Organic geochemical compositions of Mesoproterozoic source rocks in the Yanliao Rift, Northern China

Abstract

Organic-rich Mesoproterozoic rocks from the Xiamaling and Hongshuizhuang formations, with a moderate thermal maturity within the oil window, are well preserved in the Yanliao Rift, Northern China. Their organic geochemical compositions, especially biomarkers, organic matter origin, and depositional environment, are discussed in detail using GC, GC–MS and GC-MS-MS techniques. The results indicate that the Xiamaling and Hongshuizhuang samples share similar molecular compositions, which are distinct from those of Phanerozoic bitumens, suggesting a unique microflora in the North China sea during the Middle Mesoproterozoic (1.45–1.30 Gyr). The biomarkers in the saturated fraction were dominated by 13α(n-alkyl)-tricyclic terpanes (13αNATTs) and drimanes. The 13αNATTs appeared to be diagnostic biomarkers for special, ancient microbes in the Proterozoic, as they were only detected in Meso–Neoproterozoic bitumens and were absent in Phanerozoic sequences. Abundant drimane series, including extended drimanes, may be sourced from the ring C cleavage of 13αNATTs or may directly originate from the same precursor of 13αNATTs, based on the observation that the concentrations of drimanes and 13αNATTs exhibited a good correlation. Elevated C19 and C20 regular tricyclic terpanes together with abundant rearranged hopanes, which are usually typical fingerprints for terrestrial rocks, are distinct features of the Xiamaling and Hongshuizhuang marine rock samples. These compounds are considered to be derived from unique prokaryotes that were present in the Mesoproterozoic sea, which could synthesize abundant precursors of rearranged hopanes and low molecular tricyclic terpanes. Pristane (Pr) and phytane (Ph), mainly originated from the phytol side-chain of chlorophyll, were only present in very low abundances, even in the low maturity and organic-rich Xiamaling oil shales (total organic carbon >20%). This may indicate a contribution of phototrophs with other molecular proxies, rather than phytanic lipids in their chlorophylls. An absence or trace amounts of steranes in the samples may reveal a low contribution of eukaryotes, although previous studies have found a wide occurrence of eukaryotes in the Xiamaling and Hongshuizhuang formations, as well as in even more ancient rocks in the Yanliao Rift. Low Pr/Ph ratio values (<1.0), together with an absence of dibenzothiophenes and aryl isoprenoids, may indicate a common anoxic environment with relatively low sulfate concentrations and a low contribution of green sulfur bacteria. Several maturity indictors, such as Tmax, C31 22 S/(22 S + 22 R), and the methylphenanthrene index, all suggest an early oil-window thermal maturity for the Xiamaling rocks and a middle oil-window maturity for the Hongshuizhuang rocks.