Phanerozoic strata of the Mackenzie Plain region, Northwest Territories, record a complex, multi-phase history of burial, exhumation and deformation which resulted in the development of several major unconformities. In this study, we combine two methods, organic maturity and sonic porosity, to characterize two major unconformities within sediments of the Brackett Basin: (1) the Devonian-Cretaceous unconformity which separates Middle Devonian strata of the Paleozoic Keele Arch and Root Basin from overlying Cretaceous-Tertiary strata of the Brackett Basin (an approximately 270 m.y. hiatus); and, (2) the near-surface post-Cretaceous unconformity, a regionally extensive Cenozoic erosion surface. The magnitude of erosion at these unconformities and the prevailing geothermal gradient at maximum burial were estimated using shale sonic porosity (φs) and vitrinite reflectance data for wells in the southern part of the Brackett Basin. These data were also used to constrain preliminary thermal and burial history models for the study region. Shale compaction profile (CP) and maturity profile (MP) data indicate that maximum burial occurred during the Cenozoic (culmination of Laramide Orogeny) followed by erosion of between 1 to perhaps 3.5 km of Cretaceous-Tertiary strata. CP estimates, which we believe to be more sensitive to paleoburial depth, restrict Cenozoic erosion to between 1.5 to 2.5 km in the study wells (Tate J-65, Cloverleaf 1-46 and the Keele South wells). This implies that the maximum pre-erosion thickness of Cretaceous-Tertiary strata was about 3 km. A major maturity discontinuity exists between Cretaceous and Devonian strata in the Paleozoic Root Basin portion of the study area, indicating about 0.5 to 2 km of erosion at this unconformity and high geothermal gradients (about 55-65°C/km) sometime prior to mid-Cretaceous time. The absence of a corresponding discontinuity in shale compaction implies deeper Cenozoic burial at lower temperatures, and that shale compaction is more sensitive to overburden pressure than to temperature. There is no maturity discontinuity evident to the north, suggesting lower pre-mid Cretaceous geothermal gradients and thermal overprinting by Cenozoic burial in this part of the basin. Results indicate that φs-based erosion estimates and present geothermal gradients can be used to reconstruct the Cenozoic thermal history of the Brackett Basin. This is advantageous because well logs and temperatures are readily available from existing exploratory wells, whereas organic maturation data are sparse. Furthermore, except for the Paleozoic Root Basin, Cenozoic temperatures during maximum burial probably controlled hydrocarbon generation from potential Lower Cretaceous and Paleozoic hydrocarbon source rocks. Within the Root Basin, Paleozoic rocks appear to be overmature with respect to liquid hydrocarbon generation, and MP data are needed to reconstruct the pre-mid Cretaceous burial and thermal history. Although potential Cretaceous source rocks appear to be low to moderately mature with respect to the oil window (0.5-0.6 %Ro), significant differences in Cenozoic geothermal gradient among the study wells (28 to 37°C/km) suggest that prospects for oil generation are good in regions of higher geothermal gradient (>30°C/km).
|Number of pages||17|
|Journal||Bullentin of Canadian Petroleum Geology|
|State||Published - 1 Dec 1996|