Abstract: In order to study the origin of organic matter enriched siliceous shale from WufengLongmaxi Formations in the Middle and Upper Yangtze region, and its relationship with shale gas accumulation, rock features, mineral composition, micromorphology, structural characteristics and chemical elements were analyzed by Xray diffraction, scanning electron microscope (SEM) and energy disperse spectroscopy methods. Quartz content was found to be relatively high, mostly above 45% and 88% being the highest, while clay mineral content was relatively low in the Wufeng Formation and lower section of the Longmaxi Formation.  Abundant biological residue and debris can be seen in the siliceous shales under SEM, they acted as skeleton particles and bound tightly with the mineral matrix, resulting in a highly compact rock structure; clay mineral enriched shales, whereas, developed horizontal beddings. Under SEM, the observed circular layered growth structure and needle structures of the biogenic materials clearly showed the characteristics of radiolarian morphology. The chemical compositions of these structures were shown to be mainly consisted of silicon and oxygen elements by energy spectrum analysis, further providing proofs to their radiolarian origin. Moreover, surface distribution of elements indicated the terrigenous input and hydrothermal activity were very limited, implying silica was mainly from radiolarian skeleton. Altogether we now have sufficient evidence to support a biogenic origin for siliceous shales in the region. High soluble silica concentration is prerequisite for radiolarian enrichment, it creates an environment for algae proliferation which leads to organic matter accumulation. Biogenic silicon was also favorable for the preservation of primary inorganic and secondary organic pores. Enrichment of biogenic silica, therefore, plays an important role in shale gas accumulation in the Wufeng and Longmachi Formations.

Key words: biogenic, silica, quartz, radiolarian, siliceous shale