Climate-Provenance Effect on the Organic Matter Enrichment of the Chang 9 Source Rocks in the Central Ordos Basin, China

Abstract

The Upper Triassic Chang 9 organic-rich sediments have been considered as effective hydrocarbon source rocks for the Mesozoic petroleum system in the Ordos Basin. Previous studies on the Chang 9 member mostly focused on the influence of their paleoproductivity and paleoredox conditions on the organic matter (OM) enrichment, whereas there are few studies on the influence of the paleoclimate condition and sediment provenance on the OM enrichment. In this study, a series of geochemical analyses was performed on the Chang 9 core samples, and their hydrocarbon generation potential, paleoclimate condition, and sediment provenance were assessed to analyze the effect of paleoclimate-provenance on OM enrichment. The Chang 9 source rocks are characterized by high OM abundance, type I−II OM type, and suitable thermal maturity, implying good hydrocarbon generation potential. Based on the C-values and Sr/Cu ratios, the paleoclimate condition of the Chang 9 member was mainly semihumid. In addition, the Th/Co vs. La/Sc diagram and negative δEuN indicate that the Chang 9 sediments were mainly derived from felsic source rocks. Meanwhile, the paleoweathering intensity of the Chang 9 member is moderate based on moderate values of CIA, PIA, and CIW, which corresponds to the semihumid paleoclimate. The relatively humid paleoclimate not only enhances photosynthesis of the primary producer, but also promotes chemical weathering intensity, leading to suitable terrestrial clastic influx to the lacustrine basin, which is beneficial for OM enrichment.

Provenance and Tectonic Setting of the Lower Cretaceous Huanhe Formation in the Northwestern Ordos Basin and Its Implications for Uranium Mineralization

“…Previous research suggested that the granite plutons deposited during the Variscan period had a uranium content of 7.8 × 10 –6 , and multiple uranium mineralization events have been identified within these plutons. 93 , 94 The Daqing Mountain Granite Pluton has an overall higher uranium content (4–8 × 10 –6 ), indicating that it has a uranium-enriched structure. 93 , 94 Scholars have identified an average Th/U ratio of 4.2 for mainland Chinese rocks.…”

“… 93 , 94 The Daqing Mountain Granite Pluton has an overall higher uranium content (4–8 × 10 –6 ), indicating that it has a uranium-enriched structure. 93 , 94 Scholars have identified an average Th/U ratio of 4.2 for mainland Chinese rocks. A Th/U ratio greater than 4.2 is considered indicative of significant uranium loss in rocks.…”