Paleoenvironmental characteristics and organic matter enrichment mechanisms of the upper Ordovician-lower Silurian organic-rich black shales in the Yangtze foreland basin, South China

Abstract

The Wufeng Formation-Longmaxi Formation (O3w-S1l1) black shales within the Yangtze foreland basin (South China) hold significant potential as unconventional oil and gas resources. However, several challenges, including sluggish sedimentation rates, thin sedimentary layers, and notable homogeneity, hinder a comprehensive grasp of the organic matter enrichment (OME) mechanism within these shales. These challenges impede the exploration of unconventional oil and gas resources. This study aims to identify the O3w-S1l1 black shales in the Weiyuan area through a combined approach of organic and inorganic geochemical analyses. By doing so, it delves into the controlling mechanism behind OME in the black shales of the Yangtze foreland basin. The findings of this research reveal that the O3w-S1l1 black shales primarily consist of sapropelic kerogen and exhibit signs of over-maturation, with TOC content ranging from 0.43% to 8.21%. These shales are classified as organic-rich, mainly composed mixed and siliceous lithofacies. The presence of silica in the shales originates mainly from biogenic sources, and the highest TOC content coincides with a Sixs contribution of around 30% to the total silica content. During the late Katian, a global sea level drop led to an oxic paleoenvironment, transitioning into a euxinic paleoenvironment as sea levels rose in the early Rhuddanian. Notably, TOC content exhibited a significant correlation with paleoceanographic conditions (e.g., V/Cr, Ni/Co) and paleoproductivity levels (e.g., P/Al, Babio), while displaying a negative correlation with paleoclimate conditions (e.g., CIA, C-value), terrigenous detrital input intensity (e.g., Sixs, Ti/Al), and sedimentation rate ((La/Yb)N). Specifically, favourable conditions for OME encompass robust reducing seawater conditions, high paleoproductivity, a humid climate, reduced influx of terrigenous debris, and relatively elevated sedimentation rates. Regional tectonic movements (Guangxi movement) and fluctuations in relative sea levels exert influence over the deposition of O3w-S1l1 black shales. This study provides a valuable case study for exploring and developing shale gas resources within the Yangtze foreland basin.

Rare Earth Element Characteristics of Shales from Wufeng–Longmaxi Formations in Deep-Buried Areas of the Northern Sichuan Basin, Southern China: Implications for Provenance, Depositional Conditions, and Paleoclimate

“…This indicates that the provenance supply or sedimentary-tectonic setting may have changed during the sedimentary period of the Wufeng Formation shale, resulting in the opposite trend of variation of the LREE and HREE contents and the failure to maintain the same ratio. 10 During the sedimentary period of the Longmaxi Formation shale, it may have been under a relatively stable provenance supply and sedimentary-tectonic background. Although the contents of ∑REE, LREE, and HREE vary from bottom to top, the ratio of LREE to HREE is relatively stable, maintaining the overall characteristics of light rare earth enrichment and heavy rare earth depletion (Figure 2).…”

“…34 Surrounded by these old land and uplifts, the Upper Yangtze Platform evolved in the Middle Ordovician sea area with extensive characteristics into the semilimited sea area adjacent to the northern part of the Qinling Sea in the Late Ordovician, forming a large area of low-energy, under-compensated, and anoxic sedimentary conditions (Figure 1b). 10,35 The Sichuan Basin is located on the Upper Yangtze Platform. 36 Well XX1, whose geological location is in Tongjiang County, Bazhong City, Sichuan Province, is located in the northern part of the Sichuan Basin (Figure 1b).…”

“…The vitrinite reflectance Ro of the Wufeng Formation shale of Well XX1 is 2.8%, while the vitrinite reflectance Ro of the Longmaxi Formation shale is 2.2%. 10 Although there are few measuring points for vitrinite in the shale, combined with their current burial depth exceeding 6000 m, this indicates that the evolution of organic matter in the Wufeng−Longmaxi Formation shale has entered a stage of high maturity to overmaturity. Diagenesis has entered the phase of quasimetamorphism.…”

“…Some scholars also believe that the deposition of this set of shales is the result of the synergistic effect of paleoclimate and paleoceanic factors caused by geological time such as volcanic eruption, undercurrent intrusion, and terrigenous detrital input. 10,13,16 In summary, there is still a great deal of controversy regarding the formation environment of the Wufeng−Longmaxi Formation shale, and there is also less research on the source background. Currently, previous studies on the geochemical characteristics of the Wufeng− Longmaxi Formation black shales buried less than 3500 m have been conducted in depth.…”

“…The end of the Ordovician and the beginning of the Silurian was an important period in Earth history, − and it is marked by marine mass extinction, large-scale glaciation, sea-level change, extensive volcanism, ocean anoxic event, and extensive deposition of marine black shale. − Black marine shale, as a source rock for oil and gas, is widely accumulated in paleogeographic environments. − To this day, such a black shale has been widely developed as unconventional reservoirs in many countries, such as the United States, Canada, Australia, China, Germany, Russia, etc. − Thus, the study of the sedimentary environment and the mechanism of organic matter accumulation for the marine shales of the Upper Ordovician Wufeng Formation and the Lower Silurian Longmaxi Formation in the northern Sichuan Basin not only can provide important information about the climatic and paleoenvironmental change of the Ordovician–Silurian but also provides evidence for the prediction of favorable areas for oil and gas exploration.…”