Influence of palaeoclimate and hydrothermal activity on organic matter accumulation in lacustrine black shales from the Lower Cretaceous Bayingebi Formation of the Yin’e Basin, China

Abstract

The lacustrine black shales of the Lower Cretaceous Bayingebi Formation are an important hydrocarbon source rock in the Yin’e Basin. It is important to understand the mechanism of organic matter accumulation in these shales to reconstruct the palaeoenvironment and palaeoclimate in the Early Cretaceous in northwestern China and search for potential oil and gas resources. Based on petrological, elemental geochemical and organic geochemistry analyses of typical field profile samples, the influence of palaeoclimate and hydrothermal activity on organic matter accumulation in different sedimentary units (Unit I, II and III) is revealed. Unit II has the highest total organic matter abundance (TOCavg = 5.95%), mainly type I kerogen (lamalginite), which is in the immature to low mature stage. Organic carbon isotope data (δ13Corg), element ratios (Sr/Cu and C-value) and weathering indexes (including CIA, CIW and PIA) together revealed that the region experienced a semihumid to semiarid palaeoclimate in the middle of the Early Cretaceous. The increase in rainfall in Unit II enhanced the abundance of lake algae and surrounding terrestrial vascular vegetation and elevated the lake level, which contributed to adequate sources and favourable preservation conditions for organic matter accumulation. Several lines of evidence, including high sulfur contents, the combination of vein pyrite and micritic dolomite, positive Eu anomalies, and element geochemical correlations, confirm the existence of intermittent low–temperature white smoker–type hydrothermal activity. The decrease of oxygen content in lake bottom water may be related to the input of hydrothermal fluid, which provided necessary nutrient elements for biological productivity. According to palaeontological and radioisotope dating data, it can be inferred that the black shales of the Bayingebi Formation are the product of the combined effect of global palaeotemperature rise in the early Aptian stage of the Early Cretaceous and intermittent low–temperature hydrothermal activity.