Thermal Maturity Trends from Rock-Eval and Vitrinite Reflectance Analysis on Potential Source Rocks in Eastern Nevada

BARKER, CHARLES E., JAMES A. PETERSON, and FORREST G. POOLE, U.S. Geological Survey, Denver, CO

Organic geochemical and petrographic analyses of potential source rock samples taken from 30 drill holes penetrating Cenozoic basins of the Basin and Range province in eastern Nevada indicate two distinct trends in thermal maturity, as expressed by vitrinite reflectance equivalent (VRE). VRE is any coal rank or other thermal maturity parameter converted to its equivalent in vitrinite reflectance units using correlation charts.

One trend is an increase with depth from about 2% VRE at 500 ft to 5% VRE at 12,000 ft, which is supermature with respect to oil generation. The other trend consists of mature rocks that have a nearly uniform VRE of about 0.7% (+/-0.2%) from the near surface to a depth of 12,000 ft. Both the lower and the higher range geothermal gradients measured across the area, about 0.4 to 2.5 degrees F/100 ft (8 to 45 degrees C/km), correspond with the mature and supermature trends, respectively.

We attribute these trends to two major thermal environments common within these Cenozoic basins. The supermature trend results from high geothermal gradients found near igneous intrusions, faulted contacts of uplifted metamorphic core complexes, or other heat sources. The mature trend characterized by near-zero VRE gradients results from groundwater recharge which produces cool temperatures at depth and low geothermal gradients. These generalizations should be applied with caution to any specific area because they are based on a composite VRE depth curve and, in some localities, may not represent realistic interpretation. However, VRE profiles within individual drill holes within the region generally follow one of these trends.

AAPG Search and Discovery Article #91010©1991 AAPG Rocky Mountain Section Meeting, Billings, Montana, July 28-31, 1991 (2009)