Conventional global oil reserves may amount to 4 trillion bbl excluding tar sands, twice the size generally thought.
Surface seepage rates are far too large to be supported under equilibrium conditions by the approximate 2 trillion bbl of global oil reserves predicted by analog models.
Richard G. Miller of BP Exploration Operating Co. Ltd., London, advances the ideas in an article published in the April 1992 issue of the American Association of Petroleum Geologists Bulletin.
Implications of a global oil system concept to exploration are that 50% of all economic oil was created within the last 29 million years and almost 90% within the last 90 million years.
"The best exploration prospects are where there is still active oil generation and where there has been no time for seepage to empty the reservoir," Miller wrote.
The global oil system concept depends on two major geological assumptions: that the global rate of oil generation and expulsion equals the rate of natural loss and that oil loss can be described by a natural decay law.
GLOBAL OIL SYSTEM
The global oil system contains all the expelled oil that existed before human intervention--the resource--and a portion of this oil forms the reservoired oil system.
The two systems are isolated.
Most of the oil continuously generated and expelled from source rocks is eventually lost by seeping directly to surface, degrading in reservoirs, or experiencing thermal cracking.
Global oil generation is estimated at 2.7 million bbl/year. Limited surface seepage data indicate at least 800,000 bbl/year enters reservoirs, but most of the generated oil escapes to surface without being trapped.
If the global system is in equilibrium, then the reservoir filling rate cannot be less than the rate at which reservoirs seep, Miller suggests.
SYSTEM ELEMENTS
The article assumes that on a global basis oil is destroyed exponentially with time in analogy with radioactive decay.
The age distribution of known oil supports this assumption: the current contents of the reservoired oil system define a precise half life.
The half life of the global expelled oil system will not be the same as that of the reservoired oil system because their sizes are different.
"The disproportionately large oil reserves in young rocks may exist simply because less oil has seeped out or cracked, rather than because they have more or better quality reservoirs or because source rocks are more abundant in younger sequences," Miller writes.
Assumed threshold depths are 2.5 km for the onset of oil generation, 4 km for oil cracking and gas generation, and 5.5 km for over-maturation. And 44% of sediment is immature, 20% lies within the oil window, 16% is in the gas and oil cracking window, and 20% is over-mature.
This sedimentary mass has a definable half life of 130-150 million years, which is compatible with a constant rate of sediment loss from all depths by erosion or metamorphism.
If global sedimentation occurs at the global erosion rate of 10 quadrillion g/year before human intervention, then maturing sediments generate and expel 2.7 million bbl/year of oil. Global oil seepage probably exceeds 1.5 million bbl/year.
At 2.7 million bbl/year, 2 trillion bbl of oil could have been emplaced in less than I million years.
The world's 1,300 sedimentary basins would therefore generate and lose at least 2,000 bbl/year/basin. The mean loss rate could be as much as 18,000 bbl/year for the 150 basins that have produced commercial oil or gas. For comparison, the Coal Oil Point seep off California flows 33,000 bbl/year.
HOW MUCH OIL?
Factors in the global volume of oil are a global oil generation rate of 2.7 million bbl/year, cracking rate of 700,000 bbl/year, and seepage rate of roughly 2 million bbl/year of which 1.2 million bbl/year migrates directly to surface after generation and expulsion.
Therefore, Miller writes, 1.5 million bbl/year enter the global expelled oil system and 800,000 bbl/year seep from it.
Some 700,000 bbl/year of the seepage and 100,000 bbl/year of cracking loss is assigned to large reservoirs. The other 800,000 bbl/year are thought to enter the reservoired oil system, while 700,000 bbl/year remain elsewhere in the global system.
With a half life of 29 million years for the reservoired oil system, the total oil in place there is 33 trillion bbl.
Using 15% recovery and 20% shrinkage, this implies almost 4 trillion bbl of recoverable oil. The volume would be larger with higher global recovery factors.
WHERE OIL MUST BE
Most of the oil implied in Miller's global oil volume excess to that in analog estimates will be in unconventional places, including unconventional traps in maturely explored basins.
"We could, for example, speculate about traps sealed by permafrost or volcanic flows, synclinal traps sealed updip by depth-controlled diagenetic cement, or traps defined entirely by hydrodynamic conditions.
"Most of the excess, however, especially the remaining large accumulations, presumably remains in untested areas."
Tar sands do not resolve the discrepancy, he noted.
Miller notes that there are probably major flaws both in our understanding of the global oil cycle and in the migration, cracking, and seepage processes and in estimates of global oil reserves. He also says the model may also be unrealistically oversimplified.
Copyright 1992 Oil & Gas Journal. All Rights Reserved.
