Peculiar Berriasian “Wealden” Shales of northwest Germany: Organic facies, depositional environment, thermal maturity and kinetics of petroleum generation

Abstract

This study provides new organic geochemical, organic petrological and isotope data on the Berriasian Wealden Shales, northwest Germany as well as new kinetic data on petroleum generation determined both for kerogen concentrates and whole rock samples. The kinetics were applied in numerical petroleum system models for two wells: one in the western Lower Saxony Basin (LSB) and the other one in the Pompeckj Block further north. The study increases the understanding on rate, conversion and timing of petroleum generation and is consistent with oil fields and prospects in the area, which was not the case in previous studies.The first facies of the Wealden 3–4 members is partly very rich in organic matter (TOC up to 21.85 wt%), the organic matter being composed almost exclusively of thin lamalginites. The second organic facies is characterized by lower TOC values (3.86–10.73 wt%) containing both Botryococcus algae and lamalginites. High atomic H/C and C/S ratios as well as the occurrence of Botryococcus algae prove a brackish–freshwater depositional environment with high HI values, which are typical of type I or type II kerogen. Broad n-alkane distributions without odd-even predominance are surprising in view of vitrinite reflectance, HI and Tmax data, which are typical of immature or early mature kerogen.Several biomarker indices indicate dysoxic/anoxic bottom water conditions and possibly water stratification, while the low steranes/terpanes ratio suggests a significant bacterial contribution to the organic matter. Low δ13C values (−28.5 to −34.5‰) are in line with previously obtained data for the Wealden and support the presence of microbial-derived kerogen.Low Pristane/Phytane, low Pristane/nC17 and Phytane/nC18 ratios and the absent odd-even predominance are regarded as a result of kerogen being derived from microbial rather than algal biomass; it is not related to thermal maturation. Lamalginite is in this case probably derived from microbial mats producing no chlorophyll and little sterane precursors. Deeper in the water or in the sediments, denitrification of the bacterial biomass took place leading to extraordinarily low nitrogen/carbon ratios and extraordinarily high ẟ15N values. The occurrence of H2S in the lower water column resulted in excellent organic matter preservation leading to very high TOC values (up to 21.8 wt%). Abundant framboidal pyrites indicate a microbial pyrite origin.The burial, thermal and maturity history calibrated by Easy%RoDL reveals that more erosion is required than assumed in previous studies to explain the present-day maturation in the western LSB and Pompeckj Block. In both wells (A, B), the Wealden source rocks experienced sufficient temperatures for hydrocarbon generation and primary migration. Kinetics obtained from kerogen concentrates indicate earlier generation (i.e. at slightly lower temperatures) than those from whole rock samples. The Botryococcus algae facies reveals lower transformation ratios at a given temperature history, suggesting that most of the oil was generated by the lamalginite facies. Significant and late (~40 million years ago; Lutetian) kerogen-to-oil conversion in the northern part of the study area lowers the charge risk in future conventional oil exploration.