Biogenic origin and hydrocarbon significance of siliceous shale from the Wufeng-Longmaxi formations in Fuling area, southeastern Sichuan Basin

Drill core samples were collected from the Fuling area in the southeastern Sichuan Basin to study the origin of siliceous shale rich in organic matter in the Wufeng-Longmaxi formations. X-ray diffraction, optical microscopy, scanning electron microscopy and energy dispersive spectroscopy were applied to study the mineral components, micro-morphology and structure of the samples. Quartz is dominant in the lower part of Wufeng-Longmaxi formations, and decreases gradually upwards, while clay content increases. Abundant biological residue and debris with circled growth structure and needle structure were clearly observed, especially in the Wufeng Formation and the lower part of Longmaxi Formation. The circled structure has a skirt-like boundary, and was filled with microcrystalline quartz. Smaller quartz grains among biological residues are like irregular balls or ellipsoids. They are not idiomorphic, and are authigenic quartz via silica precipitation from siliceous organism dissolution. Debris scattered in mineral matrix, either in complete or clastic form. The enrichment of siliceous organisms and siliceous recrystallization provide direct biogenic evidence for siliceous shale in the Wufeng-Longmaxi formations, which has an important significance for potential hydrocarbon recovery, water sedimentary environment trace, micro-pore and fracture structure, and rock mechanics analysis.