The relationship of hydrocarbon occurrences to geothermal gradients and time-temperature indices in Mesozoic formations of southern Alberta

Mesozoic oil pools in southern Alberta occur within the inferred zone of thermal maturity of potential Carboniferous (Exshaw Formation) and Cretaceous (Colorado Group) clastic, iron-rich Type II source rocks. The majority of Mesozoic gas pools occur east of the inferred time-temperature index threshold for the onset of significant gas generation from either Type II or Type III source rocks. Biogenic gases generated during relatively early stages of burial are inferred to be the main source for the gas pools. These gases migrated under the influence of the groundwater recharge and migration system that was established during uplift and erosion of the Alberta syncline. This groundwater circulation system is reflected in the present geothermal environment. All stratigraphic families of gas pools exhibit, at their mean, a higher geothermal gradient than those areas lacking gas accumulations. This results from the mass migration pattern that was responsible for the concentration and entrapment of the gases. Oil pools in the Mannville Group exhibit a similar positive anomaly at their mean in comparison with the areas where accumulations do not occur. This indicates migration of oils from Exshaw Formation sources into the Mannville Group. Oil pools found in younger Cretaceous reservoirs have a negative mean gradient anomaly when compared with the areas devoid of oil accumulations, and we can therefore conclude that only local migration occurred.