Potential causes of depleted δ13Ccarb excursions in Ordovician marine carbonates, Ordos Basin, China

Li, Yining; Liu, Wenhui; Jing, Xianghui; Liu, Peng; Zhang, Dongdong; Luo, Hao; Wang, Qingtao; Pei, Lixin; Zhai, Guanghui

doi:10.1016/j.marpetgeo.2021.105331

Cited by 4 publications

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“…Recently, several major exploration breakthroughs have been gained in marine carbonate sequences in China. , Although marine carbonate rocks in Chinese petroliferous basins are characterized by high evolution levels, deep burial, and low total organic carbon (TOC) contents, widespread carbonates are considered to be potentially effective hydrocarbon source rocks. − Recently, several giant gas fields, including the Daniudi and Jingbian gas fields, have been discovered during exploration in the Lower Paleozoic carbonate sequences in the Ordos Basin. , Gas geochemical evidence indicates that the natural gas in the above giant natural gas fields was sourced from the Lower Paleozoic carbonate sequences. , However, TOC content, which is considered the main evaluation indicator for effective source rocks, is low in Majiagou carbonate rocks, ranging from 0.01 to 0.83 wt %. , An obvious discrepancy of over 0.7 in the TOC contents of carbonate rocks can be observed in the Majiagou Formation . The TOC contents of several intervals are over 0.5 wt %, which achieves the minimum TOC standard of effective hydrocarbon source rocks…”

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confidence: 99%

Paleoenvironment and Organic Matter Enrichment of the Middle Ordovician Marine Carbonates in the Ordos Basin of China: Evidence from Element Geochemistry

Liu

et al. 2021

ACS Earth Space Chem.

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The Middle Ordovician Majiagou marine carbonates in the Ordos Basin, China, are important natural gas exploration targets and potential gas source rocks. However, the hydrocarbon generation potential of marine carbonates is difficult to evaluate due to their old ages, high thermal maturities, and low total organic carbon (TOC) contents. The enrichments of organic matter deposited in the sedimentary period are of significance to the subsequent hydrocarbon generation process. Integrated petrology and geochemistry studies, including mineral and maceral contents, major and trace element contents, TOC, and organic carbon isotope signatures, were carried out on the Majiagou Formation. The detrital input, paleoproductivity, and preservation conditions of organic matter, which are essential control factors for the enrichment of organic matter, were discussed in detail. The results showed that sea level changes regulated the conditions for the enrichment of organic matter in the Majiagou sedimentary period. The third member of the Majiagou Formation (O2m3) resulted in high paleoproductivity (high Ba/Al, Cu/Al, and TOC values), terrigenous input (high Ti/Tiavergae shale and TCP/TOC values), and an anoxic (V/(V + Ni) > 0.6) and salty environment (high anhydrite contents) for the preservation of organic matter. Conversely, there is little organic matter input in the fourth member of the Majiagou Formation (O2m4) under the transgressive environment. Gypsum not only plays a potentially important role in the blooming of special organisms (halophilic archaea) but is also beneficial for the preservation of organic matter.

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