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This research reveals the depositional environment can affect the organic matter enrichment. The major elements and trace elements are important indexes to reconstruct the depositional environment of Ordovician subsalt marine formations in the central-east Ordos Basin, combining organic parameters with inorganic parameters. Based on the microfacies, the study area is divided into gypsum-bearing dolomite flat, dolomite flat, gypsum-salt depression and limy dolomite flat. The higher Ti/Al ratios and the characteristics of trace elements show that there were detrital inputs in the sedimentation process. When the amounts of detrital inputs are large enough, they can obviously adsorb the elements and dilute the organic matter. Therefore, gypsum-bearing dolomite flat and dolomite flat have lower organic matter accumulation with higher detrital input. Lower P/Ti and Ba excess indicate the deficiency of paleoproductivity. Sr/Ba is higher than 0.6, indicating marine-continental transitional and marine environment. Even the paleosalinity is higher, previous researches have demonstrated the descending sea levels and strong hydrodynamic conditions are unfavorable for organic matter preservation. Lower paleoclimate index "C" and higher Sr/Cu illustrate arid and hot conditions. The value of V/(V + Ni) suggests that the limy dolomite flat is under the oxidizing conditions, and the three other environments are reductive to a certain extent. The Mo-U covariation shows that the water body is under strongly restricted conditions. These conditions are not conducive to biological growth and the accumulation of organic matter, causing low ancient productivity. The partial correlation analysis shows: the organic matter enrichment is affected by multiple factors; paleoproductivity, paleosalinity and paleoclimate have greater influence on the results of organic matter enrichment, and detrital input and oxidation conditions can decrease the paleoproductivity, indicating multiple factors are interrelated and interact with each other. • Major and trace elements are used to reconstructs the depositional environment of four types microfacies. • Paleoproductivity, paleosalinity and paleoclimate have greater influence on the organic matter enrichment. • Depositional environment has direction relationship with low paleoproductivity, causing low organic matter enrichment. [ABSTRACT FROM AUTHOR]

The origin and potential source of the presalt natural gas in the Lower Paleozoic Ordovician carbonates within the Ordos Basin, China, is heavily debated, and the causes of carbon isotope reversal are currently unclear. This study analyzes the source and origin of the presalt natural gas and determines the reasons for carbon isotope reversal. The results show that the nonhydrocarbons contain CO 2 , H 2 S, and N 2 and some gas samples have high H 2 S contents of 9.02%–23.58%. The dryness coefficient (C 1 /C 1+) of 91.06%–99.99% (ave. 98.34%) suggests that the presalt gas is mainly a dry gas with some wet gas and that it originates from a high-to over-mature source. However, the results show that the presalt natural gas is primarily a wet-gas-cracking gas, rather than a kerogen-cracking gas, with a certain amount of coal-type gas mixing and it mainly originated from oil-prone marine carbonate source rocks rather than the Permian–Carboniferous coal measures. The gas has different types of carbon isotope reversal patterns, and the δ13C 2 values become negative at Ro equ > 1.5%. The mixing between the oil-cracking and wet-gas-cracking gas at Ro equ > 1.5% results in carbon isotope reversal (δ13C 1 > δ13C 2), rather than Rayleigh and migration fractionation. The occurrence of thermochemical sulfate reduction (TSR) is supported by the high abundance of H 2 S in the presalt gas, reservoir bitumen in dissolved pores, secondary calcite with negative δ13C carbonate of −21.1‰ to −9.7‰, and diagenetic cubic pyrite. It is evident that the heavy δ13C 2 values (greater than −28‰) and the carbon isotope reversal of δ13C 2 > δ13C 3 of the presalt natural gas at Ro equ < 1.5% are related to TSR. This study also establishes a new model of the carbon isotope reversal of the natural gas, which is primarily controlled by its thermal maturity and TSR. Six carbon isotope patterns can be well explained and are supported by real gas samples from the presalt strata, and two types of carbon isotope reversal are predicted. • The presalt gas is mainly oil-derived gas and dry gas with some wet gas. • Heavy δ13C 2 values (>−28‰) and carbon isotope reversal of δ13C 2 > δ13C 3 of are caused by TSR at low maturity. • The δ13C 2 values become greatly more negative at Ro equ > 1.5%. • A new model of carbon isotope reversal is established firstly, controlled by maturity and TSR. [ABSTRACT FROM AUTHOR]