Organic matter inputs and depositional palaeoenvironment recorded by biomarkers of marine‐terrestrial transitional shale in the Southern North China Basin

Abstract

In recent years, significant breakthroughs have been achieved in shale gas exploration and exploitation of marine and continental shales in petroliferous basins of China. As an important petroliferous basin in China, marine–terrestrial transitional shale was widely distributed among the Southern North China Basin (SNCB). However, biomarkers distribution characteristics of marine–terrestrial transitional shale in the SNCB, which can reveal the organic matter inputs and paleoenvironmental characteristics, is not widely concerned. In general, organic matter in a transitional environment can be affected by the amounts of organic matters input from fresh water and aquatic organisms from the sea. To systematically investigate the organic geochemical characteristics of biomarkers in marine–terrestrial transitional shale from SNCB, core samples from the Upper Shihezi, Shanxi and Taiyuan formations were obtained from a single borehole in the SNCB. Gas chromatography–mass spectrometry (GC–MS) analysis was conducted on the saturated hydrocarbons. n‐Alkanes, acyclic isoprenoid alkanes, terpanes, and steranes were detected to study the change in organic matter inputs and sedimentary paleoenvironment of the marine–terrestrial transitional shale. The results reveal that organic matter inputs obviously changed during the sedimentary period in the study area of the SNCB. In addition, the terrestrial higher plants contributed more organic matter to shale from the Upper Shihezi Formation compared to the Taiyuan Formation and the Shanxi Formation. The shift of organic matter inputs is consistent with marine regression in the Early Permian period, which may significantly influence the organic matter input characteristics. Meanwhile, the marine–terrestrial transitional shale, including samples from the Taiyuan, Shanxi and Upper Shihezi formations, was deposited in a relatively oxic condition during the depositional process.