The following discussion is a formation by formation summary of the data presented in these appendices, and in part displayed in contoured form as Overlays V through XII. The reader is encouraged to refer to these appendices and overlays while using this section. It may also be helpful to refer to the Stratigraphy, while considering the geochemistry of an individual formation.

In general, the best oil-generating capability appears to be in the Elko and Woodruff Formations and in specific organic-rich facies of the Chainman, Diamond Peak, Pilot and Vinini Formations. The organic-lean portions of these formations most probably contain low hydrogen organic matter and gas-only generating kerogen, or have been reduced by severe subaerial weathering and oxidation. Significant percentages of terrestrial organic matter were observed only in a limited number of samples of the Chainman, Pilot and Webb Formations. Thermal maturity varies substantially within these formations, with most samples showing mature to locally overmature surface maturity. Limited and specific areas with rocks showing the proper thermal maturation and organic content to be considered hydrocarbon source rocks, have been determined during this evaluation. The results are discussed for each formation below.

Overlay IV shows the distribution of the 6 outcrop samples of the lacustrine Elko Formation which were taken near Elko, and in the Adobe and Carlin-Pinon Ranges. Laminated, calcareous, tuffaceous, and organic shales within the Elko Formation were selected for geochemical analysis.

Total organic carbon within the Elko Formation ranges from 0.06 percent in a sample of water lain tuff, to 25.09 percent in the oil shale member of the formation. TOC averages 3.37 percent for the entire data set. The dominant kerogen in the organic-rich Elko Formation is amorphous, Type II aquatic material, with minor amounts of spore and pollen related exinite or Type I kerogen. S₂ values range from 15.93 to 190.81 mg/g suggesting good source potential. Hydrogen indices are 477 or greater indicating oil-prone source beds.

TAI values vary from 1+ to 2, indicating that surface exposures of the Elko Formation are dominantly thermally immature but are locally mature. T-max values range from 433 to 441 degrees centigrade suggesting the Elko Formation is within the low range of oil generation.

These parameters suggest that as a unit the Elko Formation is a good potential source rock which will generate oil, perhaps a high-wax oil, where thermally mature. Overlays V (TOC) and VI (TAI) show that the Elko Formation is capable of hydrocarbon generation only in northern Pine Valley, the Suzie Creek drainage, Dixie Flats, western Lamoille Valley, and in the Elko East and West Quadrangles.

Overlay IV shows the distribution of the 7 outcrop samples of the lacustrine Sheep Pass Formation which were taken in the Grant, Egan and Schell Creek Ranges. Fetid, thinly bedded and massive, ostracodal, silty and sandy limestones within the Sheep Pass Formation were selected for geochemical analysis. No mudstones or shales were found at the type section or any of the other Sheep Pass exposures examined by Western Cordillera geologists.

Total organic carbon ranges from 0.03 to 0.22 percent in these limestones, and averages 0.07 percent for the entire data set. Of the 7 samples analyzed, only sample 278-SP contained enough organic material to perform kerogen typing and pyrolysis. The dominant kerogen type in sample 278-SP is amorphous Type II aquatic material with a possible minor bitumen fraction. A TAI of 3- was determined for this sample. The very low organic content of the Sheep Pass Formation samples coupled with the low S₂ and hydrogen indices suggest that the Sheep Pass limestones are not viable source rocks.

The Sheep Pass Formation however has been considered the most probable source for the Eagle Springs oil field in Railroad Valley. Poole and Claypool (1984) report organic-rich Sheep Pass calcareous mudstones, siltstones, and limestones from cores in the Shell Eagle Springs Unit No. 1-35 in the Eagle Springs Field. TOC ranges from 0.43 to 7.40 percent with an average of 1.66 percent for the 7 core samples analyzed. R_o data ranges from 0.44 to 1.35 and T-Max values are from 418 to 435 degrees centigrade (Poole and Claypool, 1984). These parameters suggest immature to mature thermal conditions. An acceptable gas chromatographic correlation can be drawn between Sheep Pass rock extracts from cores of the Shell Eagle Springs Unit No. 1-35, and oil from the Eagle Springs Draycutt No. 45-36 well (Poole and Claypool, 1984).

Our observations of the Sheep Pass Formation in the Grant, Egan and Schell Creek Ranges suggest that the organic-rich facies in the Eagle Springs Field is areally restricted, and may be present only in the subsurface of Railroad Valley. We did not find correlative facies or source beds in the surrounding surface exposures.

Triassic sediments including the shallow marine, Lower Triassic Dinwoody and Thaynes Formations, and various unnamed Triassic lithologies were sampled in the southern Pequop Mountains, Medicine, East Humboldt, and Adobe Ranges, the Wildhorse area, Windermere Hills, Leach Mountains, and the northeastern corner of Elko County. Laminated, platy to massive mudstone, silty and calcareous shale, siltstone, and silty or shaley limestone were selected for geochemical analysis. The 20 samples analyzed are shown on Overlay IV.

Total organic carbon in these Triassic Sediments ranges from 0.07 percent in siltstones to 1.95 percent in a siliceous mudstone. TOC averages 0.30 percent for the entire data set. The dominant kerogen types are amorphous, Type II aquatic material, and solid bitumen. S₂ values are all below 0.67 mg/g and hydrogen indices are less than 41. TAI values cluster around a value of 3, and T-max determinations indicate 465 to 470 degrees centigrade.

These geochemical parameters, in part displayed on Overlays V (TOC) and VI (TAI/Ro), suggest that although the Triassic sediments are thermally mature, they are generally organic lean with only minor amounts of gas prone kerogen, and are not capable of hydrocarbon generation.

Overlay IV shows the distribution of 7 outcrop samples of the Diamond Peak Formation taken in the HD, Pancake and Adobe Ranges, and in the Diamond Mountains. Platy to blocky, silty mudstone, and silty shale to shaley siltstone were selected for analysis and represent the most attractive facies observed within the Diamond Peak Formation. Most of the unit is composed of quartzite and sandstone or conglomerate. The Diamond Peak Formation has been interpreted with the Chainman Formation on these overlays since they represent an interfingering depositional sequence.

Total organic carbon in the Diamond Peak Formation ranges from 0.28 percent in a shaley siltstone to 1.55 percent in a silty shale. The entire unit averages 0.72 percent TOC. The kerogen is dominated by amorphous Type II kerogen, but also contains varying portions of vitrinite, bitumen, inertinite, and exinite in order of decreasing abundance. This indicates a mix of marine oil-prone and terrestrial gas-prone kerogens within these deltaic sediments. Very low S₂ and hydrogen index values in the Diamond Peak are probably the result of both weathering and thermal maturation rather than an indication of poor source potential.

Ro values range from 0.75 to a questionable 1.99, and TAI values are from 2 to 4 indicating mature surface samples. T-max varies from 438 to 444 degrees centigrade suggesting the samples are at the lower threshold of oil generation.

Overlays VII (TOC) and VIII (TAI/R_o) indicate that the Diamond Peak is a source rock capable of generating hydrocarbons along the eastern flank of Smoky Valley, within the North Fork of the Humboldt River drainage, and in the HD Range area.

Overlay IV shows the distribution of the 102 outcrop samples of the Chainman Formation which were taken throughout the evaluation area. Lithologies sampled include finely laminated or massive shale, silty or siliceous shale, fissile to blocky silty mudstone, calcareous and quartzose siltstone, shaley siltstone, and coal.

Total organic carbon in the Chainman Formation ranges from 0.05 percent in a silicified silty mudstone to 81.60 percent in a coal seam. On average the Chainman Formation is organic-rich with about 1.34 percent TOC. The majority of the kerogen in the Chainman is amorphous Type II kerogen with varying proportions of bitumen, vitrinite, intertinite, and exinite in order of decreasing abundance. This indicates a mix of marine oil-prone and terrestrial gas-prone kerogens within the deltaic Chainman Formation. Neglecting the coal sample, S₂ yields range from 0 to 22.4 mg/g and hydrogen indices vary from 0 to 312. Both pyrolysis parameters have probably been affected by surface weathering and thermal maturation.

TAI values vary significantly from a questionable 2- to a 5, and R_o values range from a questionable 0.5 to a 3.95. T-max values are between 431 to 520 degrees centigrade. These maturation indices show that surface exposures of the Chainman Formation in the evaluation area are locally immature to overmature, and are dominantly thermally mature.

We feel that the geochemical data show that the Chainman Formation is a good to excellent source rock in restricted areas. The relationship of the Chainman Formation to oil fields in Pine and Railroad Valleys is discussed in a later section on Gas Chromatographic Analysis.

Overlays VII (TOC) and VIII (TAI/R_o) show that the Chainman Formation is capable of generation hydrocarbons only in the following areas: Lake Valley, southernmost Spring Valley, northern Dry Lake Valley, the east flank of central Cave Valley, Coal Valley, northern Garden Valley, Railroad Valley, northern Big Sand Springs Valley, northern Little Smoky Valley, southern Newark Valley, northern Jakes Valley, Long Valley, Butte Valley, the western flank of central Steptoe Valley, southern Ruby Valley, Dixie Flats-Huntington Valley, northernmost Diamond Valley, northern Pine Valley, the Elko West Quadrangle, and along the North Fork of the Humboldt River drainage.

The Eleana Formation is a southern facies equivalent of the Chainman and Diamond Peak Formations. Overlay IV shows the distribution of the 4 samples of the silty shale, quartzite, and argillite of the Eleana selected for geochemical analysis. These samples represent the best potential source facies observed by Western Cordillera geologists within the Eleana Formation.

Total organic carbon within the Eleana Formation ranges from 0.49 percent in a silty shale to 1.16 percent in a thinly bedded shale. The entire data set averages 0.53 percent TOC. The dominant kerogen type in the Eleana is amorphous Type II kerogen with significant percentages of bitumen and Type III vitrinite. This once again indicates a mix of marine oil-prone and terrestrial gas-prone kerogen sources. The pyrolysis yielded negligible S1 and S2 in the Eleana, probably as a result of weathering and thermal alteration, making the computation of further pyrolysis parameters meaningless in this case. TAI varies from 2 to 3+, and questionable R_o values range from 1.05 to 1.19 indicating that thermal conditions for surface exposures of the Eleana Formation are within the hydrocarbon generation window.

Overlay VII (TOC) and VIII (TAI/R_o) indicate that the Eleana Formation may be capable of generating hydrocarbons in the northern portion of Hot Creek Valley and eastern Little Fish Lake Valley, in the western portion of the evaluation area.

The locations of the 7 samples for the Webb Formation are shown on Overlay IV. These silicified shales, silty and silicified mudstones, and blocky to laminated siltstones were taken in the Carlin area, and in the southern portion of the Fish Creek Range.

Total organic carbon in the Webb Formation ranges from 0.06 percent in a silty mudstone to 0.72 percent in a silicified shale. The Webb Formation contains an average of 0.37 percent TOC. The dominant kerogen type in the Webb Formation is amorphous with significant proportions of bitumen and vitrinite and small amounts of inertinite, suggesting mixed marine and terrestrial kerogen sources. S₂ values are below 0.64 mg/g and corresponding hydrogen indices range from 0 to 103 probably as a result of surface weathering rather than representing a poor, gas prone, source.

TAI values vary from 2 to 3+, and Ro values range from 0.54 to 0.94. Two T-max determinations give a value of 431 degrees centigrade. These maturation data suggest that the surface exposures of the Webb Formation are thermally mature.

Overlays VII (TOC) and VIII (TAI/R_o) indicate that the areally restricted Webb Formation is capable of generating hydrocarbons only in very small areas near Carlin, along the western margin of Smoky Valley, and near the western flank of the southern Fish Creek Range.

Overlay IV shows the distribution of the 46 outcrop samples of the Pilot Formation taken across the eastern portion of the evaluation area. Calcareous siltstone, calcareous and silty shale, siliceous shale, limestone and silty limestone were selected for geochemical analysis.

Total organic carbon in the Pilot Formation ranges from 0.04 percent in a shale to 3.49 percent in a finely laminated silty and calcareous shale. The Pilot Formation is organic-rich with an average TOC of 0.99 percent. The kerogen is nearly equally divided between amorphous Type II kerogen and bitumen, with minor amounts of vitrinite suggesting a dominant marine oil-prone kerogen source. S₂ values are all below 1.84 mg/g and hydrogen indices are less than 125, commonly below 70. Both pyrolysis parameters appear to have been affected by surface weathering and thermal maturation.

TAI values in the Pilot Formation vary significantly from 2 to 5, and questionable R_o values range from 0.49 to 1.43. T-max determinations are 432 to 497 degrees centigrade. These maturation parameters show that surface exposures of the Pilot Formation are thermally mature to overmature in the evaluation area.

Overlays IX (TOC) and X (TAI/R_o) indicate that the Pilot Formation is capable of generating hydrocarbons only in the following areas: northern Newark Valley, northern Long Valley, southern Butte Valley, northern Jakes Valley, central Steptoe Valley, Spring Valley, southern Antelope Valley, Cave Valley, northern Lake Valley, and White River Valley.

The locations of the 8 samples of the Woodruff Formation taken in the Carlin-Pinon and southern Fish Creek Ranges are shown on Overlay IV. Shaley siltstones, shales, and cherty or silty mudstones were selected for geochemical analysis.

Total organic carbon in the Woodruff Formation varies from 1.12 percent in a shaley siltstone to 6.05 percent in a dense silty mudstone. The Woodruff Formation is quite organic-rich with an average TOC of 2.55 percent. The kerogen is nearly equally divided between amorphous Type II kerogen and bitumen with minor amounts of vitrinite, inertinite and exinite. S₂ ranges from 0.04 to 22.68 mg/g and the hydrogen index varies from 4 to 375. Although affected by the surface weathering problem, the Woodruff Formation appears to have a good generation potential from an organic and pyrolitic standpoint.

TAI values vary from 2 to 3+ and R_o values range from 0.52 to a questionable 1.23. T-max determinations are from 415 to 450 degrees centigrade. These thermal indices indicate that the surface exposures of the Woodruff Formation are thermally mature.

Overlays IX (TOC) and X (TAI/ R_o) show that the depositionally restricted Woodruff Formation is capable of generating hydrocarbons only in the central western portion of Little Smoky Valley, and in the northern portion of Pine Valley.

Overlay IV shows the distribution of the 17 samples of the Roberts Mountains Formation taken in the northern Toquima, Monitor, and Simpson Park Ranges, and the southern Cortez, Tuscarora, Independence and northwestern Snake Mountains. Calcareous and silty mudstone and shale, laminated silty and shaley limestone, and calcareous siltstone were selected for analysis.

Total organic carbon in the Roberts Mountains Formation varies from 0.05 percent in a recrystallized platy laminated limestone to 5.67 percent in a platy, calcareous and silty shale. The average TOC in the Roberts Mountains Formation is 0.66 percent. The kerogen is dominated by amorphous Type II kerogen and bitumen suggesting a marine or algal kerogen source. Essentially no S₁ or S₂ could be derived from any of the surface samples of the Roberts Mountains Formation making other pyrolysis parameters suspect in this case. Although thermal maturation and surface weathering have affected these samples, the uniform lack of a pyrolysiable fraction is probably an inherent characteristic of the Roberts Mountains Formation.

TAI values vary from 2+/3- to 5 and R_o range from a questionable 1.16 to 4.28 indicating the surface exposures of the Roberts Mountains Formation are thermally overmature to mature. Overlays X (TOC) and XI (TAI/R_o) show that although the Roberts Mountains Formation is locally organic-rich, it is thermally overmature and does not have the source potential necessary to generate hydrocarbons.

Overlay IV shows the distribution of the 47 samples of the Vinini Formation taken across the northern portion of the evaluation area. Lithologies sampled in the Vinini include argillites, calcareous and silty shales, silicified shale, mudstone, siltstone, and thinly bedded chert and shaley chert.

Total organic carbon in the Vinini Formation ranges from 0.04 percent in silty shale to 4.61 percent in silty mudstone. The Vinini Formation facies sampled are organic-rich with an average TOC of 0.94 percent. The kerogen is dominated by amorphous Type II kerogen and bitumen with minor amounts of inertinite indicating a marine kerogen source. S₂ values range from 0 to 8.84 mg/g and hydrogen indices vary from 0 to 192, with higher values correlating with samples which show lower thermal maturities.

TAI values vary significantly from 2 to 5, with questionable R_o values from 0.62 to 3.54. T-max determinations are from 416 to 478 degrees centigrade. These data show that surface exposures of the Vinini Formation are thermally mature to overmature.

Overlays XI (TOC) and XII (TAI/ R_o) indicate that the Vinini Formation is capable of generating hydrocarbons only in the following areas: northern Monitor Valley, western Kobeh Valley, west-central Diamond Valley, Dixie Flats, the Suzie Creek drainage north of Carlin, and along the North Fork of the Humboldt River drainage east of the Adobe Range. The Vinini and Valmy Formations are interpreted together on the overlays since they are facies equivalents and part of the same depositional assemblage.

Overlay IV shows the distribution of the 17 outcrop samples of the Valmy Formation taken in the northern portion of the evaluation area. Lithologies sampled within the Valmy included shaley siltstone, laminated and thin-bedded chert, mudstone and silicified mudstone, argillitic and clay shale, and fine-grained platy quartzite.

Total organic carbon in the Valmy Formation ranges from 0.06 percent in silicified siltstone and quartzite to 3.32 percent in silicified mudstone. The Valmy facies sampled are organic-rich with an average TOC of 0.83 percent. The kerogen is dominantly amorphous Type II kerogen and bitumen indicating a marine kerogen source. Both S₁ and S₂ values are extremely low ranging from 0 to 0.03 mg/g, making other pyrolysis parameters suspect for the Valmy Formation.

TAI values vary from 3- to 5, and questionable Ro values range from 1.39 to 5.34 indicating thermal conditions within the gas generation zone to thermal overmaturity.

Overlays XI (TOC) and XII (TAI/R_o) indicate that surface exposures of the Valmy Formation are locally organic-rich but are commonly thermally overmature or within the dry gas window. Thus an optimistic scenario suggests that the Valmy Formation may very locally be a source of dry gas along the western and eastern flanks of the northern Independence Mountains.