Abstract
A suite of source rock consists of mudstone and shale, with great thickness and continuous deposition was found in the well LK-1 in Lingshan island in Ri-Qing-Wei basin. In order to evaluate the hydrocarbon generation prospects of these source rock and find the mechanism of organic matter enrichment, shale samples were selected from the core for TOC (total organic carbon) and element geochemistry analysis. The results show that organic matter abundance of the source rocks are generally high with average TOC content of 1.26 wt%, suggesting they are good source rocks. The geochemical features show that the sedimentary environment is mostly anoxic brackish water to salt water environment with arid to semiarid climate condition. The enrichment mechanism of organic matter varied with the evolution of the basin, which was divided into three stages according to the sedimentary characteristics. In the initial-middle period of rifting evolution (stage 1 and early stage 2), paleoproductivity is the major factor of OM-enrichment reflecting by high positive correlation between the TOC contents and paleoproductivity proxies. While with the evolution of the rift basin, redox condition and terrigenous clastic input became more and more important until they became the major factor of OM enrichment in the middle stage of rift evolution (stage 2). In the later stage of rift evolution (latest stage 2 and stage 3), besides terrigenous clastic input, the effect of paleoclimate on OM-enrichment increased gradually from a minor factor to a major factor.
Highlights
As the most important type of source rock, shale is widely distributed in almost every depositional setting in different tectonic settings (Verma and Armstrong-Altrin, 2013)
This study focuses on the core sample of LK-1, using Total Organic Carbon (TOC) content and element contents of shale samples to reveal the mechanism of organic matter enrichment
The TOC, major element, trace element and rare element contents of LK-1 shales analyzed in this study are listed in the Supplementary Table S1
Summary
As the most important type of source rock, shale is widely distributed in almost every depositional setting in different tectonic settings (Verma and Armstrong-Altrin, 2013). Previous researches (Yan et al, 2018; Gallego-Torres et al, 2007; Mort et al, 2007) indicate that restricted basin sedimentary conditions are good for organic matter (OM) enrichment and there are two typical models for OM enrichment: the paleoproductivity model and the preservation model. While the preservation model stresses the importance of the sea level fluctuation and the restricted effect of basin, proving an anoxic environment for OM accumulation (Arthur et al, 1998; Mort et al, 2007; Tang et al, 2020). The geochemistry of shale could reflect the sedimentary environment such as Ba, P, Cu for paleoproductivity (Paytan and Griffith, 2007; Algeo et al, 2011), Mo, U, V for redox condition and Al, Ti, Zr for terrigenous clastic input (Tribovillard et al, 2006)
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