Deciphering the Source and Formation Mechanism of Immature Crude Oils in the Paleocene Linhe Formation, Hetao Basin

Abstract

The discovered crude oil in the Linhe Formation of the Hetao Basin is a typical immature oil, with proven petroleum geological reserves exceeding 200 million tons, making it one of the most successful cases of low-maturity oil exploration in China in recent years. However, the organic matter sources and hydrocarbon generation mechanisms remain incompletely understood because of limited data. Geochemical analyses were systematically conducted on 52 core samples and 6 immature oil samples from the Linhe Formation in the Bayannur Sag of the Hetao Basin, employing organic petrography, trace element analysis, and Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) to evaluate maceral composition, sedimentary paleoenvironmental conditions of source rocks, and generation mechanisms of immature oil. The results indicate that the source rocks are generally of good quality (TOC > 1%), containing predominantly Type I–II1 organic matter at an early maturation stage. Organic petrographic analysis reveals that liptinite (55.4–90.2%), comprising alginite, sporinite, and bituminite, constitutes the primary organic component with alginite and sporinite serving as the key hydrocarbon-generating macerals. FT-ICR MS analysis shows that the oils are enriched in oxygen-bearing compounds (N1O1, O1, O2, and O3), particularly in crude oils with lower maturity, where O2 species with a double bond equivalent (DBE) of 1 (e.g., readily degradable fatty acids) are especially abundant. Geochemical indicators suggest that deposition of these high-quality source rocks occurred in brackish to saline water under reducing to strongly reducing conditions, with some intervals experiencing sulfidic environments. The study identifies early hydrocarbon generation from liptinite under highly reducing conditions as the primary mechanism for immature oils in the Linhe Formation.