Geochemical characteristics of lacustrine sediments : the Miocene Nördlinger Ries crater lake, Germany and the Cretaceous Songliao Basin, northern China

Zhao, Zibin; Littke, Ralf (Thesis advisor); Sachsenhofer, Reinhard (Thesis advisor)

Aachen : RWTH Aachen University (2022)
Dissertation / PhD Thesis

Dissertation, Rheinisch-Westfälische Technische Hochschule Aachen, 2022

Abstract

Many studies have been conducted on lacustrine sediments due to their economic importance, e.g. as petroleum source rocks, and as important archives of paleoenvironmental and paleoclimatic conditions. This thesis focuses on the characterization of two sets of lacustrine sediments having distinctly different thermal maturities; immature Miocene Nördlinger Ries sediments and the highly mature Cretaceous sedimentary rocks from the Songliao Basin, northeastern China. For the latter, comprehensive geochemical, petrographic and numerical modeling analyses are presented based on 56 shale core samples, which were obtained from the Middle-Lower Qingshankou Formation (K2qn1+2, Upper Cretaceous) in the eastern Changling Sag, Songliao Basin. A warm climate and increasing paleo-temperatures are deduced based on decreasing K/Al ratios upwards the K2qn1 section. Bottom water anoxia prevailed during deposition of the upper K2qn1 and lower K2qn2 as revealed by low total organic carbon/total sulfur (TOC/TS) values. The highest rate of siliciclastic input occurred in the lower K2qn1 as revealed by high TiO2/TOC and Al2O3/TOC ratios which also indicate high nutrient supply. The studied section is within the late oil window with vitrinite reflectance values ranging from 1.03-1.09% and Tmax (peak pyrolysis yield temperature) values above 445 °C; about 78% of the initial kerogen has been transformed to petroleum as calculated based by open-system pyrolysis kinetics and numerical modeling. OM (organic matter) consisted originally of type I-II kerogen, but has lost a large percentage of its original hydrocarbon generation potential upon maturation. The oil saturation index (OSI) values suggest that the studied layers show a fair shale oil potential but are regarded as good conventional petroleum source rocks. A newly proposed shale oil potential index (SOPI) takes also thickness into account, suggesting that the lower K2qn2 holds the highest shale oil potential. In addition, 176 immature Miocene post-impact sediments from the Nördlinger Ries crater basin were studied by a series of detailed organic geochemical, petrologic and other methods. The marginal Miocene post-impact sediments are correlated with previously defined subunits in the central crater based on chemostratigraphic profiles. Deposits from the central well are much thicker and contain mainly hydrogen-rich type I kerogen with a mean hydrogen index (HI) of 447 mg HC/g TOC, whereas the marginal sequence contains type II-III kerogen with an average HI of 206 mg HC/g TOC. A decreasing paleo-temperature trend is suggested by overall decreasing retene/(retene+cadalene) ratios, ending up with an annual mean temperature of 25 °C as suggested by data on glycerol dialkyl glycerol tetraethers. Stratified, anoxic and hypersaline water conditions prevailed in the central crater as reflected by low TOC/TS ratios (3.4 on average), δ13C values (-26.7‰ on average for phytane), low α-/total methyl-trimethyltridecyl chromans (<0.6), pristane/phytane (Pr/Ph <0.2), and the occurrence of des-A-lupane. Salinities decreased during deposition of the uppermost Miocene units and were also lower towards the crater margin. Halophilic red algae/plankton prevailed in the bio-community as revealed by abundant C27 steranes, while terrestrial material and aerobic bacteria significantly contributed to the OM only in the latest stages of lake evolution as deduced from abundant vitrinite/inertinite, high DTPs (diterpenoids)/ C27 sterane (up to 255) and low C27-29 regular steranes/C29-32 hopanes (<0.5) values. Alkaline conditions possibly enhanced the vulcanization of OM. Thirty four kerogen concentrates from the Miocene post-impact lacustrine sediments in the Nördlinger Ries were studied by elemental, pyrolytic, infrared spectroscopy and mineralogical methods. Presence of type I kerogen is confirmed by high atomic H/C ratios (1.6 on average), HI values (mostly >600 mg HC/g TOC), and spectral A- and C-factor values (mostly >0.8), but low oxygen index values (OI mostly < 25 mg CO2/g TOC). Type III kerogen occurs in the uppermost central and the lowermost marginal crater lake sediments with low H/C (<1), HI (<300 mg HC/g TOC), spectral A- and C-factor values (<0.6), but high OI values (>40 mg CO2/g TOC). The mineral matrix effect induced by abundant clay minerals and zeolites significantly reduces the HI values of the whole rock; those of kerogen concentrates are much higher on average. Type I sulfur-rich kerogen occurs in the basal central units, and occasionally in the marginal setting with high atomic Sorg/Corg > 0.04 and pyrolytic thiophenes/toluene > 13.5. Abundant pyrolytically generated alkanes/alkenes, high spectral A-factors and CH2/CH3 ratios suggest prominent algal input to OM for most of the samples. The aliphatic chain length of kerogen decreases and the branching degree increases from the older, deeper to the younger, shallower deposits. While oxygen-bearing organic constituents are common, nitrogen-bearing moieties are rare. A structural scheme is finally established for type I sulfur-rich kerogen.

Institutions

• Division of Earth Sciences and Geography [530000]
• Institute for Geology and Geochemistry of Petroleum and Coal [532410]