Thermal History of Alberta Deep Basin: A Comparative Study of Fluid Inclusion and Vitrinite Reflectance Data

Barbara J. Tilley, Bruce E. Nesbitt, Fred J. Longstaffe

The thermal history of the Alberta Deep basin, the gas-saturated westernmost part of the Western Canada sedimentary basin, has been studied by analyzing fluid inclusions in diagenetic cements and comparing the results with coal maturity data. Analyses of fluid inclusions in diagenetic quartz and calcite cements from Lower Cretaceous conglomerates and sandstones indicate that the fluids which precipitated these minerals attained temperatures of at least 190°C. These fluids had salinities of 2-3 wt. % equivalent NaCl and were CH₄ saturated. Time-temperature calculations for vitrinite reflectance data from coal interbeds using the Lopatin-Waples method indicate maximum burial temperatures of only 145°-155°C. The discrepancy in the results from the t o types of temperature determinations suggests that either fluids in the conglomerates were 40°C hotter than the ambient rock temperature or the correlation of coal maturity with maximum burial temperature is inaccurate. If the first scenario is correct, hot fluids would have had to have moved through permeable conglomerate beds and bedding-plane fractures at a rate fast enough such that their heat was not substantially dissipated along the pathway. If the second scenario is correct, a paleogeothermal gradient of 38°C/km (vs. the present-day 27°C/km) is indicated and a time-temperature index can be calibrated to vitrinite reflectance data specifically for the Alberta Deep basin (1.4 % R_o correlates to 190°C). With the available data, neither scenario can be conclusively proven. In either case, unexpectedly high temperatures (190°C) indicate the redistribution of heat by fluid flow in the Falher and Cadotte Members in the Alberta Deep basin.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.