Definition of Top-crystalline basement in the Upper Austrian Molasse Basin

Gloria Thürschmid

Research output: ThesisMaster's Thesis

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Abstract

In cooperation with Rohöl-AufsuchungsAG (RAG) a new log-based method is developed to identify precisely the depth of the top of the crystalline basement (Top-XX) within the Austrian Molasse basin. Currently, different geophysical logs are used within the company with different emphasis. This results in varying depth estimates for Top-XX. Among the logs available (GR, SP, photoelectric effect, caliper, sonic, porosity, resistivity, density and FMI logs), resistivity logs are predominantely used to define the boundary. Within the frame of this diploma thesis, six representative wells have been analyzed, whereof two wells provide each a crystalline basement overlain by sandstone horizons of Eocene (HIER-002A, MLRT-003C), Cenomanian (BH-N-001, BH-N-002) or Jurassic age (KH-003, V-037). Based on detailed core inspections and thin sections, Top-XX as well as the lithology above and below that boundary have been determined. To determine log depths of the top of the crystalline basement, (total and spectral) core gamma ray (Core-GR) measurements were performed. The result of this study indicates, that the well logging signal is influenced by several factors, like the lithology, as well as heavy mineral contents at the base of the overlying sedimentary succession, but also the lithology and the degree of weathering of the crystalline basement (magmatic versus metamorphic). Therefore, a general log pattern across the Top-XX in the investigated wells cannot be observed. However, close inspections show that all wells including Mesozoic sandstones overlying plutonic rocks exhibit high values of total GR at the base of the sandstones and significantly lower values of total GR within the uppermost part of the crystalline basement. Spectral Core-GR measurements indicate that high GR values are caused by heavy minerals (high contents of Th, U), whereas low GR contents result from weathering of the crystalline basement (removal of K). In contrast to Mesozoic sandstones, Eocene sandstones provide a positive correlation of total GR values and the content of potassium. In this case, heavy minerals are not dominating GR values. According to the lithology of the crystalline basement, GR values are either increasing (metamorphic basement) or do not show significant changes (plutonitic rocks). Further investigations of different wells are necessary to determine if this is a general trend. In summary, significant changes at the top of the crystalline basement are primarily visible in the GR log.
Translated title of the contributionBestimmung der Oberkante des Kristallins im Untergrund der oberösterreichischen Molassezone
Original languageEnglish
QualificationDipl.-Ing.
Supervisors/Advisors
  • Troiss, Wilma, Co-Supervisor (external), External person
  • Sachsenhofer, Reinhard, Supervisor (internal)
Award date25 Jun 2010
Publication statusPublished - 2010

Bibliographical note

embargoed until 17-05-2015

Keywords

  • top-crystalline basement Upper Austrian Molasse Basin log patterns

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