Mixed lithofacies shale and depositional environment of Longmaxi Formation in southern margin of Sichuan Basin

Marine shale is well developed in the southern margin of Sichuan Basin, especially the Upper Ordovician Wufeng Formation and Lower Silurian Longmaxi formation, which is a good reservoir for shale gas exploration and development.In order to study the shale facies and sedimentary environment, taking Longmaxi formation shale(member L1) of Well Y5 in the southern margin of Sichuan Basin as the research object, based on two main factors affecting shale gas exploration and development, shale lithofacies division scheme based on TOC content and shale mineral composition is proposed in this paper. By using X-ray diffraction analysis and TOC content test data, shale facies division is carried out, and the properties of shale with different lithofacies were analyzed; the sedimentary environments of different lithofacies shale were studied by the analysis of major and trace elements; the development characteristics of organic pores of different litho facies shale were found out by observing FE-SEM images of different lithofacies shale, extracting organic pores with Image J software and the calculation method of organic porosity proposed by predecessors; finally, the sedimentary environments for different lithofacies shale were summarized the control of the abundance of organic matter and the development of organic pores. The results show that the organic rich clayey siliceous mixed shale(OR-M-1) and organic rich calcareous siliceous mixed shale(OER-M-3) are mainly developed in the L1 member; the trace elements V/Cr, Ni / Co and U/Th indicate that the OR-M-1 shale in the lower Longmaxi member is formed in oxygen rich anoxic sea water, while the OER-M-3 shale is formed in anaerobic water, and the Ba/Al,P/Ti and Cu+Zn+Ni indicators indicate two lithofacies depositsare little difference in environmental paleomarine productivity. The percentages of Al and Ti in the samples indicate that the input strength of terrigenous clasts of OR-M-1 shale in sedimentary period is greater than that of OER-M-3 shale. Based on the Paleoenvironment of water body stratification caused by sea level change and upwelling current, the organic matter formation conditions of OER-M-3 shale in sedimentary period are better than that of OR-M-1 shale, and the ratio of organic matter pore volume of OER-M-3 shale to total pore volume of shale Higher, more conducive to shale gas adsorption.