Biological sources, paleoenvironment, and organic matter enrichment in source rocks of the Ordovician Majiagou Formation, Ordos Basin, China:Evidence from biomarkers,microfossils, and inorganic geochemical analyses  

作　　者：Jing-Jing Cao Wen-Zhe Gang Shang-Ru Yang 

机构地区：[1]State Key Laboratory of Petroleum Resources and Prospecting,China University of Petroleum(Bejing),Beijing 102249,China [2]College of Geosciences,China University of Petroleum(Bejing),Beijing 102249,China [3]Oil and Gas Technology Research Institute,PetroChina Changqing Oifield Company,Xi'an 710018,Shaanxi Province,China

出　　处：《Journal of Palaeogeography》2024年第1期92-115,共24页古地理学报（英文版）

基　　金：supported by the PetroChina Changqing Oilfield Company (Qitan 2019-022)。

摘　　要：Carbonate deposits in the Ordovivian Majiagou Formation are significant source rocks for natural gas generation in the Ordos Basin, northwestern China. Previous studies mainly focused on the organic matter enrichment mechanism of shales rather than carbonate rocks. The biological sources and paleoenvironment of carbonate source rocks, and the main controlling factors of organic matter enrichment in the carbonate source rocks were studied in this paper in combination with evidence from biomarkers, microfossils and inorganic geochemistry analysis. The results show that four types of microfacies were identified in the Majiagou Formation, respectively as: mud flat microfacies, mud-dolomite flat microfacies, dolomite flat microfacies, and open marine microfacies. The biological sources of organic matter are chiefly planktonic algae, followed by bacteria. The mud flat and mud-dolomite flat contain a high abundance of terrigenous detrital inputs, as indicated by the high content of Al_(2)O_(3), TiO_(2), Th, and Zr. The low Sr/Cu and high Rb/Sr values reveal warm and humid paleoclimate conditions in the mud flat and mud-dolomite flat, whereas the dolomite flat and open marine were likely formed in hot and arid paleoclimate conditions. The mud flat and mud-dolomite flat deposits were characterized by high paleoproductivity of the Majiagou Formation. Low Sr/Ba values were found in the mud flat samples, indicating fresh to brackish water condition, whereas samples of other facies have a relatively high degree of salinity. Based on U_(auth), Mo_(auth), Cr_(auth), Co_(auth), δCe, and δEu values, the mud flat microfacies was formed in a suboxic and anoxic environment, whereas the mud-dolomite flat, dolomite flat and open marine microfacies were within dysoxic to oxic conditions. A model of organic matter enrichment in the Majiagou Formation is thus established. The level of terrigenous detrital inputs is the principal factor of organic matter enrichment in the Majiagou Formation, secondly are redox condition and then paleopr

关 键 词：Biological source MICROFACIES Trace elements Organic matter Majiagou Formation Ordos Basin 

分 类 号：P618.13[天文地球—矿床学]