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

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.