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Van Tuyl Lecture: Petroleum Systems Analysis in the Alpine Foreland Basin
February 8, 2018 @ 4:00 pm - 5:00 pm
Reinhard Sachsenhofer, Montan Universitat Leoben
241 Berthoud Hall
“Petroleum Systems Analysis in the Alpine Foreland Basin”
Abstract: The North Alpine Foreland Basin (Molasse Basin) in Central Europe is a mature oil and gas province. Nevertheless, significant new discoveries have been made in the recent years. The improved understanding of petroleum systems contributed significantly to the exploration success.
Two petroleum systems are distinguished in the Austrian part of the NAFB: a Lower Oligocene – Mesozoic/Eocene oil and thermogenic gas system and an Oligocene-Miocene microbial gas system. In the last years, the work of several research groups from both academia and industry furthered the understanding of different petroleum geological aspects in its Austrian sector. Based on these studies, the present contribution provides a comprehensive overview of the petroleum systems.
Lower Oligocene organic-rich successions (up to 12 %TOC; HI: 400-600 mgHC/gTOC), capable of generating slightly more than 1 t of hydrocarbons/m², are the source rocks for the thermogenic petroleum system. Their distribution is controlled by submarine mass movements, redeposition of transported material beneath the Alps and tectonic erosion. Hydrocarbon generation commenced during Miocene time, with the oil kitchen being located south of the Alpine thrust front. Hence, lateral migration distances of up to 50 km were required to charge the mainly Eocene and Cenomanian non- and shallow-marine sandstone reservoirs. Hydrocarbons are mainly trapped in structures related to E-W trending normal faults, while differences in source rock facies led to the development of a western and an eastern oil family. Surprisingly, associated gas of most fields contains varying (and partly very high) percentages of primary and secondary microbial methane. In addition to biodegradation, the oil composition of some oil fields is influenced by water washing. Finally, post-Miocene basin uplift had further effects on biodegradation and the consequent formation of secondary microbial gas, and resulted in re-migration as well.
The Puchkirchen Group and the Hall Formation provide both, source and reservoir rocks, for the microbial petroleum system. TOC contents (<1.0 %) and HI values (<140 mgHC/gTOC) of the pelitic source rocks are typically low. Microbial gas was generated through organic matter decomposition during early diagenesis and subsequently fixed in gas hydrates. Decomposition of gas hydrates during deeper burial led to the initial filling of reservoirs, while later mixing of microbial gas with thermogenic gas and condensates is a common feature. However, biodegradation of most gas samples precludes the determination of the fraction of thermogenic hydrocarbons. As the reservoir rocks were deposited along a deep-marine channel belt along the basin axis, reservoir quality strongly depends on the position within the channel belt. Gas is generally trapped in compaction anticlines or at the channel margin pinch-out. Additional traps are formed by imbrication structures.