In a few decades, there will not be enough gas left on Troll to maintain production, and the field will be shut down. Will natural processes fill the oil and gas fields up again, over time? How long would that take?
Mjøsa map source: www.norgeskart.no (Kartverket-Geodata AS)
My interest in the time scale for petroleum started at the Norwegian Petroleum Directorate, where Svein Eggen was one of the pioneers in the 1980s to start making and using simple 1D basin modelling tools to calculate formation, inflow and storage of oil and gas.
We realised that the considerable loading of sediments during the ice ages centrally in the North Sea and the Halten terrace (Norwegian Sea) had to have great significance for the petroleum systems we see today.
Then the attention shifted to the areas in the Barents Sea and coastal areas, where large volumes of sediments were removed during the ice ages, with the consequence of unloading and cooling the petroleum systems. It was observed that vast volumes of hydrocarbons had leaked out in the discoveries that were made.
The ice ages in the northern hemisphere lasted for approx. 2.8 million years. We drew the conclusion that a time scale for petroleum had to have scale lines shorter than 1 million years. But several questions still remained unanswered: Was it mostly oil or mostly gas that seeped out in the eroded regions? How and when did this happen? Was this a seepage that is also ongoing today?
Oldest and youngest
The contrasts are significant as regards the time span for inflow of oil and gas into a trap. For example, the chalk deposits in Valhall and Ekofisk may have been in contact with gas and oil as early as immediately after they were deposited, 60 – 70 million years ago.
The gas in the Peon discovery, however, is located in sand that was deposited in front of glaciers in the Norwegian Channel about 700 000 years ago, and must therefore have flowed in later than this.
A lake upside down
An oil or gas field can be compared with a lake that is upside down. Rivers and streams that flow into the lake are analogous to the inflow paths for hydrocarbons. The horizontal water surface is the contact between water and air, analogous to the contact between gas and water in a gas field.
Water spills out from the outlet of a lake, in the same way as oil and gas will find its way below the spill point and flow away from a full trap.
The water that evaporates from the surface of a lake can be compared with gas that escapes by seeping upwards through the field's cap rock. The fluid surface lies at a constant level when there is a balance between the volumes of fluid flowing in and fluid that escapes.
If the water surface drops, one can see the marks on the beach where the highest water level has been, just as one can observe how residual oil is trapped underneath the oil zone in many oil fields.
One important difference between the lake and the hydrocarbon trap is that in the subsurface both hydrocarbons and water are contained in a pore system. Movement occurs according to Darcy's law, where pressure gradients force the fluids to flow through the pores.
Also, a sedimentary package is divided into layers, it is fractured, and therefore contains geological obstacles to the flow of oil, gas and water. An additional obstacle is that oil and water compete for space and flow paths in the pores.
A dynamic system
How old is the Troll field?
The reservoir sandstones were deposited in the Late Jurassic. A sedimentologist could say this is a Jurassic field that is 160 million years old. A structural geologists can prove that the fault blocks came into place approx. 150 million years ago, when the North Sea rift system took its final shape.
However, Troll did not become a trap for hydrocarbons until the structure was completely sealed by clay, which could have happened at the earliest in the Palaeocene Age, approx. 100 million years later. And the entire structure has been rotated during multiple events that took place even later. The Troll landscape, the trap, has changed its shape and consequently its capacity to trap hydrocarbons has changed.
The oil and gas in Troll originate from organic rich shales which in Late Jurassic time were deposited in thick layers in the deep Viking Graben west of the field. Oil and gas was formed when there was sufficient burial of the shales to generate temperatures exceeding 80-100 degrees C. The hydrocarbons moved up to Troll through migration paths that we can compare with streams and rivers. The shales in Viking Graben currently have a pressure and temperature which indicates that hydrocarbons are still being formed. Even though gas seeps out from the top of the structures and is trapped in fracture systems and young layers above the field, there are no indications that the gas volume in the trap has been reduced by this leak.
This setting implies that the petroleum system is dynamic, and that Troll is an example of a field which has a balance between hydrocarbons that migrate in and that leak and migrate out. One would expect such a balance for all fields that are filled down to the spill point. We have the same situation in a lake like Mjøsa: The water is gradually replaced because it has inflow and outlets, but the water level remains fixed. Oil and gas are also replaced, but move slowly in the subsurface, so the time that hydrocarbons spend in a field will be orders of magnitude longer than for the water in a lake.
The sediment basins in the central North Sea have undergone rapid sedimentation, subsidence and temperature increase during the ice ages over the last 2.8 million years. Most of the fields in the North Sea therefore are parts of active petroleum systems.
Troll contains a mixture of oil and gas molecules, however, that may originate from a long period of migration into the structure. In some of the shale packages in Viking Graben, oil was formed before the Quaternary, back to 20-30 million years, and the first filling in Troll may have started earlier than the Miocene.
Figure 1. Model of the top surface of the Troll field reservoir, viewed from the north. The field is more than 50 km long from north to south The yellow and red colours show the shape of the part of the field that contains gas. The green surface is the contact between gas and oil, the blue surface shows the top of the water level where the oil zone is very thin or does not exist. The top surface defines a hilly landscape, where the hills are connected with narrow passages.
Mjøsa map source: www.norgeskart.no (Kartverket-Geodata AS)
Mjøsa is a deep lake carved out by glaciers, almost 100 km long. During deglaciation it was connected to the sea, and as the sea water was gradually replaced by fresh water some marine organisms have adapted and can still be found in the lake.
Distribution and redistribution of oil and gas
For a while around 2005, I worked with different geological models in the Norwegian Petroleum Directorate's Troll group. We were particularly interested in how quickly oil and gas could flow between Troll East and Troll West, and whether it could be possible to recover oil from the thin oil zone in Troll East. We tried to map how thick this oil zone was, and wanted to understand why there was oil in the western part of Troll East, but not in the eastern part.
At that time, Idar Horstad and Steve Larter had already published a geochemical study on the redistribution of oil and gas at Troll. We agreed with them that the distribution could be correlated with tectonic movements in Neogene (the last 20 million years). Could these movements be dated more precisely?
From Troll to Hardangervidda
Residual oil is located underneath the oil zone that is being produced in Troll. Residual oil is trapped between the grains of sand and can fill up to approx. 20 per cent of the pore volume, the rest of the pore volume is filled with water. A residual oil zone is a remnant of a former oil accumulation which has been flushed by water. Most of the oil which was originally present in a residual zone is now replaced by water.
The profile shows that the zone with residual oil is up to 60 metres thick in the west, and gradually becomes thinner to the east. The base of the residual oil zone is an inclined surface with a slope of approx. 1 metre per 200 metres, up towards the east.
The tilting shows that the oil-water contact in the west was once deeper down in the stratigraphic sequence than today. The western part of the structure must at that time have been higher than the eastern part. Troll East did not have a significant trap volume then. After this early filling of oil (and gas), the structure was tilted up in the east. Oil and gas was redistributed in the structure and could also enter into Troll East.
The tilting of the old oil-water contact (Paleo-contact) can be correlated with, and is consistent with, tilting of the young layers over the field. The unconformity in the Middle Miocene (mmu), 16 million years old, is a horizon that is tilted up towards the east.
Figure 3. West-east profile of the northern North Sea and the Scandinavian mountain range. The tilting that led to redistribution of oil and gas at Troll is correlated with the youngest phase of uplift of Southern Norway, and the last phase of subsidence in the North Sea. Red line: Current topography. Blue line: Summit level from mountain areas combined with base Cretaceous unconformity surface in the North Sea. Orange line: The “Palaeogene” surface east of the southern Norwegian high mountains, combined with Middle Miocene unconformity surface in the North Sea. Gray: Quaternary sediments in the North Sea.
The surfaces we interpreted on Troll display a segment of a much larger geological structure. The top of the basement east of Troll was covered by sediments after the Late Jurassic and defines a sloped surface that can be drawn further towards the top level of the highest mountain areas in Southern Norway. The Middle Miocene unconformity (orang line) can be correlated up towards an erosion surface that reaches up to a height of more than 1000 metres east and south of Hardangervidda. These surfaces are features in the landscape that can be used to assess different phases of crustal uplift of the current Scandinavian mountain range.
The water that ran away
Oil and gas that migrated into Troll displaced the water that the pores originally contained. In a water-filled pore volume (an aquifer) which is closed so that the water cannot escape, an additional water volume of 1 per cent will build up approx. 100 bar overpressure.
Such overpressures are not observed around Troll. The water has hydrostatic pressure. A total water volume corresponding to the volume of oil and gas in Troll must therefore have escaped from these formations as oil and gas migrated into the field.
Correspondingly, we see that there is naturally hydrostatic pressure in the water zones that surround the major gas fields Ormen Lange, Frigg-Heimdal and Snøhvit.
Even though we do not know the pressure development through time, this indicates that the processes of migration and leaks are slower than the process of pressure equalisation in water zones that are not closed.
Figure 4. The main reservoir in the Troll field belongs to the Sognefjord formation which covers the yellow area in the figure. The Sognefjord formation has about the same extent as the underlying Fensfjord and Krossfjord formations. These formations contain water that is in communication with hydrocarbons in Troll and several of the discoveries north of Troll. The blue line shows where these formations are located in direct contact with Quaternary layers in the Norwegian Channel.
One field – many processes
The fluid contacts in the different segments on Troll are horizontal. The fluids in each individual segment appear to be close to equilibrium after the tilting of the field, but the oil zones in different segments have varying thickness and different contact levels.
In these cases, it is common to assume that the faults are barriers that have prevented equilibration of the oil zones. Fault zones often have much lower permeability than the reservoir, and capillary forces can help to keep oil in place on one side of the fault when there is gas or water on the other side.
Experience from production on Troll indicates that the faults are permeable in several places. Gas and water can flow through them also in the engineer's time scale. This is in accordance with the pressure in the gas and water zones being in equilibrium before production started.
A lack of equilibration of oil zones between segments does not necessarily imply that the segments do not have communication. It can also be a question of the degree of communication, a question of how long time it takes to equilibrate.
Figure 5. Schematic profile which shows a field with two segments and two reservoirs where oil and gas migrate in, water and gas flow out of the structure. The sketch illustrates several effects of oil and gas not reaching complete (static) equilibrium: Different contacts in different reservoir zones. The composition of oil may differ in the two segments, and the pore pressure will be different. The aquifer may be affected by production from nearby fields. The fault between the two segments has such a low relative permeability that it forms a baffle between oil on the one side and water on the other: A boundary in a time scale for petroleum.
The figure shows that, in a typical field, there are many types of equilibrium to be checked. Troll illustrates some of them.
Craig Smalley and his colleagues have worked on simplified models for several such equilibrium processes and have established formulas that provide an indication of the time periods they need. Different processes have different velocities. When this type of work is calibrated against fields where the geological framework conditions are well known, we advance one step further in dating the various epochs of the history of the hydrocarbons in the fields.
The end is a beginning
Every oil and gas field has a story to tell. A story consisting of many other interwoven stories. A good understanding of this story is useful for managing the fields today and in the future. Those who work on managing the resources should also keep in mind that the stories never end. Even fields that are shut down have a future, which is determined by processes that are in a time scale for oil and gas.
This is the last article about seven mysteries that have inspired me in my geological work. Geoscience is developing. The view of petroleum as humanity's most important source of energy is changing. The future has become more uncertain.
But we can be certain that we will need more science, better technology and more understanding in order to find good paths forward.
The message is that science advances. But not just because of more data and new grants. It lives when we observe, wonder – and take pleasure in seeing how our mysteries are gradually being solved.