Geochemical characteristics of the Chang 7 Member in the southwestern Ordos Basin, China: the influence of sedimentary environment on the organic matter enrichment

Abstract

The enrichment of organic matter is of great significance to the formation of source rocks, while its mechanism in the Chang 7 Member (Chang 71, Chang 72, and Chang 73 periods) of the southwestern Ordos Basin has not been studied in depth. Forty-eight samples obtained from Chang 7 Member were subjected to rock pyrolysis, gas chromatography-mass spectrometry (GC-MS) analysis, and determination experiments of major and trace elements to evaluate the hydrocarbon potential of the source rock, to reconstruct the sedimentary environment, and to analyze the factors of the organic matter enrichment. The results showed that the average total organic carbon (TOC) contents were 0.70%, 5.48%, and 7.70% for Chang 71, Chang 72, and Chang 73, respectively. From Chang 73 to Chang 71 period, the contents of TiO2, Al2O3, Fe2O3, and P2O5 represented a gradual increase of terrigenous input and a gradual decrease in volcanism; the Chemical Alteration Index and the Sr/Cu values reflected warm and humid paleoclimate conditions with a gradual decrease in temperature; the V/(V+Ni) and the Mo-U covariation revealed an anoxic environment with a weakened reduction; and the P/Ti-Cu/Ti model indicated a gradual decrease in paleoproductivity. The linear relationship between indicators of paleoenvironment and TOC indicated that a small amount of terrigenous input, volcanism, warm and humid climate and reducing conditions were more conducive to the enrichment of organic matter; thus we propose a Chang 73 organic matter enrichment production model and a Chang 71 organic matter enrichment preservation-dilution model.