Knowing where and when hydrocarbons are generated and where they finally end up seems so basic, yet it took years for the concept of the petroleum system to become an accepted practice. Now, using fast computers and innovative software, all exploration data including wells, seismic lines, geochemical data on the source rock and known hydrocarbons can be incorporated into petroleum system models. This concept provides the geoscientist with a new understanding of how a basin’s rocks and fluids change over time, helping to reduce hydrocarbon exploration risks.
Leslie B. Magoon, Emeritus Scientist, US Geological Survey, Menlo Park, California, has spent most of his career “mapping fluids, collecting and analysing oil and gas samples”. He first presented his work on the petroleum system concept as a brochure and poster in 1986. This was after an earlier paper on the subject was rejected by three prominent petroleum geologists, possibly not understanding his approach, who said “we already do this”. With ever increasing computer power over the past decade, his original concept is now being applied to present and future petroleum provinces around the world.
What it Really Means
“Nature’s distribution of hydrocarbon fluids is the petroleum system,” says Les. “Deposition of sedimentary rock into a basin provides the setting and once a hydrocarbon fluid network forms, it can then be modelled as a petroleum system.”
To really understand the definition of a petroleum system, it is important to break it down. Les explains, “The essential elements of a petroleum system are the source rock, reservoir rock, seal rock, and overburden rock. The two processes that are key to understanding a petroleum system are the trap formation and the generation-migration-accumulation of hydrocarbons. These essential elements along with the processes control the distribution of petroleum in the lithosphere.”
“Genetically related hydrocarbons give the explorationist an idea about the correlation between the source rock and the petroleum occurrences. This can range from just having a source in the same geographic location (very speculative correlation) to a positive petroleum-source rock correlation (known correlation). As for shows, seeps and accumulations, any amount of oil or gas is proof of a petroleum system. Finally, we use the term active source rock to denote when that actually occurs, not what stage of maturity the source rock may be at today.”
“The definition and a breakdown of some of these elements are needed to visualise the concept,” continues Les. “We also had to refine and extend some vocabulary and create a series of graphic diagrams as a folio sheet. It is important for geoscientists to understand that generation-migration-accumulation need to be modelled at the time it happens, which we call the critical moment.”
Development of the Concept
Like all science and most new concepts, the petroleum system was developed over a period of time. A foundation of principles in geology dating back to the 17th century and much more recent 20th century developments in organic geochemistry are two key disciplines necessary to formulate the petroleum system concept. Discoveries in the geosciences over the last 50 years have greatly added to our knowledge about the earth and the dynamics of the earth’s systems.
It was near the beginning of this recent period of scientific discovery (1966) that Les Magoon was hired by Shell Oil Company to study source rocks in the Santa Barbara Channel, California. This was the beginning of a chain of events and experiences that would eventually lead to the concept of the petroleum system.
“When I was working for Shell, we would do source and migration studies,” explains Les. “The explorationists for Shell would say to me, ‘We already know there is oil here, why do we need to do more basin analysis?’ This was when I started to realise that we needed a better way to look at both the geology or the rocks and the geochemistry or the fluids.”
“While I was attending the AAPG Annual Meeting in Denver, Colorado in 1972, I listened to presentations by Wally Dow,” says Les. “He and Jack Williams at Amoco Research presented papers on the geochemistry of oil they collected in the Williston Basin. They were able to correlate crude oils to specific source rocks, which were key ingredients in their concept of oil systems.”
Les went on to work for the US Geological Survey in 1974, concentrating on oil and gas resource assessment. He quickly found that geology and geochemistry are trumped largely by statistics. By 1982, this led him to start developing a concept to help rank prospective areas.
“At first, the petroleum system concept was not well received,” says Les. “It met early resistance, but others would comment ‘this is important, pursue it’. I essentially started over and presented it as a series of poster sessions. The first was in 1987 at another AAPG Annual Meeting. Wally and I then organised a half-day session on the petroleum system for the 1991 AAPG Annual Meeting.”
The 1991 session was the big turning point for the concept. Three years later Magoon and Dow published AAPG Memoir 60, The Petroleum System – From Source to Trap. The memoir, designated a classic by AAPG, is now out of print but can be purchased on CD.
A Powerful Tool
“Using static snapshots like fairway maps fails to account for the timing of petroleum system events,” says Ken Peters, consulting professor at Stanford University. “Basin and petroleum system modelling software allows us to quantify the petroleum system concept. It can explain why traps are barren or filled with hydrocarbons and is a powerful tool in assessing exploration risk.”
An example from Alaska’s prolific North Slope will help to demonstrate how petroleum system modelling, through the use of event charts, can be used as a prediction tool. The prospect was named Mukluk and, prior to drilling, expectations ran high. The prospect was right on trend with the Prudhoe Bay Field along the Barrow Arch Fairway. The entire structure was leased in 1982 with the total high bids exceeding US$1.5 billion. At that time, a consortium of companies headed by BP touted that it contained more than 1.5 Bb of recoverable oil (any connection to the bidding?). In 1983, the consortium built a gravel island and drilled a $120 million well; still the most expensive dry hole ever drilled.
So What Happened?
“Our models indicate that oil accumulated in the Mukluk prospect, Prudhoe Bay, and other structures along the Barrow Arch, starting about 97 Ma,” says Ken Peters. “Overburden Brookian deposition (Cretaceous and Tertiary in age) occurred from the south-west to north-east across the North Slope. These episodes of uplift and burial caused eastward tilting along the Barrow Arch starting at 67 Ma. Another key to our modelling was the mapping of sandstone bodies deposited on the Lower Cretaceous Unconformity. The sands served as thief zones for the re-migration of hydrocarbons.”
“When modeling a basin or prospect, it is important to visualise what is actually happening through time,” says Ken. “Cross sections through the Mukluk High show present-day closure as well as one drawn at 75 Ma. However, the 41, 55, and 60 Ma sections show that petroleum migrated up dip along sands (Kuparuk ‘C’) deposited above the Lower Cretaceous Unconformity to the south-east, towards the present-day location of the Kuparuk Field. At Prudhoe Bay, the Ivishak Formation reservoir sandstone is in superposition with shale across the Lower Cretaceous Unconformity (LCU) that trapped the oil. Had this thief sand been present, Prudhoe oil could have ended up somewhere else as well.”
Donovan Krouskop, State of Alaska geophysicist, says, “The data around Mukluk was good quality and it’s offshore enough that permafrost/velocity issues are not a problem. The vertical resolution of the data is the limiting factor. They (BP’s geoscientists) could not see separate top and bottom reflectors of the Kuparuk sand, but would have seen an amplitude anomaly at the LCU level. I do not think that would have been enough to affect the decision to drill.”
Les Magoon agrees, saying, “Mukluk would have been drilled regardless of the timing because the prospect was so large. Exploration is full of risk and some prospects just beg to be drilled because one cannot be sure any evaluation is correct (until drilled).”
These two geoscientists have pointed out that a weakness in the petroleum system concept is the inability to actually predict volumes and the secondary processes that act over geologic time. The current state of art for 4D petroleum system modelling is that it is a great tool to better understand subsurface hydrocarbon generation, migration and accumulation. Using this approach, geoscientists can better predict the pod of active source rock and the timing of petroleum generation, thermal maturity, and migration pathways to possible traps, as has been pointed out in this article.
Ken Peters addresses the concept’s shortcomings this way, “Current 4D petroleum system modelling is limited in predicting volumes, compositions, and secondary processes. As seen in the Mukluk example, we can predict these things very accurately after the fact. We are hoping to address these questions with our industrial affiliates Basin and Petroleum System Modelling (BPSM) programme at Stanford University through long-term research.”
Ken concludes, “Computerised 4D modelling considers the relative timing of petroleum system events, processes and dynamics of associated fluids to better assess whether past conditions were suitable to fill reservoirs and survive to the present day. Understanding the total process through time could have a major impact on economies throughout the world.”
Author’s Note: I first met Les Magoon during the summer of 1980 working on the Alaska Peninsula. Under his guidance, we examined petroleum seeps, source rocks, and potential reservoirs rocks in an effort to understand the overall petroleum potential of the Shelikof Strait prior to a Federal OCS lease sale. After this work, I was certainly not surprised by his future publications on petroleum system and the overall acceptance of the concept.