A Practical Approach to Reconstructing Burial and Temperature Histories: An Example from the Mobil O'Connell Well, Piceance Basin, Colorado

Matthew M. Laughland

The O'Connell well was drilled about five years ago as part of a deep basin, tight gas play. The well penetrates a Devonian through Paleocene age section of carbonate and clastic lithologies. Fifty-five m.y. old strap of the Wasatch Formation are exposed at the surface. Corrected measurements of bottom-hole temperature are as high as 240°C (464°F), and the present-day geothermal gradient is ~40°C/km. (2.20°F/100ft.) Measurements of %R₀ increase semi-continuously from ~0.5% at 0.9 km. subsurface depth (2,950 ft) to ~6.0% at a total well depth of 5.6 km. (18,422 ft).

A multidisciplinary empirical approach was used to determine the magnitude and timing of maximum temperature and paleogeothermal gradient, as well as uplift and erosion, independent of numerical basin modeling. Apatite fission track analysis (AFTA) indicates that 1.36 km (4,460 ft) of section were eroded beginning at ~5 Ma, which was synchronous with the timing of uplift of the Colorado Plateau. The timing of maximum temperature at (approx.) 27 Ma was determined using ⁴⁰Ar/³⁹Ar thermochronology and was synchronous with the timing of local igneous activity and mineralization of the Colorado Mineral Belt. Maximum paleotemperatures based on %R₀ and pressure corrected homogenization temperatures from fluid inclusions in quartz veins are as high as 340° C (644°F), or about 100°C higher than present-day maximum temperature. Calculations of maximum paleogeothermal gradient equal ~52°C/km (3.03 degree C/100ft)

This multiple thermal indicator approach to the analysis of cuttings and core limits the number of possible burial and thermal history scenarios, thereby reducing much of the uncertainty in basin history reconstructions. Entering the empirically determined values for the magnitude and timing of maximum paleogeothermal gradient, as well as uplift and erosion, into numerical basin models yields a match between model predictions of %R₀, and measured values, giving the reconstruction of burial and temperature history a stronger degree of confidence than a reconstruction based solely on the traditional industry approach of matching model fits of %R₀ to measured values.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California