Log-facies and log-correlation of Cenomanian sediments in the Molasse Basin (Upper Austria)

Arno Rech

Research output: ThesisMaster's Thesis

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Abstract

The Cenomanian in the Upper Austrian part of the Molasse Basin is an important clastic reservoir horizon for oil. In the present thesis this formation is characterized on the basis of geophysical well logs. For this purpose log facies and well log correlation profiles have been interpreted. The thesis mainly concentrates on well log data for 58 wells kindly provided by Rohöl-Aufsuchungs AG. In addition, spectral core gamma measurements have been performed using cores representing Cenomanian units from 30 wells. In a first step, base and top of the Cenomanian sandstones (“Regensburg Formation" sensu Niebuhr et al., 2009) as, well as a maximum flooding surface within overlying Upper Cenomanian to Turonian marls (Eibrunn Formation sensu Niebuhr et al., 2009) have been defined and laterally correlated. Considerably varying log patterns of the Cenomian sandstones from west to east indicate lateral facies changes. Based on the detailed correlation between well gamma log and core gamma data, cores together with measured total and spectral core gamma logs were exactly depth shifted. Thus, changes in lithology observed in the cores could be directly related to their well log response. This allows an interpretation of lithology and subunits in boreholes where log data, but no cores are available. The Cenomanian of the Upper Austrian Molasse Basin overlies different geological units. In the easternmost area the Cenomanian sediments overlie crystalline basement units of the Bohemian Massif, whereas in the western area they overlie Upper Jurassic carbonates. Locally terrestrial sediments, partly representing karst deposits (Schutzfels Formation), are intercalated between the Jurassic carbonates and the Cenomanian sandstones. Correlation panels and thickness maps demonstrate a basinward (SW-ward) increase in thickness of the oldest Cenomanian units (“Saal Member” sensu Niebuhr et al., 2009). Within the study area the thickness of the Saal Member ranges generally between 10 and 30 m. Within the Saal Member the gamma log is a poor tool for the estimation of shale contents. This is because the gamma response is strongly influenced by varying glauconite contents of the near shoreline marine sandstones. Moreover detailed investigations of spectral gamma logs suggest that the total gamma values are often dominated by increased thorium and uranium contents. On the basis of sonic and resistivity logs the Saal Member correlates with the informal units CE3 and CE2 of Rohöl-Aufsuchungs AG, at least in the Trattnach and Voitsdorf fields. The Saal Member is overlain by lower energetic sandy-silty, often carbonate cemented sandstones (Bad Abbach Member sensu Niebuhr et al., 2009). Its base is defined by a characteristic marker bed in the area of the Trattnach Field. So far it has also been called “red layer”, due to colorful red shaded gravels. The exact correlation of core and well gamma logs shows that the red layer may be part of the marker bed, but does not necessarily define it. The Bad Abbach Member is characterized by high sonic velocity, high resistivity and in most wells by gamma log values, which are significantly lower than those from the Saal Member. The thickness of the Bad Abbach Member reaches a maximum (12-15 m) along a SE-NW trending zone and decreases towards the basin (SW-ward) and towards the paleo-shore line (NE-ward). The overlying Eibrunn Formation represents the uppermost Cenomanian to Lower Turonian units and comprises a maximum flooding surface. It is located 4 to 15 m above the Cenomanian sandstones of the Regensburg Formation. The sediment package between Top Regensburg Formation and the maximum flooding surface shows, for most parts of the study area, a remarkable increase in thickness from SW to NE.
Translated title of the contributionLog-Fazies und Log-Korrelation im Cenoman des Molassebeckens (Oberösterreich)
Original languageEnglish
QualificationDipl.-Ing.
Awarding Institution
  • Montanuniversität
Supervisors/Advisors
  • Sachsenhofer, Reinhard, Supervisor (internal)
Award date16 Dec 2011
Publication statusPublished - 2011

Bibliographical note

embargoed until 19-09-2016

Keywords

  • Molasse Basin
  • Cretaceous
  • Cenomanian
  • Log correlation
  • Spectral Gamma Log
  • Log facies
  • Austria
  • Regensburg Formation
  • Eibrunn Formation
  • Trattnach Field

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