A paleolatitude approach to assessing surface temperature history for use in burial heating models

Calculations using heat flow theory as well as case histories show that over geologic time scales (10⁶ years), changes in mean annual surface temperature (T_s) on the order of 10°C penetrate kilometers deep into the crust. Thus, burial heating models of sedimentary basins, which typically span kilometers in depth and persist over geological time frames, should consider T_s history to increase their accuracy. In any case, T_s history becomes important when it changes enough to be detected by a thermal maturation index like vitrinite reflectance, a parameter widely used to constrain burial heating models. Assessment of the general temperature conditions leading to petroleum generation indicates that changes in T_s as small as 6°C can be detected by vitrinite reflectance measurements. This low temperature threshold indicates that oil and gas windows can be significantly influenced by T_s history. A review of paleoclimatic factors suggests the significant and geologically resolvable factors affecting T_s history are paleolatitude, long-term changes between cool and warm geological periods (climate mode), the degree to which a basin is removed from the sea (geographic isolation), and elevation or depth relative to sea level. Case studies using geologically realistic data ranges or different methods of estimating T_s in a burial heating model indicate a significant impact of Ts when: (1) continental drift, subduction, tectonism and erosion significantly change paleolatitude, paleoaltitude, or paleogeography; (2) strata are at, or near, maximum burial, and changes in T_s directly influence maximum burial temperature; and (3), when a significant change in T_s occurs near the opening or closing of the oil or gas windows causing petroleum generation to begin or cease. Case studies show that during the burial heating and petroleum generation phase of basin development changes in climate mode alone can influence T_s by about 15°C. At present, T_s changes from the poles to the equator by about 50°C. Thus, in extreme cases, continental drift alone can seemingly produce T_s changes on the order of 50°C over a time frame of 107 years.