| Based on drilling and logging data, the law of organic matter accumulation in shale of Wufeng Formation and
Longmaxi Formation in southern Sichuan was studied with cyclostratigraphic and geochemical methods. According to the
GR series of Well Y8, the cyclostratigraphy of the Late Ordovician Wufeng Formation-Early Silurian Longmaxi
Formation was studied. The long slope period (1.2 Ma), long eccentricity period (405 ka), and short eccentricity period (100 ka)
signals recorded in sediments were identified, and the astronomical time scales floating in this geological history period
were established. Geochemical analysis shows that the total organic carbon content (TOC) at the bottom of the Wufeng
Formation-Longmaxi Formation is relatively high, with an average content of 3.16%, and becomes lower and tends to be
stable upward. Combining organic carbon isotopes (δ13Corg) and trace element ratios such as U/Th, V/Cr, Ni/Co, V/(V+Ni),
the sea level change and paleo-ocean sedimentary environment in Late Ordovician-Early Silurian were restored in
southern Sichuan. The long slope period (1.2 Ma) has a good correlation with the trend of global sea level change in Late
Ordovician-Early Silurian. The accumulation of organic matter in Wufeng Formation-Longmaxi Formation in southern
Sichuan is closely related to the long-term fluctuation of sea level driven by the long slope period and the redox condition
of water body: the sea level rises in the early Katian, late Hirnantian, and early Rhuddanian in southern Sichuan, and the
semi deep-deep sea environment is favorable for the formation and preservation of organic matter; during the deposition
of Guanyinqiao carbonate strata, the sea level dropped, and the shallow sea environment was not conducive to the
formation and preservation of organic matter. |
