Abstract
In the Carboniferous–Permian period, several organic-rich black shales were deposited in a marine–continental transitional environment in the Linfen area on the eastern margin of the Ordos Basin. Integrated sedimentological and organic geochemical analyses are performed on an outcrop in order to clarify the relationship between paleoenvironment and organic matter accumulation. The results of this study show that the marine–continental transitional strata of the Upper Carboniferous Benxi Formation to Lower Permian Taiyuan and Shanxi Formation exposed in the Linfen area are composed of sandstone, shale, coal, and limestone. Total organic carbon (TOC) contents of the studied samples were mainly distributed in the range of 0.59%–35.4%, with an average of 7.32%. From Benxi Formation to Shanxi formation, the humidity gradually increased, and the climate gradually changed from hot and humid to warm and humid during Carboniferous to Permian. The deposition of the Shanxi Formation ended with the climate returning to hot and humid, having an oxic-suboxic conditions and a high paleoproductivity. Paleoredox conditions and paleoproductivity are the two vital factors controlling the formation of organic matter in black shales. The transitional environment characterized by oxic-suboxic, relatively high deposition rate, and various source of organic matter, although different from the marine environment, provides a good material basis for the deposition of organic-rich shales.
Highlights
Unconventional resources have become an important sector in the world energy; shale gas, as a vital proportion of unconventional natural gas, has drawn tremendous attention from both academia and industry [1,2,3,4]
A few authors argued that transitional shales were deposited under lagoon environment [20,21], while others argued that transitional shales were dominated by prodelta and delta plain facies, and determining which sedimentary environment favors the deposition and accumulation of organic-matter-rich shales requires further research
The results show that the Benxi Formation lies within the area of active continental margin, the Taiyuan Formation lies within the area of passive continental margin, while the Shanxi Formation lies within the area of active and passive continental margin
Summary
Unconventional resources have become an important sector in the world energy; shale gas, as a vital proportion of unconventional natural gas, has drawn tremendous attention from both academia and industry [1,2,3,4]. Thin sandstones, limestones, and coals are usually embedded within transitional shales, forming a vertical lithological assemblage that is different from the vertically continuous marine shales (~200 m thick) in the Sichuan Basin. This is attributed to the frequent variations of depositional facies, while marine shales were deposited within stable shelf environments [1]. A few authors argued that transitional shales were deposited under lagoon environment [20,21], while others argued that transitional shales were dominated by prodelta and delta plain facies, and determining which sedimentary environment favors the deposition and accumulation of organic-matter-rich shales requires further research
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