Geological Controls from and Geochemical Interpretations of Reservoir Fluid Heterogeneity in Northern Alberta Heavy Oil and Tar Sand Fields
Biodegradation is believed to be the primary cause for heavy oil and oil sands formation over geological timescales. Net biodegradation of petroleum fractions in reservoirs is primarily controlled by reservoir temperature, charge and mixing history, the chemical compounds being degraded and relationships between oil-water-contact area and oil volume. When crude oils are progressively biodegraded in subsurface reservoirs, they become denser and more viscous. While other factors such as porosity, permeability and pressure fields determine the producibility of a reservoir, it is the oil density and especially viscosity that dictate the optimal production approach.
The measured viscosity from several case studies with frozen cores in northern Alberta heavy oil and oil sand fields shows dramatic variations up to two orders of magnitude difference at reservoir temperature throughout the oil column with better oil occurring at the top and worse oil at the bottom of the reservoir section. Increasing oil viscosity downwards suggests that oil biodegradation and the proximity of the sample to the oil/water transition zone is the critical control for oil physical properties. However, viscosity profiles of individual wells reflect the geological framework and reservoir structure. When oil fills to the underseal, the oil is generally less biodegraded and well preserved due to removal of the source of nutrients and free water to the microorganisms. When thick basal water is available which creates a large burn out zone, the oil has overall worse quality throughout column and is extremely heavy at the bottom.
In some areas of the studied fields, shale zones and sand pinch-outs are common. Whether or not a shale zone acts as a barrier or baffle is critical to thermal recovery processes but the integrity of these low permeability zones is commonly ambiguous based on pressure or geological data. Varying molecular compositions across the shale bands can be applied to diagnose the integrity and extent of these baffles or barriers. Geochemical analyses and interpretations, when integrated with geological and geophysical data provides important input to development and production planning in heavy oil and bitumen recovery.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009