From Deep Water Exploration to Tar Sand Production: Bugs; Biodegradation, and the Origin of Heavy Oil

Steve Larter, University of Calgary, Alberta, Canada

Microbial action has destroyed or altered most of the world’s petroleum resources, producing heavier and more acidic crudes. While this has been known for a long time, the rates at which this happens, the reactions and organisms involved, and the nature of the alteration process have remained shrouded in mystery. We now know that the major reactions involve microorganisms carrying out hydrocarbon-water reactions producing methane as a major end-product as well as heavy oil. We know that the processes take place under anaerobic conditions without the need for free oxygen and we know how fast the process happens. We have even isolated some of the DNA from organisms that carry out the processes. We think that the oil degradation floor at 80°C probably represents the base of life in the crust and it seems the microorganisms in the reservoirs are slowly evolved descendants of those deposited with the reservoir rather than organisms carried down from the surface into the reservoir. It typically takes up to about 10Ma to destroy the n-alkanes in crude oils causing commercial problems in deep water production settings and the Canadian Tar sands could have reached the state they are in around 35Ma or less after reservoiring. The principle control on fluid properties such as viscosity or API gravity is the mixing of fresh oil into actively degrading oil contained in reservoirs. I describe how, using these concepts, fluid properties can be predicted predrill using basin modeling approaches and how the processes of heavy oil origin produce systematic variations in fluid properties that are predictable on reservoir thickness, field and even regional scales. These in-reservoir fluid heterogeneities can be used to design and enhance production programs and solve many practical problems associated with heavy oil exploration and production. Further, since the production of methane is often a common process during biodegradation of oils in deep reservoirs this raises the exciting possibility that if this process could be accelerated, heavy oil or tar sand bitumens, or even residual oils in conventional oilfields, could be recovered as methane on production timescales.