New Techniques for New Discoveries – Results from the Lisbon Field Area, Paradox Basin, Utah

Seneshen, David M.¹, Thomas C. Chidsey², Craig D. Morgan², Michael D. Vanden Berg² (1) Direct Geochemical, Golden, CO (2) Utah Geological Survey, Salt Lake City, UT

Innovative surface geochemical techniques were tested over Mississippian Leadville Limestone reservoirs of the Lisbon oil-gas field and a new Leadville gas discovery in the Paradox Basin, southeast Utah. The traps for both fields are elongate northwest-trending anticlines bounded on the northeast flanks by large normal faults. The objective of this study is to test low-cost geochemical methods for detecting hydrocarbon microseepage over productive hydrothermal dolomite reservoirs, especially in areas with extensive outcrops.

Approximately 400 soil samples were collected along grid lines over 15 mi² and analyzed for C₁-C₁₂ hydrocarbons, heavy aromatic hydrocarbons, 53 major and trace elements, and seven anion species. In addition, vegetation tissue and soils from joints in outcrops of the Jurassic Navajo and Wingate Sandstones were sampled to compare with the soil results. Free gas samples were collected from 6-ft depth over the new discovery and known non-productive areas off the structures.

Results to date are very encouraging. Productive and non-productive areas can be distinguished based on absolute concentrations of hydrogen, propane, nButane, nPentane, and carbon dioxide in free gas samples. Microseepage in soils is also different in terms of synchronous scanned fluorescence spectral patterns. The discriminant function separating microseepage over productive and non-productive areas correctly predicts the location of the new production southwest of Lisbon. Sulphate and chloride increase along suspected faults near production, which probably reflects the ascent of brines to the surface from the underlying Pennsylvanian Paradox Formation. The results of other analyses (i.e., C₁-C₁₂ and major/trace elements) will also be presented.