Biomarker paleo-reconstruction of the German Wealden (Berriasian, Early Cretaceous) in the Lower Saxony Basin (LSB)

Abstract

During the Early Cretaceous (Berriasian; Wealden 3–4), Northwestern Germany was covered by an east–west elongated tentatively brackish lake in which locally more than 700 m-thick black shales were deposited. While the distribution of organofacies’ in the basin is relatively well documented, the paleoenvironmental conditions in the basin center (e.g., occurrence and spread of water column stratification) and the spatial record of biomarkers in Wealden 3–4 shales (and coals) are rarely known. We here present respective data from the entire basin. In large areas, total organic carbon (TOC) contents are above 5 wt% and HI values above 700 mg hydrocarbons (HC)/g TOC, supporting the high potential of shales in the central basin as petroleum source rocks. Furthermore, bulk geochemical data as well as biomarkers clearly mirror the Wealden 3–4 facies distribution with the differentiation of a predominantly terrestrial setting east of the Weser River and an aquatic and brackish lake setting in the west. Certain biomarkers such as isorenieratane, specific for green sulfur bacteria, indicate that the basin consisted of a permanently stratified water column with a brackish/marine deep water body and an oxic–anoxic transition zone in the photic zone. In the southwestern gate of the lake (including the Isterberg area) and towards the east, no water column stratification developed. Characteristic of Wealden 3–4 black shale organic matter are: high relative abundances of saturated versus aromatic hydrocarbons (most likely due to high Botryococcus algal input), highly negative δ13C values in the extract fractions, low isotopic “canonical variables” (sensu Sofer in AAPG Bull 68:31–49, 1984), and high gammacerane, dinosterane, and C35-homohopane relative abundances. Interpreting those data, the different sub-facies of the environmental setting can be excellently documented. Particularly, in the western part of the basin, Wealden 3–4 shales are important petroleum source rocks. However, an overlap of biomarker signatures with those from Jurassic Posidonienschiefer Formation (“Posidonia”) shales from the same area shows that oil–source rock correlations in this area remain challenging.