An empirically verified theory provides petroleum geologists with a tool to make it easier to explore oil and gas.
The idea behind “The Golden Zone”, which includes 90% of the oil and gas resources of the world, is easy to understand and is in the process of being accepted among petroleum geologists.
The principal message in the theory is so simple that everyone can understand it:
“Most of the hydrocarbons in a sedimentary basin – both oil and gas – are located at a depth where the temperature is between 60 and 120 °C.”
This new understanding represents perhaps one of the main advances in petroleum geological thinking in recent years. All petroleum geoscientists should therefore take note of the expression “The Golden Zone”.
“The theory is, in principle, applicable everywhere, that is to say in all types of sedimentary basins,” says Per Arne Bjørkum in Statoil. Together with Paul Nadeau he has, over a ten-year period, developed rather hazy ideas into a well thought out theory, which is now in the process of becoming firmly rooted in petroleum circles. But it was necessary for an American, Bill Maloney, to give it a name, attractive both for colleagues and for Statoil’s management.
“The Golden Zone” contains 90% of the world’s oil and gas resources. The idea behind the theory is easy to grasp, and more and more geoscientists believe it is worth while to pursue t his concept when exploring for additional hydrocarbons. Image credit: Statoil.
“The Golden Zone”
“We have developed a synthesis involving several different fields of scientific study. The premise is that oil and gas occur in a definite pattern despite many geological differences from basin to basin,” says Per Arne Bjørkum.
Dr. Per Arne Bjørkum, Chief Exploration Scientist for Statoil, with a Master degree and a doctorate in sandstone diagenesis
All sedimentary basins give the appearance of being unique. They all have their own history of development that gives them special features. Nevertheless, it is possible to discern a pattern, which is common to all of them, and it is also apparent that they show great similarities as regards the location of hydrocarbons. The traditional sub-division into types of sedimentary basin becomes thereby less relevant seen in the light of the new theory.
Petroleum geologists without long and habit-forming work experience will have no difficulty in accepting the existence and significance of ‘The Golden Zone”. When, however, the two geologists first introduced the theory many refused to be convinced. This was largely due to traditional thinking regarding generation and migration of oil and gas suggesting a basin-specific distribution of hydrocarbons rather than the universal pattern as suggested by Bjørkum and Nadeau.
“But the opposition was also due to a certain doubt regarding a petroleum geological world, which was easy to understand but required a radically different way of thinking. It was necessary, first of all, to eliminate “all” preconceived lines of thought,” say the two scientists.
Paul Nadeau has a PhD in geology and works for Statoil in the field of international exploration.
“We have searched for a simple explanation and for a robust theory independent of details or information that is difficult to access,” says Bjørkum.
Now the two research scientists go even further in their theoretical studies. Mechanisms that come into force at 120 °C can possibly also explain other phenomena. “We believe that overthrusting, for example, in which thrust sheets or nappes were pushed across Norway during the Caledonian episode of mountain folding, can be more easily understood if we take into consideration the effect of temperature on the geological processes involved. The same temperature and the processes we have described can also largely control the late phase of subsidence of sedimentary basins,” says Bjørkum.
“Our theory is a contribution to our understanding of the earth’s dynamic behaviour,” he maintains.
It is tempting to believe him. He has the capacity to be convincing without overwhelming us with technical arguments. Instead he offers observations from many basins around the world, which document the pattern, together with philosophical considerations based on his study of scientific thought over many years.
“We must find the patterns in Nature – the fact that the sun rises once every 24 hours is one of them. We must look for organised patterns since these reflect Nature’s inherent orderliness. We foresaw the universal pattern in the “Golden Zone” from theoretical considerations before we had the data to support it. This is the fundamental scientific philosophical approach. Geologists have a habit of being fascinated and engaged in the unusual, that is to say, in cases which are exceptions to the rule and which cannot therefore form the basis for a pattern.
Oil first, gas later
“Geologists have been negligent regarding temperature. Only geochemists have really bothered to understand what happens when the temperature in a basin increases. It is therefore geochemists that have developed the theory that the formation of oil and gas is controlled by temperature,” says Per Arne Bjørkum.
With increasing temperature oil is formed first (real speed in the process does not occur before 120-130 °C) and then gas (with a maximum around 150 °C) from decayed organic material (kerogen) incorporated in shales or carbonates (source rocks). This understanding as to how oil and gas form has also determined how geologists have perceived the distribution of hydrocarbons with increasing depth, oil 38 GEO ExPro September 2004 “uppermost”, gas “lowermost”. And it was just this argumentation which Per Arne Bjørkum and Paul Nadeau ultimately became involved in, though they did not start with it.
“When working on my main subject in geology I studied the diagenesis of sandstones and the reduction of the porosity of rocks with increasing depth. Later, when working in Statoil and cooperating with Paul Nadeau we began to think about the consequences of our combined knowledge. Paul was an expert on shales and I knew something about sandstones, which I had acquired together with Olav Walderhaug, now technical adviser in Statoil. Slowly a pattern emerged which we were not looking for at the outset.”
“This is a classic approach for research scientists. We attack one problem, solve it and thereby gain insight into another area, which we had not thought about to begin with. This is often the way new interdisciplinary insight emerges,” explains philosopher Per Arne Bjørkum.
“The Golden Zone” – explained!
The theory is based on two main patterns that can be observed in sedimentary basins worldwide:
The pore pressure in the water phase
The spatial distribution of hydrocarbons in sedimentary basins.
Experience has shown that there is a connection between the two patterns, i.e. they are related because they have a common cause.
Normally, the pore pressure is plotted against depth. Each basin will then have its own curve. By plotting the increase in pore pressure against temperature the profiles combine to a universal curve that is common to all sedimentary basins. The increase in pore pressure (the pressure ramp) starts at around 80-90 ·c and reaches hydrofracture pressure at around 120°C. Image credit: Statoil
In nearly all sedimentary basins the pore pressure exceeds the hydrostatic pressure (pressure of the water column) beneath a certain depth. The depth at which overpressure first appears varies from basin to basin and it is impossible to say in advance where this pressure increase above the “normal’ will be found. By comparing pore pressure with temperature – instead of depth – a distinct pattern becomes apparent. Increase in pressure above hydrostatic starts typically at 80-90°C and increases rapidly until the temperature reaches 120°C.
“We have replaced basin specific pore pressure profiles with a universal profile, and with the help of th is it is possible to say something about the increase in pressure in a basin before drilling starts,” explains Per Arne Bjørkum in Statoil.
Likewise, the distribution of oil and gas resources can be plotted against temperature instead of depth. In this way a universal pattern emerges, that is to say we find the pattern in all sedimentary basins independent of the way they formed, their history of development, and age. We obtain a distribution that shows an accumulation of oil and gas between 60 and 120°C (“accumulation zone”) commonly referred to as “The Golden Zone”. Surprisingly, gas also occurs in precisely the same temperature interval as oil, despite the fact that it forms much deeper and at different temperatures compared to oil.
“The volume of hydrocarbons in reservoirs falls exponentially with temperature when it rises above 110-120°C, and the total volume of oil and gas that geologists can find at temperatures above 120°C is only about ten percent of the total amount in a given basin. This we have proved by data from the whole world ,” says Bjørkum.
The zonation of a sedimentary basins based on temperature boundaries. The expulsion zone applies to both source and reservoir rocks. Image credit: Statoil
“Most of the hydrocarbons form in source rocks where temperatures exceed 120°C. Since oil and gas do not collect in reservoirs at such high temperatures, we conclude that oil and gas are driven outwards and upwards to lower temperatures than those prevailing in the source and reservoir rocks. The zone where temperature exceeds 120°C we call the “expulsion zone”, which is characterised by a pore pressure that lies around the pressure needed to fracture the host rock. Hydrocarbons will then stream upward through hydraulically formed cracks, that is to say from the expulsion zone and into the “The Golden Zone”.”
“When the temperature is lower than 60°C, there are few hydrocarbons left. We call this the “Sealing Zone” which is also the zone where microorganisms devour most of the hydrocarbons which have migrated into it.”
The pattern established here is independent of the type of basin, its age, rate of subsidence, temperature gradient, the total amount of oil and gas and the size of individual fields. Therefore only temperature can say something about the distribution of hydrocarbons at depth. “Isn’t it fantastic to see how well organised Nature is?” asks Per Arne Bjørkum.
By plotting the volume of hydrocarbons against temperature a pattern emerges which is similar to sedimentary basins all over the world. There are upper and lower limits at 60 and 120 °C, respectively, with an approximate norm around 90 °C. Note the exponential reduction in volume of both oil and gas at around 120 ·c where the pore pressure is around hydrofracturing pressure. Image credit: Statoil
“The reason why the pattern exists and is so robust and stable is because the basic processes that operate in the expulsion zone are controlled by one parameter only- the temperature. Water, oil and gas, are driven out by thermally controlled processes and we can regard the expulsion zone as a thermally powered chemical pump in which the fluids are forced upwards into the overlying accumulation zone at the same time as the pore volume progressively disappears due to ‘cementation’ in the expulsion zone,” explains Per Arne Bjørkum.
“The temperature is also responsible for much more than has so far been realised. The more mechanical understanding of a basin controlled by stress factors is replaced by one in which dynamical forces are related to fluid flow and the subsidence of a basin due to thermally controlled chemical processes. Therefore, it is logical for the universally accepted pattern to become apparent only if one relates these factors to the parameter that controls the processes, namely, the temperature.”
“We have managed to create a new and logical order in the spatial distribution of hydrocarbons in sedimentary basins. We have done this by identifying – and describing – a few controlling processes related to reduction in porosity in sandstones and permeability in shales thus providing a basis for the stable dynamic patterns now documented. We believe that in this way we have simplified understanding as to how oil and gas behave – and thereby also simplified the exploration process for hydrocarbons,” says Per Arne Bjørkum.
