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.
Highlights
The Ordos Basin, as the main continental petroliferous basin in central China, shows tremendous potential for oil and gas resources in the Mesozoic petroleum system [1, 2]
Previous studies on the Chang 9 organic matter (OM) enrichment mechanism were mainly based on the paleoproductivity and paleoredox conditions, and the results have shown that the Chang 9 organic-rich sediments were deposited in a suboxic semideep lacustrine environment of fresh-brackish water mass with
The black shales in three units are characterized by high total organic content (TOC) varying between 3.79% and 28.96% (Unit I black shales averaging 6.8%, Unit II shales averaging 14.45%, and Unit III averaging 4.31%), whereas the gray mudstones have low TOC ranging from 0.37% to 2.04%
Summary
The Ordos Basin, as the main continental petroliferous basin in central China, shows tremendous potential for oil and gas resources in the Mesozoic petroleum system [1, 2]. The hydrocarbon resource in the Ordos Basin is mainly derived from the Yanchang Formation, which is considered as the most important hydrocarbon source rock for the Mesozoic petroleum system, especially for the Chang 7 member [3,4,5]. In recent years, an increasing number of studies suggest that the Chang 9 organic-rich sediments have a great contribution to the oil and gas resource enrichment [6,7,8]. Previous studies on the Chang 9 OM enrichment mechanism were mainly based on the paleoproductivity and paleoredox conditions, and the results have shown that the Chang 9 organic-rich sediments were deposited in a suboxic semideep lacustrine environment of fresh-brackish water mass with
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