Introduction Evaluation Prospects











Western Cordillera geologists collected 287 outcrop samples from 12 formations during late May through August of 1985. The sampled formations include the Eocene Elko and Sheep Pass Formations; Undifferentiated Triassic rocks which include the Thaynes and Dinwoody Formations as well as unnamed units in northern Elko County; The Mississippian Eleana, Webb, Chainman and Diamond Peak Formations; The Devonian-Mississippian Pilot Formation; The Devonian Woodruff and Devonian-Silurian Roberts Mountains Formations; and the Ordovician Valmy and Vinini Formations. Lithologic description and location data are presented for these samples in Appendix I, and all locations are displayed on Overlay IV.

All outcrop geochemical analyses were performed for Western Cordillera by DGSI Inc., 1544 Sawdust Road, Suite 402, The Woodlands, Texas. Samples were analyzed for the standard set of geochemical parameters. These include total organic carbon, pyrolysis data including S1, S2, S3 and T-max, vitrinite reflectance, and thermal alteration index, kerogen type, and fluorescence. The analytical procedures utilized by DGSI, and the guidelines used in the interpretation performed by Western Cordillera are presented with appendices II through VI.

Surface weathering has affected all of these parameters to some degree in most of the outcrop samples. The most consistent results were obtained with the TOC, visual kerogen, and TAI analyses. The other parameters were calibrated to these techniques. Weathering has reduced the TOC percentage to some degree and has increased the TAI and Ro data values. Pyrolysis parameters based upon S1, S2, and S3 have also been biased as a result of lower yields. As a result, S1 and S2 are artificially low and S3 is artificially high. The resulting hydrogen and oxygen indices are biased toward gas prone kerogens. In general, the entire geochemical data set could be viewed as a worst-case or minimum estimate scenario, with the overall source potential being better than indicated by surface sampling. The combined effects of high rank and subaerial weathering mask part of the oil-generating capability of these rocks.

The reader should also be aware that in calculating average total organic carbon values, we have eliminated the highest TOC value and the lowest TOC value. We feel that this approach gives a more meaningful geological average for the entire formation sampled. We have defined a hydrocarbon source rock as a rock with greater than 0.5 percent TOC which has not exceeded the dry-gas thermal preservation limit, corresponding to a TAI of 4/4+, an Ro of 3.2, or a T-max of 560 degrees Centigrade.

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Last modified: 09/12/06