Verlag des Forschungszentrums Jülich
JUEL-3094
Muscio, Gary Patrick Albert
Die Entwicklungsgeschichte von Erdöl und Erdgas in einem natürlichen System mit stark eingeengter Geometrie am Beispiel des Bakken-Erdölsystems
167 S., 1995
Abstract
When petroleum is produced from its reservoir at present day, it has already been submitted to a
complex array of natural processes, which have left their imprint on its chemical and physical
properties. These processes involve the generation of petroleum in its source rock as a function of
thermal evolution, and its migration to the reservoir. In order to achieve a better understanding of the
effects that these processes have on the nature and occurrence of petroleum, it is crucial to investigate
a study area that represents an integrated system encompassing all processes from the beginning of
crude oil formation to entrapment. The Bakken Shale petroleum system of the Williston Basin
(U.S.A./Canada) appears to fulfil these requirements as it covers a broad range of maturity
incorporating an stages of catagenesis, and source and reservoir are closely associated, i.e. Bakken
petroleum has not been submitted to long secondary migration routes. Combined with a basinwide
uniformity in kerogen type, the latter feature constrains the broad scope of potential influences, and
therefore the Bakken petroleum system is an ideal candidate to study petroleum generation, its
expulsion and migration under natural conditions.
By using a selected set of wens/core samples which were considered to be representative for the entire
Bakken petroleum system based on comprehensive screening analyses on both kerogen and bitumen,
the present study focussed on the following principal aspects: (I) Evolution of petroleum generation
as a function of maturation; (2) Primary migration and distribution of crude oil in a constrained
natural system; (3) Occurrence of gas in the immature zone; (4) Evaluation of the natural maturity
series with simulation experiments.
Mass balance calculations on Bakken Shale samples have revealed that the main phase of petroleum
formation took place very early during catagenesis (ca. 0.4% to 0.8% Ro) and that its potential has
already been realized in those areas of the basin which currently produce from the Bakken reservoirs.
Expulsion efficiencies appear to be very high as well, whereby diffusion of hydrocarbons via a welldeveloped
and continuous organic matter network may have played an important role, as suggested
by compound-specific fractionation effects encountered especially in n-alkane products. Such
efficient removal of petroleum from the shale units indicates that formations over- and underlying the
Bakken Shale may represent the principal reservoirs and that the generalized postulate of an in-source
reservoir has to be reconsidered. The hypothesized process of reimpregnation of petroleum from the
reservoir back into the source rock system may account for locally occurring high concentrations of
solvent extractable organic matter in Bakken Shales. The absence of a typical oil generation curve
during the main stage ofcrude oil formation may elucidate that abnormally high reservoir pressures
encountered in the Bakken Formation are not a basinwide phenomenon, but may be restricted to local
areas.
The discrepancy between the maturity of peak expulsion and the maturity of crude oils based on
molecular parameters implies that the high-quality, light oil character is controlled by significant
post-generation and post-expulsion in-situ thermal alteration. This process was supported by the
restricted lateral and vertical migration ranges in the Bakken petroleum system.
Despite being classified as a typical hydrogen-rich kerogen from bulk chemistry and petrology, the
Bakken Shale shows an enhanced capability of generating gaseous hydrocarbons throughout
maturation. At low levels of thermal evolution « 0.7% Ro) , the gas apparently is adsorbed on the
organic matter. The coincidence of decreasing gas yields with the main phase of hydrocarbon
expulsion suggests that low-molecular-weight compounds playa vital role in expulsion mechanisms
in that high-molecular-weight species are dissolved in the light products and removed via diffusive
transport.
The natural and artificial maturity sequences both have quantitative and qualitative features in
common. The maturity related evolution of individual ratios in the residues of both lab and natural
systems allow the calibration of the latter in terms of maturity zones. This approach is useful for the
compositional prediction of natural products in the absence of expulsion and migration effects.
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