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The Use of Epifluorescence Techniques to Determine Potential Oil-Prone Areas in the Mississippian Leadville Limestone, Northern Paradox Basin, Utah

David E. Eby1, Thomas C. Chidsey, Jr.2, and Craig D. Morgan2
1Eby Petrography & Consulting, Inc., Denver, CO
2Utah Geological Survey, Salt Lake City, UT

Potential oil-prone areas for the Mississippian Leadville Limestone were identified in the northern Paradox Basin (Paradox fold and fault belt), Utah, based on hydrocarbon shows using low-cost epifluorescence techniques. The trapping mechanisms for Leadville producing fields are usually anticlines bounded by large, basement-involved normal faults. Epifluorescence microscopy is a technique used to provide information on diagenesis, pore types, and organic matter (including “live” hydrocarbons) within sedimentary rocks. It is a rapid, non-destructive procedure that uses a petrographic microscope equipped with reflected-light capabilities, a Hg-vapor light, and appropriate filtering.

Approximately 900 cutting samples were selected from 32 wells penetrating the Leadville Limestone (six gas, condensate, and oil wells, as well as 26 non-productive wells) throughout the region. These cuttings (generally four to ten samples per depth interval from each well) display intercrystalline porosity and occasional small vugs or molds. A qualitative visual rating (a range and average) based on epifluorescence evaluation was applied to the group of cuttings from each depth interval in each well. The highest average and highest maximum epiflourescence from each well were plotted and mapped.

As expected, productive wells (fields) are distinguished by generally higher epifluorescence ratings. However, a regional southeast-northwest trend of relatively high epifluorescence parallels the southwestern part of the Paradox fold and fault belt while the northeastern part shows a regional trend of low epifluorescence. This implies that hydrocarbon migration and dolomitization were associated with regional northwest-trending faults and fracture zones, which created potential oil-prone areas along the southwest trend.

AAPG Search and Discovery Article #90092©2009 AAPG Rocky Mountain Section, July 9-11, 2008, Denver, Colorado