Page 4 - The Petroleum System From Source to Trap

PREFACE

Wallace G. Dow analytical results and our interpretation demon­
strated that oils from different source rocks can be
Tie petroleum system concept was first different and that oil-source rock correlations are
eveloped in 1970 at the Amoco research geologically meaningful.
laboratory in Tulsa, Oklahoma. In my first In the early 1970s, it was generally known, but
major geochemical study, I described three "oil largely ignored, that traps, reservoirs, seals, and
systems" in the Williston basin based on analytical source rocks were all required to make an oil accu­
data generated by Jack Williams and the geochem­ mulation. Most geologists knew a lot about traps
ical research group, headed by Jim Momper. The and reservoirs, little about seals, and virtually
purpose was to reduce risk by predicting the most nothing about source rocks. A few source rock
likely places where oil would be found and where papers had appeared in the 1960s by workers now
it most likely would be absent. accepted as pioneers in the field-Hunt, Philippi,
After graduating from Rutgers University in Tissot, and Vassoyevich-which served as a foun­
1959 with a B.A. in geology, three years in the dation for our work. We did the best we could in
military, and receiving an M.S. in geology from the an era before biomarkers, vitrinite reflectance,
University of North Dakota under Wilson Laird, I Rock-Eva! pyrolysis, capillary gas chromatog­
took a job with the Pan American Petroleum raphy, and most of the analytical techniques we
Corporation (now Amoco) in their Denver Division take for granted today. Despite these difficulties,
office. I was an exploration geologist, alternating Jack's oil-source rock correlations have survived
between project and well site work in the northern the test of time.
Rocky Mountain area. Jim Momper, a senior My job was to find ways to make this new
geologist I knew in Denver, had been transferred to geochemical information useful to Amoco's explo­
the Tulsa geochemical research group and asked ration effort in the Williston basin and eventually
me to collect crude oils whenever possible because to all petroleum provinces around the world. I
they needed samples to analyze. 1 collected over reasoned that if we knew where the oils came from
250 oil samples between 1966 and 1969, about half and how they migrated, we could better predict
of which were from the Williston basin. Because of where they would be found in the future.
my interest in geochemistry, Jim offered me a Geochemistry could then be used to high-grade
transfer to the Tulsa Research Laboratory to help areas in which to concentrate exploration activity,
him bring geologic insight into the then new thereby reducing risk.
science of petroleum geochemistry. I accepted the The first step was to map the stratigraphic and
challenge and arrived in Tulsa with my family on a areal distribution of each oil type. We were
snowy New Year's Eve in 1969 with little idea of fortunate to start with the Williston basin because
what the future would bring. I have been involved the three oil types are distinct here and the accumu­
in geochemistry ever since. lations of each type are isolated by evaporite seals. I
Jack Williams analyzed the oils I had collected identified three source-reservoir packages that I
using techniques that were far less sophisticated called "oil systems" and named them after their
than those in use today. The Williston basin oils principal source and reservoir rocks. Each oil
were clearly divided into three major genetic types system had an area of mature source rock,
with several subtypes and mixtures. The oil migration pathways, reservoirs, traps, and seals.
compositional differences indicated that three The concept depended on the ability to separate
separate source rocks were involved. Cores from oils into genetic types, correlate each type of a
all available organic-rich rocks in the basin were specific source rock, estimate the quantity of oil
solvent extracted, and the extracts were analyzed generated and expelled from the source rock, and
with the same techniques used on the oils. Ordovi­ map the vertical and lateral migration pathways
cian rock extracts positively correlated with the oil through which the oil moved. This study led us to
found in Ordovician and Silurian reservoirs rocks, conclude that the combination of geology and
Bakken shale extracts were very similar to the oils geochemistry would become a powerful explo­
in Mississippian and Devonian reservoirs, and ration tool. We did not know it at the time, but our
extracts from Tyler shales compared favorably to work had predicted most of the successful
oils produced from Pennsylvanian Tyler reservoirs. Williston basin oil plays of the 1970s and 1980s and
Extracts from other organic-rich rocks lacked simi­ that little or no oil would be found in areas that we
larity to any of the oils I had collected. These considered high risk.

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