25th International Meeting on Organic Geochemistry IMOG 2011

O-48
Evolution of petroleum composition during generation and
expulsion - a case study on the Bakken Formation
Philipp Kuhn, Brian Horsfield, Rolando di Primio
GFZ German Research Centre for Geosciences, Potsdam, Germany (corresponding author:kuhn@gfzpotsdam.de)
The Devonian-Mississippian Bakken Formation of the
Williston Basin in central North America is generally
seen as a prime example of a self contained
petroleum system. It consists of the productive
reservoir member encased within the two source
rocks of the upper and lower Bakken Formation. A
distinguishing stratigraphic property of the Bakken
petroleum system is the immediate juxtaposition of
source and reservoir. This, coupled with negligible
influence of biodegradation, secondary cracking and
mixing from external sources make the Bakken
Formation a natural laboratory for studying expulsion
and retention phenomena, and their effects on
petroleum composition.
Based on a core sample set of more than 250
samples of all members of the Bakken and adjacent
formations from 22 wells, covering a natural maturity
range from 0.35 to 1.1% Ro as well as 27 produced
oil samples, the generation behaviour, the mass and
composition of the retained, expelled and produced
bitumen/petroleum was analyzed.
As a first step the transformation of the kerogen and
the evolution of the properties of the generated
hydrocarbon throughout the entire oil window were
characterized by Pyrolysis gas chromatography,
Rock-Eval pyrolysis (RE) and TOC measurements.
Further, the mass and composition of the retained
bitumen and its changes with maturity were
monitored: based on the S1 of RE and the bulk yield
of Thermovaporization gas chromatography a
maturity independent, low variability of the retained
bitumen per mass unit TOC was documented, which
remains below 8% of the original generation potential.
Thus, the Bakken source intervals are excellent
expellers, in agreement with earlier mass balancing
results [1+2] .
The extracted bitumen was further separated into four
fractions namely asphaltene (ASP) by precipitation,
and the aliphatic (SAT), aromatic (ARO), and hetero
compound (NSO) fractions (together comprising the
maltenes) using medium pressure liquid
chromatography [3] .
With increasing maturity the NSO yield falls from 60%
of the maltenes to about 30%. The SAT fraction
displays the exact opposite trend, with values
increasing from 20% of the maltene yield at the lowest
maturity to beyond 50% in the well exhibiting the
highest maturity. The ARO yield of the maltenes
varies between 15 and 30% without a significant trend
throughout the examined maturity range. The
asphaltene concentration reached a maximum of 30%
at a maturity of 433°C Tmax then dropping to 3% of the
retained bitumen at the highest maturity.
In contrast to the retained composition, extracted
material from the reservoir member did not display
any maturity related trends. In all sample extracts the
SAT fraction yield was dominant with ~60%, followed
by the ARO and NSO (10–30% each) and a low
percentage of ASP fraction (