient results.
WHITE RIVER VALLEY
White River Valley has been actively drilled for many years because of the similarity of lithologies, areal extent of the valley and proximity to Railroad Valley. The temperature/depth plots for White River Valley show a linear clustering of points. The trend is quite remarkable considering the total data plot represents 20 wells from Townships 6 North to 13 North, or 42 miles between the northernmost and southernmost wells.
The gradient data in White River Valley show a very even distribution throughout the Low and Medium values and vary from Low to low High gradients with no clear maxima. By analogy with Railroad and Pine Valleys, White River Valley does not show the Medium to High and local Very High gradients that seem to be associated with known hydrocarbon accumulations.