Characteristics of microorganisms and origin of organic matter in Wufeng Formation and Longmaxi Formation in Sichuan Basin, South China

To evaluate the characteristics of microorganisms and the origin of organic matter of the Wufeng Formation and Longmaxi Formation (WF-LMX) in Sichuan Basin (South China), the type and number of microorganisms were determined in this study using scanning electron microscope (SEM) microphotographs and microbial extraction. The contribution of various microorganisms to organic matter was analyzed in combination with the data of biomarkers and total organic carbon (TOC). It can be deduced from the paleogeography and pristine/phytane (Pr/Ph) ratio that the microorganisms of WF-LMX resided in 60 m-200 m deep shelf with anoxic bottom water. From SEM microphotographs and microbial microscopy analysis, it was demonstrated that the microorganisms in the WF-LMX can be divided into 3 types: planktonic algae (mainly including acritarchs and green algae), fungi, and benthic algae. Acritarchs were the most common microorganisms in the WF-LMX with the average relative content of 60.5%. The average relative contents of benthic algae, fungi, and green algae were 17.6%, 18.0% and 3.9%, respectively. Since the acritarchs and green algae in the Wufeng Formation and the Longmaxi Formation basically belong to planktonic algae, the relative content of planktonic algae in them is about 64.4%. The strata of WF-LMX can be divided into 3 sections based on the distribution characteristics of various microorganisms. The relative content of acritarchs gradually decreased upwards, while that of benthic algae gradually increased. The multiple proxies of biomarkers proved that the planktonic algae was the main source of organic matter in the WF-LMX. Combining the statistical data of microorganisms, it was considered that the organic matter in WF-LMX was mainly derived from acritarchs, which might be the components of planktonic algae. The fungi had a significant contribution to the organic matter in organic-rich layers (TOC > 2.0%) rather than in organic-poor layers, while green algae and benthic algae made little contribution to the organic matters in WF-LMX.