Depositional environment and petroleum potential of Oligocene rocks in the Waschberg Zone (Austria)

Abstract

Oligocene successions in the North Alpine Foreland Basin (NAFB) and the Western Carpathians reflect Paratethys-wide paleogeographic changes, which also control their petroleum potential. Whereas these rocks have been studied in detail in both areas, the transition zone is still under-researched. In order to fill this gap, the Oligocene succession in the Waschberg Zone, comprising the Ottenthal Formation (NP21–23) and the overlying Thomasl Formation (NP23–24) has been studied using outcrop (Waldweg section) and borehole samples (Thomasl, Poysdorf) and a multidisciplinary approach. The Ottenthal Formation is subdivided from base to top into marls and shales (Ottenthal Mbr.), diatomaceous shales (Galgenberg Mbr.) and marlstones (Dynow Mbr.). Biogenic silica contents, determined using atomic absorption spectroscopy, reach 30 wt. % in the carbonate-free Galgenberg Member, but also in the Dynow Members, which is characterized by upward decreasing productivity of calcareous nannoplankton. Close lithological relations exist with the Oligocene succession in the NAFB, but diatoms are largely missing in the latter. Organic matter contents are surprisingly low in the Ottenthal and Thomasl formations in the Waldweg section, which therefore are poor hydrocarbon source rocks. In contrast, the Thomasl Formation, encountered in the Thomasl and Poysdorf boreholes, holds a fair to good hydrocarbon potential (~ 2.2–2.5 wt. % TOC; type III and type II kerogen) and may generate 1.0 to 1.6 tons of hydrocarbons/m2. Obviously TOC contents of borehole samples are significantly higher than in outcrop samples. Because of severe indications of weathering (e.g., presence of gypsum and jarosite), a detrimental effect of weathering on the samples from the Waldweg section cannot be excluded. Biomarker data suggest a nearshore depositional environment with changing oxygen-availability and salinity. Vitrinite reflectance measurements show that the investigated sections are thermally immature.

Geochemical characteristics, thermal maturity and source rock potential of the Oligocene Šitbořice Member of the Menilite Formation in the Ždánice Unit (Czech Republic)

“…The pre vail ing im ma tu rity was sup ported by vitrinite reflectance (R r ) mea sure ments and Rock-Eval T max val ues in all of Mod i fied ac cord ing to Krhovský et al (2001) for Czech ter ri tory and ac cord ing to Wag ner (1996,1998) for Aus trian ter ri tory; stages of the Cen tral Paratethys are ac cord ing to Rögl (1998); NP Zones are ac cord ing to Berggren et al (1995) the Eggerding, Zupfing and Thomasl for ma tions (Sachsen hofer et al, 2010;Pupp et al, 2018). Ox y gen-re stricted con di tions were in di cated by the TOC/TS ra tio in the Eggerding and Zupfing for ma tions (Sachsenhofer et al, 2010) and sup ported by biomarkers in the Thomasl For mation within the Ottenthal sec tion (Pupp et al, 2018). Strongly oxy gen-de pleted con di tions were in di cated by biomarkers in the same for ma tion in the Thomasl well (Pupp et al, 2018).…”

“…Ox y gen-re stricted con di tions were in di cated by the TOC/TS ra tio in the Eggerding and Zupfing for ma tions (Sachsenhofer et al, 2010) and sup ported by biomarkers in the Thomasl For mation within the Ottenthal sec tion (Pupp et al, 2018). Strongly oxy gen-de pleted con di tions were in di cated by biomarkers in the same for ma tion in the Thomasl well (Pupp et al, 2018).…”

“…Both the Eggerding and the lower part of the Zupfing for ma tions are con sid ered to be mi nor source rocks (Schulz et al, 2002;Sachsenhofer et al, 2010). Ac cord ing to Pupp et al (2018), the Thomasl For ma tion encoun tered in the Poysdorf and Thomasl wells holds a "fair" to "good" source rock po ten tial which is in con trast with the "poor" po ten tial of the same for ma tion of the Ottenthal sec tion.…”

“…Kerogen type II and III within the lower part of the Eggerding For ma tion (Schulz et al, 2004;Sachsenhofer et al, 2010) and pre dom i nantly of type III within the rest of the Eggerding (Schmidt and Erdogan, 1996) and Thomasl for ma tions (Pupp et al, 2018) are in di cated by Rock-Eval hy dro gen in dex (HI) values. The pre vail ing im ma tu rity was sup ported by vitrinite reflectance (R r ) mea sure ments and Rock-Eval T max val ues in all of Mod i fied ac cord ing to Krhovský et al (2001) for Czech ter ri tory and ac cord ing to Wag ner (1996,1998) for Aus trian ter ri tory; stages of the Cen tral Paratethys are ac cord ing to Rögl (1998); NP Zones are ac cord ing to Berggren et al (1995) the Eggerding, Zupfing and Thomasl for ma tions (Sachsen hofer et al, 2010;Pupp et al, 2018). Ox y gen-re stricted con di tions were in di cated by the TOC/TS ra tio in the Eggerding and Zupfing for ma tions (Sachsenhofer et al, 2010) and sup ported by biomarkers in the Thomasl For mation within the Ottenthal sec tion (Pupp et al, 2018).…”

“…The Køepice-5 bore hole gamma log in clud ing its strati graphic de scrip tion (Stráník, 1979;Stráník et al, 1981) is at tached for more de tailed sub di vi sion of the for ma tion; cores from Nevojice-1 and Mouchnice-2 bore holes (sym bol ized as col oured rect an gles) were cor re lated to the pro file of Mouchnice-1 as an analogousical se quences trian Molasse Ba sin and Waschberg Zone (Schmidt and Erdogan, 1996;Sachsenhofer et al, 2010;Gusterhuber et al, 2013;Pupp et al, 2018). The to tal sul phur con tent of the sam ples stud ied ranges from 2.1 to 4.4 wt.% and does not show a sin gle trend.…”