Post-Early Devonian thermal constraints on hydrocarbon source rock maturation in the Keele Tectonic Zone, Tulita area, NWT, Canada, from multi-kinetic apatite fission track thermochronology, vitrinite reflectance and shale compaction

Abstract

Abstract New thermal maturity (%Ro, Rock-Eval® pyrolysis), shale compaction and apatite fission track (FT) data were integrated into thermal history models for the East MacKay I-77 petroleum exploration well located approximately 80 km southeast of Norman Wells, Northwest Territories. The study well is in the Keele Tectonic Zone where multiple phases of anomalous Phanerozoic subsidence and uplift have resulted in an Upper Cretaceous–Cenozoic foreland succession resting unconformably on Devonian strata. This major unconformity, which developed during pre- and post-Albian time, displays a thermal maturity discontinuity (0.55-0.75%Ro) and represents approximately 270 m.y. of missing geological record. Linear shale compaction across this unconformity suggests that maximum burial occurred during the Cenozoic, whereas thermal maturity data imply that maximum temperatures were reached sometime between the Devonian and Early Cretaceous. Detrital apatite grains from a single sample from the Upper Devonian Imperial Formation of the I-77 well yielded two FT age populations (90.4±6.1 Ma and 222.2±22.5 Ma; ± one standard deviation) with different thermal annealing properties based on their chlorine content. An inverse multi-kinetic FT annealing model was developed and used to determine thermal histories that are consistent with the FT parameters and other geological constraints. Model results suggest that hydrocarbon generation from Devonian rocks at the I-77 well location occurred during the early Mesozoic prior to the development of Late Cretaceous-Cenozoic structures. Peak FT model temperatures are 124±10°C within the Early Triassic to Middle Jurassic (250–170 Ma), <75°C during the Albian (112–100 Ma) and 97±9°C during the Paleocene-Early Eocene (65–50 Ma). The Cretaceous-Cenozoic thermal history was modelled using a simple burial history with the present geothermal gradient (32°C/km) held constant; a range of geothermal gradients (31–42°C/km) and maximum burial depths for the pre-Aptian thermal history fit Devonian maturity data. If maximum burial was during the Cenozoic, then Mesozoic peak maturity was achieved under a higher geothermal gradient than present. Although hydrocarbon generation pre-dates structural trap development near the I-77 well, Devonian source rocks retain significant hydrocarbon potential. Given the complicated geological history of the central Mackenzie Valley, deeper Cenozoic burial elsewhere in the region may have generated hydrocarbons from Cretaceous and reactivated Devonian source rocks.