Peak temperature in intracratonic basins constrained by magnetic studies; example of the Illinois Basin

Deciphering the thermal evolution of a package of sedimentary rocks through time constitutes an essential element in exploration for oil and gas. Classic geothermometers based on illite crystallinity, vitrinite reflectance, the Rock-Eval method or conodont coloration index are limited to rocks containing sufficient amounts of one of the index materials. Magnetic approaches to geothermometry have intrinsic advantages due to the quasi-ubiquitous presence of magnetically remanent grains in sedimentary environments. Previous attempts to correlate burial temperature with magnetic properties focused on the low-field bulk magnetic susceptibility Km (Hrouda et al., 2003) or on the low-temperature magnetic parameter PM in pyrrhotite-magnetite assemblages (MagEval method of Aubourg and Pozzi, 2010). We simultaneously investigate the variation of an array of magnetic parameters with temperature. These parameters include low-field magnetic susceptibility, saturation isothermal magnetic remanence, saturation magnetization, coercitive force and coercivity of magnetic remanence. Tracking multiple magnetic parameters offers the advantage of being sensitive not only to heating-induced mineralogical changes but also to heating-induced magnetic domain changes. This multi-parameter method also has the benefit of being applicable to a broad range of sedimentary lithologies. To demonstrate the principles of this method we begin examining intracontinental basins because they are broadly undeformed and their thermal histories remain, in general, relatively simple. Igneous intrusions and basinal hydrothermal fluids may, however, complicate matters. The Illinois Basin, an oil- and gas-producing basin, provides an accessible test area for the geothermometric tests. The Mount Simon Sandstone constitutes the first lithological unit investigated because it sits at the deepest level in the basin and is therefore likely to have recorded the highest burial temperatures. The proposed method consists in baking series of aliquot crushed unmetamorphosed sandstones at step-wise temperatures (11 steps) to determine their overall pattern of magnetic changes with temperature. The possible roles of basinal fluids and pressure are ignored in the current experiments. The choice of crushed material as opposed to oriented samples is motivated by the need for the new method to be applicable to drilling chips that are typically produced in oil and gas exploration. The specimens were cemented and baked at step temperatures from 75 to 400 degrees Celsius. The results of these experiments serve to build calibration curves for the method. The burial temperature of specimens of unknown burial temperature is obtained by comparison with those of the calibration runs. Although the new method provides only information on peak burial temperatures, as opposed to complete heating history, this shortfall is common to all existing low-temperature geothermometers.